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October 07 onwards

Thu, Oct 4, '07


Ed McCann, Lancaster

Electrons in bilayer graphene

 

Thu, Oct 11, '07


Benjamin Doyon, Durham

Quantum impurities in non-equilibrium steady states

With the present experimental interest in quantum dots and other mesoscopic objects submitted to electric currents, an efficient theoretical framework for studying quantum impurities in non-equilibrium steady states is much needed. After reviewing the topic of quantum impurities with relevant experiments and theoretical ideas, I will present aspects of some recent progress I made. It is a new theoretical framework that I developed recently in the interacting resonant level model (IRLM). I will explain how a non-equilibrium steady-state (Hershfield's) density matrix can be defined, why it is related the physical (Schwinger-Keldysh) construction of steady states, and how the dynamics can be encoded into conditions on the Hilbert space ("impurity conditions"). Then I will show how these simple but slightly abstract concepts immediately give the full perturbative series (as multiple integrals), without using Feynmann diagrams or Keldysh time-ordering; I will discuss the RG-improved results in the IRLM. Finally, if time allows, I will discuss how these same concepts can be used to answer the question of integrability in equilibrium and in non-equilibrium steady states.

 

Thu Oct 18, '07


Roddy Vann, York

Frequency Splitting of Compressional Alfvén Waves

 

Thu, Oct 25, '07


Matthias Schmidt, Bristol

Interfacial properties of colloidal platelet dispersions

 

Thu, Nov 1, '07


David MacKay, Cavendish

Sustainable Energy - Without the Hot Air

 

Thu, Nov 8, '07


Julia Yeomans, Oxford

Swimming with a friend at low Reynolds number

 

Thu, Nov 15, '07


Kai Bongs, B'ham & Hamburg

Cold atoms - the world of the ultracold

 

Thu, Nov 22, '07


Stephen Wells, Warwick

Rigidity and flexible motion in biomolecules

It has been known since Maxwell that a count of degrees of freedom and constraints can establish if a structure is, overall, floppy, rigid or stressed (overconstrained). The "pebble game" algorithm can rapidly identify the rigid and stressed regions of a two-dimensional framework. The "Molecular Framework Conjecture" states that the "pebble game" is valid for networks with nearest-neighbour and next-nearest-neighbour constraints, or equivalently, to frameworks with fixed bond lengths and angles and variable dihedral angles. So, we can apply rigidity analysis to protein structures (as obtained from X-ray or neutron crystallography) and identify rigid substructures. Rigidity analysis provides a natural coarse-graining for a simplified model of protein motion. This in turn allows us to address flexible (slow, low-energy) motions in proteins using the technique of "geometric simulation". Finally, I will show an example of synergy between molecular-dynamics simulation and geometric simulation in the field of protein structure prediction.

 

Thu, Nov 29, '07


Seminar has been postponed

Thu, Dec 6, '07


Arash Mostofi, Imperial

Maximally-localised Wannier functions as building blocks of electronic structure

We combine large-scale, ab initio electronic structure calculations and the maximally-localised Wannier function (MLWF) approach in order to study the electronic properties of complex nanostructures. MLWFs provide an accurate, localised, minimal basis set in which to diagonalise the Hamiltonian. In the MLWF basis, Hamiltonians for large, complex systems can be constructed directly from the short-ranged Hamiltonians of smaller constituent units by performing full first-principles calculations on either periodically-repeated or isolated fragments. We apply our approach to the case of DNA helices. This work has led to the development of a new open-source code called Wannier90 [1] and opens the way to obtaining a more detailed understanding of charge transport and conductance in DNA, bringing closer the prospect of engineering its electronic structure for use in nano-electronic circuits and biotechnology applications.

[1] A.A. Mostofi et al., Wannier90: A tool for obtaining maximally-localized Wannier functions, http://arxiv.org/abs/0708.0650, http://www.wannier.org

 

Thu, Jan 10, '08


James Kirkpatrick, Imperial

A multiscale approach to charge mobility simulation

A multiscale approach to simulation of charge mobility is presented. Classical atomistic molecular dynamics is used to obtain morphologies. The relative orientations and positions of molecules are then used to compute charge transport parameters using semi-empirical Quantum Chemical methods. Once the charge transfer rates are known, simulation of charge mobility are performed on a non cubic lattice. This approach is used to compute the temperature dependence of mobility in the discotic liquid crystal hexabenzocoronene. Some remarks will be made on the time dependence of charge transport parameters in the conjugated polymer polypyrrole.

 

Thu, Jan 17, '08


Petr Denissenko, Warwick

Floaters, waves, and the surface tension We argue theoretically and demonstrate experimentally that in a standing wave floating particles drift towards the nodes or anti-nodes depending on their hydrophilic or hydrophobic properties. We explain this effect as the breakdown of Archimedes law by a surface tension, which creates a difference between the masses of the floater and displaced liquid, making the particle effectively inertial. We show analytically and confirm experimentally that the drift appears as a second order effect in wave amplitude. We investigate how the inertial effects change the statistics of floater distribution in the case of random surface waves.

 

Thu, Jan 24, '08


Dimitri Gangardt, Birmingham

Hard core bosons in expansion: dynamics, correlations and relaxation

 

Thu, Jan 31, '08


Andrew Archer, Loughborough

Dynamical density Functional Theory: Dynamics of inhomogeneous liquids and glasses

 

Thu, Feb 7, '08


Neil Wilson, Warwick

Electronic transport in carbon nanotube networks

 

Thu, Feb 14, '08


Dieter Vollhardt, Augsburg

Metal-Insulator Transitions of Correlated Lattice Fermions with Disorder

Thu, Feb 21, '08


Andrea Gamba, Torino

Domain coarsening in eukaryotic directional sensing

The cells of multicellular organisms are endowed with a chemical compass of amazing sensitivity, formed as a result of billions of years of evolution. Concentration differences of the order of a few percent in attractant chemicals from side to side are sufficient to induce a chemical polarization of the membrane leading to cell migration towards the signal source. It has been realized recently that this early polarization process is the result of a phase-separation instability in a well characterized network of diffusion-controlled chemical reactions. We give a universal description of this early symmetry breaking process. Our description implies the existence of two clearly separated polarization regimes depending on the presence or absence of an anisotropic component in the activation pattern, and the existence of a sensitivity threshold for the anisotropic component. In particular, we find that the polarization time tε in the presence of an anisotropic extracellular signal depends on the anisotropy degree ε through the power law tε α ε2, and that in a cell of radius R there should exist a threshold value εth α R-1 for the smallest detectable anisotropy. Our results are in agreement with existing experimental data and explain the recent observation of a threshold in the degree of detectable anisotropy.

 

Thu, Feb 28, '08


Paul Fendley, Virginia & Oxford

Topological Quantum Computation from Non-Abelian Anyons

 

Thu, Mar 6, '08


Ana Lopez, OUCE Oxford

From climate models to impacts of climate change: can we predict anything with any confidence?

 

Thu, Apr 24, '08


Paul Tangney, Imperial

Dynamic sliding friction between coaxial carbon nanotubes

 

Thu, May 1, '08


Martin Plenio, Imperial

Entanglement: From scaling laws to simulations

 

Thu, May 8, '08


Rudolf Roemer, Warwick

Charge Migration in DNA, p53 and other speculations

 

Thu, May 15, '08


Rosalind Allen, Edinburgh

Simulating the flipping of the bacteriophage lambda genetic switch
Bacteriophage lambda is a virus that infects the bacterium Escherichia coli. It is a paradigm for developmental biology because it has two alternative modes of living - the lysogenic state, in which it integrates its DNA into the E. coli chromosome and lives stably, and the lytic state, where it replicates and kills the E. coli cell. We have constructed a stochastic simulation model for the genetic regulatory network that controls the transition from lysogeny to lysis. This network is bistable and subject to random fluctuations, but it has been found experimentally to be extremely stable, flipping spontaneously less than once in 10^9 bacterial generations! We have used the Forward Flux Sampling rare event simulation method to calculate the spontaneous flipping rate. Our results highlight the need to consider nonspecific DNA binding, DNA looping and macromolecular crowding in order to get the "right" answer, leading to some general conclusions about stochastic modelling of gene regulatory networks.

 

Thu, May 22, '08


David Dunstan, QMUL

Quenching of the Inversion Transition of Ammonia - A Classical or Quantum Problem?
In chemistry, molecules have spatial configurations while in physics they do not. Getting from the physics to the chemistry is an example of the problem of measurement, or Schroedinger's cat. Ammonia appears to bridge the gap, being physical at low pressure (as evidenced by its inversion transition) and chemical at high pressure. Consequently, the behaviour of the inversion transition has been a problem of lively interest for over seventy years. We will describe a simple quantum model of the ammonia molecule. Perturbed by collisions with ideal gas molecules, its time evolution is an problem in stochastic mechanics which we address by numerical calculation. The model does not display the behaviour expected from quantum-mechanical theories, but it does display the essential features of the experimental data. To understand this better, we have constructed purely classical models of perturbed oscillators and these do display behaviour similar to our model and to experimental data. However, predicting the behaviour of these stochastic models from their specifications appears to be an unsolved mathematicial problem.

 

Thu, May 29, '08


Andrew Ho, RHUL

Feshbach resonance and multiband physics in ultracold atoms trapped in optical lattices

 

Thu, Jun 5, '08


Dmitriy Krizhanovskii, Sheffield

Intrinsic decoherence mechanisms in the microcavity polariton condensate
The recent observations of polariton condensation in semiconductor microcavities have provided a new, solid state system for the study of Bose-Einstein Condensate (BEC) phenomena (J.Kasprzak et al, Nature 2006). The Bose-condensed state exhibits characteristic properties including massive occupation of the ground state and long range spatial coherence, which due to the very small mass of polaritons (10-5 of the free electron mass) occur at high critical temperatures of 20-30K. We investigated first and second order temporal coherence of polariton BEC, which are fundamental properties of systems undergoing transition into high density macroscopically occupied state. For the first time very long coherence times up to ~150-200 ps, much longer than polariton lifetime (~1.5 ps), are observed for both g1 and g2 -correlation functions. We demonstrate that the phase coherence time of polariton BEC is intrinsically limited by the combined effect of number fluctuations and interactions in the coherent state. The results are explained quantitatively by a quantum-optical model which takes into account interactions, fluctuations, and gain and loss in the system.

 

Wed, Oct 1, '08


John Dixon meeting

Scientific Meeting and Celebration in Honour of Dr. John Dixon

10.30 Conducting properties of the cytoskeleton and internal signal propagation in neurons

Jack Tuszynski, University of Alberta 

11.00 Old and new aspects of the Jahn-Teller Effect
Colin Bates, University of Nottingham 

12.00 Dynamics at the Nanoscale, as seen by Physicist and Biologist
Marshall Stoneham, University College London 

12.30 Magnetic Interactions and Magnetic Resonance: Examples from the Solid State NMR Group at Warwick
Mark Smith, University of Warwick 

 

Thu, Oct 2, '08


Stephen Powell, Oxford

Classical-quantum mappings for unconventional phase transitions in geometrically frustrated systems

 

Thu, Oct 9, '08


Robin Ball, WTG & Complexity

Quantifying Emergent Behaviour

 

Thu, Oct 16, '08


Christopher Mudry, PSI

Electron fractionalization in two-dimensional graphene-like structures
It has been generally believed that charge fractionalization in two-dimensional systems is intimately related to the notion of topological order. I will show in this paper that charge fractionalization in interacting fermionic systems with a Dirac like-spectrum -- as occurs in graphene for example -- can also be a consequence of spontaneous symmetry breaking.

 

Thu, Oct 23, '08


Suzanne Fielding, Manchester

Phase transitions in sheared complex fluids

 

Thu, Oct 30, '08


Mario Nicodemi, WTG & Complexity

On X Chromosome Inactivation

 

Thu, Nov 6, '08


Peter Krueger, Nottingham

Low dimensional physics with cold atoms

 

Thu, Nov 13, '08


Kostya Trachenko, Cambridge

Understanding liquids and glass transition on the basis of elastic interactions

 

Thu, Nov 20, '08


Magnus Richardson, Systems Biology

Mathematical models of cortical neurons

 

Thu, Nov 27, '08


Arthur Peeters, CFSA

Momentum transport due to small scale turbulence in a fusion device

 

Thu, Dec 4, '08


Nick Jones, Oxford

Dynamic Communities in Multichannel Data

Network communities are sets of nodes in a network that are connected to each other more than they are to the rest of the network. We investigate the clustering dynamics of multichannel (multivariate) time series by first representing their correlations as time-dependent networks and then examining the evolution of network communities. To do this, we employ a node-centric approach that allows us to track the functional roles of individual nodes in time without having to track entire communities. As an example, we consider a foreign exchange market network in which each node represents an exchange rate and each edge represents a time-dependent correlation between the rates. Using dynamical community detection, we find that exchange rates with strong intra-community connections are persistently assigned to communities with the same set of nodes.

 

Thu, Dec 18, '08


IoP TCM Group Meeting

Daan Frenkel, Department of Chemistry, Cambridge, 'Dense Packing and Beyond';

Jorge Kurchan, PMMH, Ecole Superieure PCI, Paris, 'Jamming versus Glass Transitions'
Igor Lerner, Birmingham, 'Jumps in current-voltage characteristics in 2D structures near a superconductor-insulator transition'
Christian Ruegg, UCL, 'Controlling Quantum and Thermal Melting in Magnets'



Thu, Jan 8, '09


Paola Carbone, Manchester

Multiscale simulation of macromolecules: static and dynamic properties from equilibrium and non-equilibrium simulations

 

Thu, Jan 15, '09


Thomas Fernholz, Nottingham

A quantum interface between light and atomic ensembles

 

Thu, Jan 22, '09


Gunnar Moeller, Cambridge

The composite particle approach to quantum Hall bilayers at filling one
The physics of the Quantum Hall bilayer systems at filling fractions near \nu=1/2+1/2 is marked by a transition from a compressible phase with strong intralayer correlation to an incompressible phase with strong interlayer correlations as the layer separation d is reduced below some critical value.? Deep in the intralayer phase (large separation) the system can be interpreted as a fluid of composite fermions (CFs), whereas deep in the interlayer phase (small separation) the system can be interpreted as a fluid of composite bosons (CBs). We present evidence for a phase with interlayer pairing occurring for d \gtrsim \ell_0 , which is continuously connected to the CF liquid at large layer separation [1]. Our understanding of this phase derives from the formulation of trial wavefunctions for the ground-state of the quantum Hall bilayer, relying on the comparison to exact results for small systems. We find that p-wave paired CF wavefunctions provide an exceedingly good description of the ground-state for d \gtrsim \ell_0 with \ell_0 the magnetic length. To capture the physics at smaller layer separation, we generalize the reasoning of [2] and introduce the idea of modified pairing wavefunctions by allowing the CFs to be replaced continuously by CBs [3]. Thus, we construct exceedingly good wavefunctions for interlayer spacings of d \lesssim \ell_0, also. The accuracy of the wavefunctions discussed here, compared with exact diagonalization, approaches that of the celebrated Laughlin wavefunction.

 

Thu, Jan 29, '09


Fabian Essler, Oxford

Local Density of States in 1D Mott insulators with a Boundary

We investigate the modification of the electronic local density of states (LDOS) in 1D strongly correlated electron systems due to a boundary or impurity. We show that the impurity LDOS can be used to probe bulk properties such as spin-charge separation of the underlying state of matter. We calculate the boundary LDOS for 1D Mott insulators and charge density wave states using methods of integrable quantum field theory.

 

Thu, Feb 5, '09


Buitelaar, POSTPONED UNTIL 30/4/09

Spin Physics in Carbon Nanotube Double Quantum Dots

 

Thu, Feb 12, '09


Sergei Fedotov, Manchester

Anomalous diffusion, reactions and random walk models

 

Thu, Feb 19, '09


Feliciano Giustino, Oxford

Phonon contribution to the photoemission kink in cuprate superconductors

 

Thu, Feb 26, '09


Mike Cates, Edinburgh

Living Colloids: Dynamics of Bacterial Suspensions

 

Fri, Feb 27, '09


Massimo Inguscio, Florence

AGN (Bham-relay)

Ultracold atoms in optical lattices

Ultracold atoms are excellent systems to quantum-simulate the physics of ideal condensed-matter systems. A fundamental tool for this investigation is represented by optical lattices, i.e. periodic potentials generated by laser standing waves. Quantum phase transitions can be induced and investigated thanks to the possibility of accurately tuning the interactions between the particles, their mobility in the lattice, the amount of disorder and the system dimensionality. We will review some of the recent developments achieved at LENS in this field by studying ultracold quantum gases and mixtures in optical lattices. We will focus on the possibility of introducing short-scale inhomogeneities in the lattice, which allows to study the interplay of interactions and disorder in the superfluid-insulator transition, one of the central problems of contemporary condensed-matter theory. We will discuss the observation of Anderson localization for a non-interacting Bose-Einstein condensate in a disordered optical lattice and the ongoing research to study the physics of localization in the presence of interactions between the atoms. In order to characterize the properties of the different quantum phases novel diagnostic techniques have to be implemented. Recently, inelastic light scattering (Bragg spectroscopy) has allowed to study the excitations of 1D bosonic gases across the superfluid to Mott insulator transition. We will discuss this technique and the perspectives of using this tool to study the physics of disordered and strongly correlated 1D systems.

 

Thu, Mar 12, '09


Yury Sherkunov, Warwick

Optimum electron entangler at low temperatures

 

Thu, Apr 23, '09


Martin McCall, Imperial

Negative Refraction: Causality, Relativity and Controversy

 

Thu, Apr 30, '09


Mark Buitelaar, Cambridge

Spin Physics in Carbon Nanotube Double Quantum Dots

In this talk, I will discuss recent measurements in which we investigate spin blockade and Kondo physics in carbon nanotube double quantum dots. Spin blockade is observed in weakly coupled double quantum dots, when electron transitions between the dots are forbidden by spin conservation. As such, this phenomenon is of considerable importance in spin-based quantum information processing schemes as a way to convert the spin degree of freedom to a much easier detectable charge state or current. We have, for the first time, investigated spin blockade in carbon nanotubes and used the developed techniques to investigate mixing between the spin-singlet and triplet states in these devices. Mixing between the various spin states is most likely mediated by spin-orbit and hyperfine interaction, an understanding of which is important as they will ultimately limit the spin-coherence times in carbon nanotubes which are generally expected to be very long. The ability to control the tunnel couplings in the nanotube devices also allows us to investigate carbon nanotube double quantum dots which are much more strongly coupled to their leads. In this case, we observe pronounced Kondo features when one of the two quantum dots contains an odd number of electrons. In the situation when both quantum dots of the double dot contain an odd number of electrons, we observe a pronounced splitting of the Kondo states in the measured differential conductance. This is direct evidence of the formation of a coherent superposition of the Kondo states of each dot, which form bonding and anti-bonding combinations. The effect has been studied by us as function of tunnel coupling, temperature and magnetic field and will be discussed during the second part of the talk.

 

Thu, May 7, '09


Daphne Klotsa, Nottingham

Structure formations of macroscopic spheres in oscillatory fluid flows

Recently there has been a lot of interest in the collective behaviour and pattern formation in granular matter [1], which can be enhanced by the presence of a fluid. We are interested in the fluid-mediated interactions between macroscopic rigid particles under sinusoidal vibration in a liquid-filled cell. Various patterns and structures in these flows had been reported [2,3] but the exact details remained unknown. At finite Reynolds numbers the oscillatory motion of a rigid object gives a non-zero time-averaged flow, called steady streaming. We have studied pairs of equal-sized spheres which are found to align perpendicular to the direction of oscillation with a well-defined gap between them [4]. Consequently multiple particles form chain patterns aligned across the direction of vibration. These systems have been investigated both in experiment and in simulation. We show that the mechanisms responsible for these effects can be traced to the streaming flows induced by the motion of the solid spheres relative to the fluid [5].

[1] I. S. Aranson and L. S. Tsimring, Reviews of Modern Physics, 78, 641 (2006); [2] R. Wunenburger, V. Carrier and Y. Garrabos, Physics of Fluids, 14, 2350 (2002); [3] G. A. Voth, B. Bigger, M.R. Buckley, W. Losert, M. P. Brenner, H. A. Stone and J. P. Gollub, Phys. Rev.Lett., 8, 234301, (2002); [4] D. Klotsa, M. R. Swift, R. M. Bowley and P. J. King, Phys. Rev. E, 76, 056314 (2007); [5] D. Klotsa, M. R. Swift, R. M. Bowley and P. J. King, Phys. Rev. E, 79, 021302 (2009).

 

Thu, May 14, '09


Alexey Kavokin, Southampton

Spin effects in superfluids of exciton-polaritons

 

Thu, May 21, '09


Mike Evans, Leeds

Statistical mechanics in complex fluids under shear: prediction and measurement

 

Thu, May 28, '09


Alessandro Troisi, Warwick

Modelling charge transport in soft materials: old and new physics

 

Thu, Jun 11, '09


Jin Zhang + Adam Swetnam

Full Counting Statistics in a Quantum Point Contact, Jin Zhang


We present details of an approach to computing the full counting statistics (FCS) of a quantum point contact (QPC) connecting two 1D wires at zero temperature. Our method gives a complete description of the charge transfer statistics of the device. We formulate the problem in terms of the eigenvalues of the density matrix for outgoing states in one of the leads. Where these cannot be computed analytically, we show that they are easily accessible numerically for arbitrary time-dependence (within the adiabatic limit) of the scattering quantum point contact and of any bias voltage applied between the leads. Two relevant potential applications, a quantum electronic entangler and single electron source on demand at low temperatures, are also discussed.

Monte Carlo Simulations of Polymers, Adam Swetnam

 

Thu, Jun 25, '09


Dr. Pragya Shukla (Indian Institute of Technology, Kharagpur, India and ICTP, Trieste)

Universality in Complexity: A Generalized Random Matrix Approach

Physical systems can be complicated. Lack of detailed knowledge of their interactions can be represented by randomness in generators of the dynamics. In waves (quantum, electromagnetic, etc.) the associated operators can be modelled by random matrices.

The choice of a suitable random-matrix model is sensitive to the nature of the complexity. The statistical analysis therefore requires exploration of a wide range of random-matrix ensembles, with the aim of identifying and analyzing common mathematical structures among their statistics. Our search leads to to a common framework: single-parametric Brownian ensembles.

This common mathematical framework suggests an analogous formulation for physical properties, and their classification into universality classes defined by the single (complexity) parameter. The analogy helps to reveal many important features of the level statistics in interacting systems e.g. a critical point different from that of non-interacting systems, the possibility of extended states even in one dimension. This further suggests a deep-rooted universality hidden underneath the world of complex systems.

Thu, Oct 7, '10

Alexei Likhtman, Reading,

Hierarchy of models for entangled polymers

I will review the multiscale approach to modelling of entangled polymers, which includes molecular dynamics (MD), single chain stochastic models (slip-springs) and the tube model. After that I will concentrate on the link between many chain (MD) and single chain models. I will report results from molecular dynamics simulations on stress relaxation and show the detailed comparison with slip-spring model. In the second part of the talk I will turn to the issue of microscopic definition of entanglement in molecular dynamics. We propose to define entanglement as a long-lived contact between mean paths of the two chains. Using this definition, we present empirical evidence and statistical properties of such entanglements, and discuss the implications for the tube theory and the slip-spring model.

Thu, Oct 14, '10

Gerardo Adesso, Nottingham,

Ground state factorization versus frustration in spin systems

We investigate the existence of factorized ground states in Heisenberg-like quantum spin models with antiferromagnetic interactions of arbitrary range and exhibiting a varying degree of frustration. After reviewing a method developed for frustration-free systems based on tools from quantum information theory, we extend it to characterize the competition between frustration and ground state factorization. Low frustration is shown to signaled by the existence of factorized ground states at specific values of the external field, while for higher frustration degrees the factorized eigenstates do not minimize the energy, leaving necessarily room for entanglement in the ground state. The compatibility threshold, characterizing the frustration-driven transition between order (factorization) and disorder (correlation), is investigated and exact analytical factorized solutions are obtained for short-range as well as infinite-range frustrated quantum magnets.Ground state factorization is thus revealed as an effective tool to probe quantum frustration in cooperative systems.

Thu, Oct 21, '10

Dr. Dmitry Skryabin, Bath,

Solitons and vortex lattices with microcavity polaritons

I am going to report our recent results on two aspects of nonlinear physics of microcavity polaritons. First, is the two-dimensional localization of exciton polaritons in a coherently pumped semiconductor microcavity operating in the strong-coupling regime. 2D polariton-solitons can exist despite the fact that the effective mass of linear polaritons has the opposite signs along the orthogonal directions in the momentum space. Nonlinearities compensating the opposing mass signs have different physical origin, but act simultaneously. They are due to repulsion of polaritons, which compensate the negative mass effects, and due to parametric four-wave mixing, which compensate the positive mass effects. Both of these nonlinearities support their respective families of one-dimensional solitons, which coexist one with another and with 2D solitons. Second part of my talk is about vortex lattices of exciton polaritons in microcavities operating in the four-wave mixing regime. These lattices can be practically seeded by a weak signal pulse formed by a superposition of three interfering beams and can be either very robust or can melt through annihilation of vortex-antivortex pairs.

Fri, Oct 22, '10

Christoph Uiberacker,

Current Conservation in Non-Equilibrium Networks

For Quantum-Hall-Systems network models have been successfully used to investigate questions of localizations as well as the distribution of electro-chemical potentials. While the well-known Chalker-Coddington network model [1] , which uses elastic single particle quantum tunneling at saddle points to obtain critical exponents, was used for the former task, in the latter the nonequilibrium network model [2-4], which describes quantities of nonquilibrium thermodynamics via the Landauer-Büttiker approach, was used. In case of local linear transport at saddles we show that the chemical potential distribution can be obtained, respecting the boundary condition of injected currents, from an inhomogeneous system of linear equations. It turns out that the solution is uniquely determined by the boundary condition, no matter how many current contacts we have. The seaming contradiction can be resolved by the fact that current is automatically conserved due to the formulation of the network. [1] J. T. Chalker and P. D. Coddington, “Percolation, quantum tunnelling and the integer Hall effect“, J. Phys. C: Solid State Phys. 21, 2665 (1988). [2] J. Oswald, “A new model for the transport regime of the integer quantum Hall effect: The role of bulk transport in the edge channel pictureâ€ン, Physica E 3, 30 (1998). [3] J. Oswald and M. Oswald, “Circuit type simulations of magneto-transport in the quantum Hall effect regime“, J. Phys.: Condens. Matter *18*, R101 (2006). [4] C. Uiberacker, C. Stecher, and J. Oswald, “/Systematic study of nonideal contacts in integer quantum Hall systems/â€ン, Phys. Rev. B *80*, 235331 (2009).

Fri, Oct 29, '10

Libby Heaney (Postponed),

Natural Mode Entanglement as a Resource for Quantum Dense Coding

Natural particle-number entanglement resides between spatial modes in coherent ultra-cold atomic gases. However, operations on the modes are restricted by a superselection rule that forbids coherent superpositions of different particle numbers.? This seemingly prevents mode entanglement being used as a resource for quantum communication. In this talk, I will demonstrate that mode entanglement of a single massive particle can be used for dense coding despite the superselection rule. I will show that the full quantum channel capacity is achieved if both parties share a coherent particle reservoir. The talk is based upon results in L. Heaney and V. Vedral, Phys. Rev. Lett. 103, 200502 (2009).

Thu, Nov 4, '10

Andrew Morris, UCL, Materials Discovery Using ab initio Random Structure Searching

Thu, Nov 11, '10

David Cobden (UW, Seattle), tba

Thu, Nov 18, '10

Manuel dos Santos Dias; Wei-Chang Lo,

Competing Interactions and Chiral Magnetism in Mn Monolayers on Transition Metal Substrates: A First-Principles Approach

and

Dynamics of Entangled Ring Polymers: A Hint of New Glassy Materials, Wei-Chang Lo

Thu, Nov 25, '10

Anthony Valentini, Imperial,

Beyond the Quantum

Thu, Dec 2, '10

Martin Speight, Leeds,

Vortex interactions in two component Ginzburg-Landau theory and type 1.5 superconductivity

Thu, Dec 9, '10

Ramin Golestanian, Oxford,
From Microswimmers to Nanoswimmers: The Role of Fluctuations

The directed propulsion of small scale objects in water is problematic because of the combination of low Reynolds number and strong thermal fluctuations at these length scales. I introduce simple prototypes of model low Reynolds number swimmers and examine their physical properties. I also discuss a number of recent experimental realizations of such devices.

Fri, Dec 10, '10

Alberto Marmodoro and Sebastian Pinski,

Thu, Jan 20, '11

Vincent Boyer, Birmingham,

Quantum optics with an atomic vapour: entangled images and super-resolution

The entanglement properties of two beams of light can reside in subtle correlations that exist in the unavoidable quantum fluctuations of their amplitudes and phases. I will review recent advances in four-wave mixing in an atomic vapour which have enabled the production and the observation of "entangled images", that is to say beams of light which are entangled "point per point" across their transverse profiles. These beams can carry quantum information not only in their average profile but also in their spatial details, opening up the field of quantum imaging. The introduction of the spatial degrees of freedom into quantum optics lets us envision novel applications for quantum light. As an example, I will present our current efforts to improve optical super-resolution beyond the standard quantum limit.

Thu, Feb 3, '11

Gabor Csanyi, Cambridge,

Efficient sampling of atomic configurational spaces using Nested Sampling

Thu, Feb 10, '11

Martin Speight, Leeds,

Vortex interactions in two component Ginzburg-Landau theory and type 1.5 superconductivity

Thu, Feb 17, '11

Mats Granath, Gothenburg,

Stripe Order and Pairing in the Cuprate Superconductors

Thu, Feb 24, '11

Lorna Dougan, Leeds,

Exploring the structural properties and molecular mechanisms of cryoprotectants

Many organisms that live in extreme environments have developed mechanisms that protect them from environmental stresses. A common mechanism involves accumulation of sugars, known as protecting osmolytes, which allow organisms to survive sub-zero temperatures. This method is widely utilized in industry, medicine and nanotechnology to prolong the storage life of specific components. One such protecting osmolyte is glycerol, a sugar alcohol with three hydroxyl group, which is a rich and complex system for the study of hydrogen bonded fluids. While much work has been done to characterise glycerol’s dynamic properties a corresponding thorough examination of the structural properties of this molecule is lacking. In particular, little is known about the structural architecture of glycerol’s hydrogen network in aqueous solution. Furthermore, the molecular mechanism by which cryoprotectants like glycerol stabilise biological molecules is yet to be elucidated. We have completed a series of neutron diffraction experiments combined with computational modelling to reveal insight into the structural properties of this important system. We have completed a range of single molecule force spectroscopy experiments to probe the mechanical stability of proteins in cryoprotectant environments. By combining these two approaches we hope to shed light onto the molecular mechanisms of cryoprotection.

Thu, Mar 3, '11

Andrew Parry, Imperial,

The Trouble with Critical Wetting

A long-standing problem in condensed-matter physics concerns the nature of the critical wetting transition in three-dimensional systems with short-ranged forces. The controversy focused originally on the discrepancy between predictions of strongly non-universal critical effects, based on renormalization group analysis of an interfacial Hamiltonian, and Monte Carlo studies of wetting in the 3D Ising model, which are instead broadly consistent with mean-field expectations. This gulf between theory and simulation was widened further by subsequent refinements of the interfacial model which appeared to show that fluctuations should necessarily drive the transition first-order. This prediction is in qualitative disagreement with the simulation studies and would radically alter the anticipated structure of the global surface phase diagram.

We review recent progress made towards overcoming these problems using a new non-local interfacial Hamiltonian. This model, which may be derived systematically from a more microscopic theory and also applied to wetting at structured (non-planar) substrates such as wedges, allows for the presence of two-body interfacial interactions in the wetting layer. These are characterised by an additional diverging coherence length, missing in previous descriptions of wetting. This serves to cut-off the spectrum of interfacial fluctuations that describe the repulsion of the interface from the wall which, in turn, slows down the onset of critical effects (non-universality) and explains why the transition is not driven first-order, therefore preserving the structure of the global surface phase diagram.

Thu, Mar 10, '11

Marzena Szymanska, Warwick,

Novel superfluid phenomena in semiconductor microcavities

Thu, Mar 17, '11

Rahul Roy, Oxford,

Topological invariants and topological insulators

Since their characterization over 80 years ago, physicists have believed that there is only one type of band insulator -- a filled valence band below the Fermi energy with a gap to excitations in the conduction band above the Fermi energy. In the past few years, it has become clear that this is not the whole story: band insulators have topological invariants that distinguish them from each other, and phase transitions must separate insulators with different values of these invariants. Insulators with non-trivial values of these invariants have come to be known as "topological insulators." In this talk I will give simple physical descriptions of these invariants, discuss their implications, and examine the experiments that have actually observed topological insulators. Application of these topological ideas has more recently led to a similar classification of topological superconductors and superfluids, some of which are well known, and others of which have yet to be observed. Finally, I will discuss further applications of topological invariants, as well as current and future directions.

Thu, May 5, '11

Jens Martin, Exeter,

Symmetry-broken QH states in Bilayer Graphene

Bilayer graphene has attracted considerable interest due to the important role played by many-body effects, particularly at low energies. The exceptional quality of suspended devices has enabled the observation of interaction-driven broken-symmetry states and the fractional quantum Hall effect. Here we report local compressibility measurements of a suspended graphene bilayer. We find that the energy gaps at filling factors nu = 4 do not vanish at low fields, but instead merge into an incompressible region near the charge neutrality point at zero electric and magnetic field. These results indicate the existence of a zero-field ordered state and are consistent with the formation of either an anomalous quantum Hall state or a nematic phase with broken rotational symmetry. At higher fields, we measure the intrinsic energy gaps of broken-symmetry states at nu = 0, 1 and 2, and find that they scale linearly with magnetic field, yet another manifestation of the strong Coulomb interactions in bilayers. Co-authors Benjamin E. Feldman, R. Thomas Weitz, Monica T. Allen, Amir Yacoby

Tue, May 10, '11

Jingsong He

Rogue Wave Solution for the NLS and DNLS-type Equations

In this talk, the variable coefficient nonlinear Schrodinger equation (VCNLSE), derivative nonlinear Schrodinger equation (DNLSE) and variable coefficient derivative non-linear Schrodinger equation(VCDNLSE) are discussed. The rogue wave solution of VCNLSE, DNLSE and VCDNLSE are given. The DNLSE is solved by Darboux transformation. The solutions of VCNLSE (VCDNLSE) are given from known solutions of NLSE (DNLSE) by a transformation developed by us recently. Several figures for these solutions are plotted to understand intuitionally its dynamical evolution.

Thu, May 12, '11

Kyriakos Porfyriakos, Oxford (POSTPONED),

On Endohedral C60

Thu, May 19, '11

Chris Pickard, UCL,

Random search: a tool for physics discovery

Thu, May 26, '11

Uwe Thiele, Loughborough,

Evaporative dewetting of suspensions - close-to-equilibrium approaches from DDFT to thin film hydrodynamics

Thu, Jun 2, '11

Kyriakos Porfyriakos, Oxford,

Endohedral Fullerenes as Building Blocks for a Quantum Computer

Endohedral fullerenes offer a unique paradigm in nature: the encapsulation of atom(s) in spherical molecular structures. The encapsulated atoms bestow extraordinary properties to the fullerene cage. Many endohedral fullerenes have unpaired electrons. Electrons can carry quantum information embodied in their spin-state. Hence endohedral fullerenes have been proposed as quantum bits or “qubitsâ€ン: the building blocks of a quantum information processing (QIP) device. N@C60 in particular is a remarkable molecule with the longest coherence time ever recorded for a molecular system (its electron spin coherence time T2 has been measured in excess of 0.24 ms 1). This property makes this molecule especially attractive for QIP. Moreover, the electronic properties of endohedral fullerenes can be tuned by appropriate chemical functionalization.

In this talk, I will endeavour to explain the basic principles of QIP and the suitability of endohedral fullerenes as building blocks for a quantum computer (see Figure 1). I will describe what a fullerene- based quantum computer might look like and how it could be made. Of particular importance is the scalability of such a device. I will show how scalable molecular structures can be built. I will review the state-of-the-art materials science with endohedral fullerenes with emphasis on N@C60.2 I will highlight the particular challenges that are involved in working with N@C60 and how these can be overcome.

Tue, Jun 7, '11

IoP TCM Group Annual Meeting 2011

PLT

Ivo Souza, San Sebastian, Orbital magnetoelectric coupling in insulators Insulators with magnetic order and lacking a center of spatial inversion can display a linear magnetoelectric (ME) effect, whereby an applied electric field induces a first-order change in the magnetization. In conventional magnetoelectrics such as Cr2O3 the ME coefficient is dominated by the spin-magnetization response, but a complete description should also take into account the induced orbital magnetization. I will describe the theoretical framework for calculating the orbital ME response. Remarkably, it contains a contribution which is purely geometric, in that it can be expressed solely in terms of the Berry potential and Berry curvature of the Bloch states in k-space. Like the Berry-phase polarization, this geometric ME coupling is only well-defined modulo a quantum of indeterminacy. While the geometric ME coupling is typically small in conventional magnetoelectrics, in strong topological insulators such as Bi2Se3 it equals half the quantum, which amounts to a rather large ME coupling. Some preliminary first-principles calculations of the orbital ME tensor will be reported.

Lorna Dougan, Leeds, Exploring the structural properties and molecular mechanisms of cryoprotectants Many organisms that live in extreme environments have developed mechanisms that protect them from environmental stresses. A common mechanism involves accumulation of sugars, known as protecting osmolytes, which allow organisms to survive sub-zero temperatures. This method is widely utilized in industry, medicine and nanotechnology to prolong the storage life of specific components. One such protecting osmolyte is glycerol, a sugar alcohol with three hydroxyl group, which is a rich and complex system for the study of hydrogen bonded fluids. While much work has been done to characterise glycerol’s dynamic properties a corresponding thorough examination of the structural properties of this molecule is lacking. In particular, little is known about the structural architecture of glycerol’s hydrogen network in aqueous solution. Furthermore, the molecular mechanism by which cryoprotectants like glycerol stabilise biological molecules is yet to be elucidated. We have completed a series of neutron diffraction experiments combined with computational modelling to reveal insight into the structural properties of this important system. We have completed a range of single molecule force spectroscopy experiments to probe the mechanical stability of proteins in cryoprotectant environments. By combining these two approaches we hope to shed light into the molecular mechanisms of cryoprotection.

Eugene Demler, Harvard, Learning about order from noiseThe probabilistic character of measurement processes is one of the most fascinating aspects of quantum mechanics. In many-body systems quantum noise can reveal the non-local correlations and multiparticle entanglement in the underlying states. In this talk I will review recent theoretical and experimental progress in the quantum noise analysis of many body states of ultracold atoms. I will discuss applications of this technique to the study of one dimensional systems in and out of equilibrium, fermionic pairing near Feshbach resonances, and magnetism in optical lattices.

Jonathan Keeling, St Andrews, Condensation, superfluidity and lasing of coupled light-matter systems,

Microcavity polaritons are a system that can show coherence in a strongly-coupled light matter system at low temperatures. As such, they connect both to Bose-Einstein condensation and also to lasing, and they currently provoke a number of questions about what properties such a non- equilibrium superfluid might have. I will present an approach to modelling such non- equilibrium condensates, and use this model to extract a number of properties addressing these questions: Firstly, I will show what ingredients allow the polariton system to show coherence while remaining in the strong coupling regime (and remaining far below the inversion required for normal lasing). Secondly, I will discuss various aspects of superfluidity in a non-equilibrium two-dimensional light matter system; in particular, power law correlations in the infinite system, and how these are modified in a finite system. I will also discuss the relation between different aspects of superfluidity that should be seen in such non-equilibrium systems, and propose an approach to answering more clearly the question of whether the polariton system is superfluid.

Tue, Jul 5, '11

Mark Ancliff, Bucheon Korea,

Solution of quasispecies models using coherent states

Quasispecies models describe the evolution of an asexually reproducing population subject to random mutation and selection. Individuals are labelled by a DNA-like string of letters of a fixed length N, and the population is described by a distribution function on the set of possible strings. Quasispecies models are a popular starting point for theoretical studies of molecular evolution, and have recently been applied to studies of virus-immune system interactions, evolution in changing environments, and extended to include sexual reproduction.

Two of the most commonly studied quasispecies models can be mapped onto a quantum spin system similar to the one-dimensional quantum Ising model, which allows the application of several techniques from statistical physics. Here I present a new method for calculating the dynamics and equilibrium population distribution in these quasispecies models by constructing a spin coherent-state path integral representation of the evolution operator. In the large N limit a semi-classical approximation gives a description in terms of a classical Hamiltonian function on a sphere. Using this method I will present several new results relevant to biological systems including evolution of the mutation rate, adaptation in changing environments, and a model of escape from adaptive conflict.

Thu, Jul 7, '11

Regis Gautier, Rennes,

How First-Principles Calculations Combined with 95Mo Solid-State NMR Can Help in the Understanding of Inorganic Materials (Joint Seminar with NMR)

Since the 1980s, the expansion of solid-state NMR has increased significantly owing to the development of new techniques that enable high resolution to be achieved even in the solid state. For inorganic compounds without protons or fluorine atoms, the two dominant interactions responsible for the appearance of the NMR spectrum are the chemical shift anisotropy and the quadrupolar interaction tensors. These parameters give information about the atomic structure of the compound under investigation. It appears that in many cases, the complexity of the experimental results require a theoretical analysis for their complete understanding. Until recently, only quadrupolar interaction parameters could be calculated using periodic DFT calculations. Pickard and Mauri presented a formalism, named GIPAW, for the ab initio calculation of all-electron NMR chemical shifts in insulators using pseudopotentials.

We present the combined application of 95Mo solid-state NMR and DFT calculations for the study of materials such molybdenum cluster compounds and nanoparticles. The power of this combined approach for the investigation of solid-state materials will be shown as well as its limitations.


Thu, Oct 6, '11

Lars Stixrude, UCL,

Protons to planets: Materials simulation as a window into planetary processes

Most planets are so large that their characteristic pressure exceeds by orders of magnitude current experimental capability. The behavior of materials in this regime is poorly understood, but likely to be rich, with important implications for our understanding of planetary formation and evolution. The discovery of exo-planets and the development of high energy density experiments motivate a closer look. We have been using density functional theory, combined with molecular dynamics and lattice dynamics to study materials in this extraordinary regime. I will explore two cosmically abundant planetary constituents at pressures up to 1 Gbar and temperatures up to 5 eV: helium and iron, focusing on changes in the electronic structure with compression and heating, including gap closure and changes in Fermi surface topology, and the connection between electronic structure and physical properties such as fluid or crystalline structure and electrical conductivity that may have important implications for the thermal evolution of planets and the generation of magnetic fields.

Thu, Oct 13, '11

Meera Parish, TCM Cambridge

PS1.28

Highly polarized Fermi gases in different dimensions


In this talk, I will consider an atomic Fermi gas in the limit of extreme spin imbalance, where one has a single spin-down impurity atom interacting attractively with a spin-up atomic Fermi gas. Such a scenario is an example of the canonical "polaron" problem, the solution of which is used to construct the low-energy behavior of many-body systems. For sufficiently strong attraction, the impurity atom has the possibility of binding one or more spin-up fermions and thus changing its statistics. I will explore the nature of these binding transitions and how they are affected by the system dimensionality.

Thu, Oct 20, '11

Jiannis Pachos, Leeds

PS1.28

Seeing Topological Order


Different phases of matter can be distinguished by their symmetries. This information is captured by order parameters that summarize the essential properties of the phase. Order parameters are usually defined in terms of local operators that can be measured in the laboratory. Topological insulators are materials with symmetries that depend on the topology of the energy eigenstates of the system. These materials are of interest because they can give rise to robust spin transport effects with potential applications ranging from sensitive detectors to quantum computation . However, direct measurement of topological order has been up to now impossible due to its non-local character. In this talk we provide a general methodology to perform a direct measurement of topological order in cold atom systems. As an application we propose the realisation of a characteristic topological model, introduced by Haldane, using optical lattices loaded with fermionic atoms in two internal states. We demonstrate that time-of-flight measurements directly reveal the topological order of the system in the form of momentum space skyrmions.

Thu, Oct 27, '11

Igor Mekhov, Clarendon Lab

PS1.28

Quantum optics of quantum many-body systems



Thu, Nov 3, '11

Aron Cohen, Cambridge

Density Functional Theory: from the Hydrogen atom to strongly correlated physics

Tue, Nov 8, '11

Glenn Martyna, IBM

PS1.28

Simulation and modelling of materials with atomic detail at IBM: From biophysics to high-tech application


Thu, Nov 10, '11

Vlado Lazarov, York

PS1.28

Atomic and Electronic Structure of Polar Oxide Films and Interfaces

Intrinsic polar materials, such as metal-oxides and compound semiconductors, are some of the most commonly used materials in electronic, magnetic, and chemical applications.It has been recognized for some time that polarity arising from chemical variations at surfaces and interfaces is the main driving force determining the structural and electronic properties of nanoscale materials. In this talk I will discuss the possible mechanism of polar oxide film growth on case of MgO(111), as well as atomic and electronic structure of polar oxide/oxide and polar oxide/semiconductor interfaces. Also I will give examples how the interface polarity can be effectively use for stabilisation of metastable thin film phases such as cub-GaN(111)/MgO(111), and large band offsets engineering at polar oxide/semiconductor interfaces.

Thu, Nov 17, '11

Fabian Essler, POSTPONED

PS1.28

tba

Thu, Nov 24, '11

Matthias Bollhoefer, TU Braunschweig

Fast Algebraic Solvers for Large-Scale Linear Systems and Eigenvalue Problems

In this talk I will discuss algebraic approaches to solving large-scale linear systems and large-scale eigenvalue problems efficiently. The techniques, which I will discuss, have in common that they implicitly use information about the analytic model, while the approach itself is algebraic and uses only a few key parameters. The talk will give an overview of two approaches. One is based on hierarchical matrix approximation techniques. The other uses multilevel incomplete factorization. For a large class of partial differential equations, and related problems, these approaches allow us to solve linear systems of equations and eigenvalue problems easily with only minor problem-specific changes.

Thu, Dec 1, '11

Nic Shannon, Bristol/Oxford

PS1.28

Angle-resolved NMR

The simple fact that nuclear and electronic spins interact makes NMR one of the most powerful probes of solid state magnetism. In particular, changes in NMR spectra provide vital information about magnetic order in cases where small sample size or extreme conditions render neutron scattering impossible. However as a probe of magnetic excitations, NMR is famously difficult to interpret, since excitations with many different momenta are mapped onto a single nuclear spin relaxation time.

Here we revisit the existing theory of the NMR T1 relaxation rate in magnetic insulators, and show how this can be extended to take account of the tensor structure of dipolar and transferred hyperfine interactions with nuclear spins. This tensor interaction makes relaxation rates sensitive to the initial orientation of nuclear spins, and as a consequence, both the magnitude and the temperature dependence of the T1 depend on the orientation of the magnetic field used to carry out the experiment.

We demonstrate that this theory is in quantitative agreement with existing data for the collinear antiferromagnets BaFe2As [1] and Li2VOSiO4, and make explicit predictions for the angle-dependence of T1 in the square-lattice antiferromagnets La2CuO4 and YBa2Cu3O6, and the triangular lattice antiferromagnet VCl2. We also explore how these ideas might be used to distinguish uncoventional forms of magnetic order, including spin nematic states which cannot be resolved by their static properties alone.

[1] A. Smerald and Nic Shannon, Europhys Lett 92, 47005 (2010), [2] A. Smerald and Nic Shannon, arXiv:1109.0384 [accepted for publication in Phys. Rev. B]


Thu, Dec 8, '11

Gareth Alexander, Warwick

On Smectics in Curved Spaces (tentative title)


Wed, Jan 18, '12

Josh Berryman, Luxembourg

PS1.28

Sampling Rare Events in Non-Stationary Dynamics


Study of rare events such as nucleation or arrest in non-equilibrium physics is ever more fashionable, but existing rare event methods intended to enhance sampling (such as umbrella sampling and FFS) are not usually practical for general nonequilibrium conditions (away from both stationary and metastable states). A novel method for calculating the time-series of the probability of a rare event is presented which is designed for these conditions. The method is validated for the cases of the Glauber-Ising model under time-varying shear flow, the Kawasaki-Ising model after a quench into the region between nucleation dominated and spinodal decomposition dominated phase change dynamics, and also for the parallel-open asymmetric exclusion process. The method requires a subdivision of the phase space of the system: it is benchmarked and found to scale well for increasingly fine subdivisions, meaning that it can be applied without detailed foreknowledge of the physically important reaction pathways.

Thu, Jan 19, '12

Fabian Essler, Oxford

Quantum Quench in the Transverse Field Ising Chain

I discuss the time evolution of observables in many-particle systems after a quantum quench, i.e. the sudden change of a parameter characterizing the Hamiltonian. I focus on the case of one dimensional systems, where recent experiments on cold atomic gases have found very unusual behaviour. I show that for the example of the transverse field Ising chain the behaviour of the system at late times after the quench can be understood in terms of a stationary state that is described by a "generalized Gibbs ensemble".


Thu, Feb 2, '12

Phil King (St. Andrews)

PS1.28

An ARPES View of Topological Insulator Surfaces

Topological insulators (TIs) are a recently discovered form of quantum matter characterized by a bulk band inversion driven by strong spin-orbit coupling. They maintain a band gap in the bulk, but unusually possess unconventional surface states which are guaranteed to be metallic. Angle-resolved photoemission (ARPES) is an ideal tool to study the detailed electronic structure of these surface Dirac fermions. I will present our recent ARPES measurements of the Bi-chalcogenide family of TIs. While several of these compounds suffer from degenerate n-type self-doping, we show that Te-rich ternary compounds can have an insulating bulk. Thus, these are model examples of true topological insulators, where only a single topological surface state intersects the chemical potential. By adsorbing n-type dopants at the surface of several TIs, we mimic the effects of an externally-applied gate voltage, of the form desirable for electronic applications. We create a two-dimensional electron gas (2DEG) that co-exists with the topological surface state and can be driven to develop a large Rashba spin-splitting, suggesting potential for its application in advanced spintronic devices such as the spin-transistor. The tuneable surface band bending also provides a novel opportunity to probe the interplay of quantization and topological order.

Thu, Feb 9, '12

Robert Best, Cambridge

Interpreting single molecule folding experiments: insights from molecular simulation

A unique capability of single molecule experiments is the unambiguous resolution of conformational sub-states of biomolecules and their rates of interconversion. However, these experiments usually probe a single observable, such as a distance, and therefore specific structural information is limited. I will describe how coarse-grained molecular simulations can be used to fill in some of the details. First, I will show how coarse-grained models can be used to suggest structures for the misfolded states of titin polyproteins which are observed in single-molecule FRET experiments. The structures of the misfolds explain their unusual stability, and are also consistent with earlier measurements by AFM. Secondly, I will consider the analysis of folding kinetics in single molecule pulling experiments, focussing mainly on the interpretation of the one-dimensional models which are commonly used to interpret experimental kinetic data.


Thu, Feb 16, '12

Martin Gradhand, Bristol

The spin Hall effect, Berry curvature, impurities, and application

After the first experimental observation of the spin Hall effect in semiconductors the topic attracted more and more interest from both experimental and theoretical point of view. The potential of the spin Hall effect to overcome the problem of spin current injection from a ferromagnet into a nonmagnetic material is an important reason for the intensive study of the effect in recent years. I will present my work, methods as well as results, on first principle calculations of the spin Hall effect in metals. This talk focuses on the semiclassical approach where intrinsic and extrinsic mechanisms can be separated naturally. The intrinsic mechanism is governed by the Berry curvature of the pure band structure whereas in the extrinsic case electron-impurity scattering has to be described quantum mechanically. We implemented both contributions where we made use of special features of the applied Green-function method and performed broad material scans to identify materials for possible applications. One particular application where the induced spin current is used to switch a ferromagnetic island I will introduce in more detail. Special features of real slabs, more relevant to the experimental situation, will be also discussed.

Thu, Feb 23, '12

Rob Smith, Cambridge

Effects of interactions on Bose-Einstein Condensation


Thu, Mar 1, '12

Gavin Morley

Quantum control of electron-nuclear spin systems

It is technologically simpler to obtain high spin polarization with electrons than nuclei. I use dynamic nuclear polarization at high magnetic fields to transfer polarization from electrons to nuclei. This provides a good starting state for a quantum computation, as well as for NMR experiments. Bismuth atoms in silicon are particularly attractive multi-qubit systems because their nuclear spin and electron-nuclear coupling are both large.

Thu, Mar 8, '12

John Morton, Oxford

Storing and manipulating quantum information using electron and nuclear spins in the solid state


Thu, Mar 15, '12

Pietro Cicuta, Cambridge

Driving potential and noise level determine the synchronization state of hydrodynamically coupled oscillators

Driving potential and noise level determine the synchronization state of hydrodynamically coupled oscillators

Synchronization has been such a central topic in science over the last 50 years that one wonders whether new breakthroughs are possible. Contrary to this expectation, recent work on cilia and flagella hydrodynamics paints a new ``shade'' of synchronization, with experimental and theoretical evidence supporting the original hypothesis by Taylor in the 50s, that coordinated beating is caused by the interactions through the surrounding fluid.

Understanding this physical problem has large biological importance, since cilia and flagella are ubiquitous in eukaryotes, key to the functionality of diverse human tissues, and possibly played a role in the evolution of multicellularity.

Central questions are how the internal engine of cilia integrates the cues coming from the fluid in order to achieve (and lose) synchronization with neighbours, and how dynamic states of many oscillators are maintained.

Current technology allows to build micron-scale active units that exhibit hydrodynamic synchronization, and are simple to describe theoretically, allowing quantitative studies. This talk will present a configuration-dependent geometric-switch feedback system, driving colloidal particles with optical traps.

We show how the internal force engine with which the active unit pushes the fluid during each beating cycle, i.e. the driving potential, determines the dynamical steady state in competition with thermal noise. In many-oscillator systems, we show how the dynamical state can be predicted on the basis of the equilibrium coupling tensor.


Thu, May 10, '12

Cedric Weber, Cavendish

PS1.28

Dynamical mean-field theory applied to linear scaling density functional theory

Interesting properties that are connected to quantum mechanics, such as magnetism, transport, and the effect of impurity atoms and disorder, and their relation to material design and energy needs are important for almost every branch of the industry. Density functional theory (DFT) was successful at making accurate predictions for many materials, in particular compounds which have a metallic behaviour. DFT combines high accuracy and moderate computational cost, but the computational effort of performing calculations with conventional DFT approaches is still non negligible and scales with the cube of the number of atoms. A recent optimised implementation of DFT was however shown to scale linearly with the number of atoms (ONETEP), and opened the route to large scale DFT calculations.

Nonetheless, one bottleneck of DFT and ONETEP , is that it fails at describing well some of the compounds where strong correlations are present, in particular because the computational scheme has to capture both the band-like character of the uncorrelated part of the compound and the Mott-like features emerging from the local strongly correlated centres. A recent progress has been made in this direction by the dynamical mean-field theory (DMFT), that allows to describe the two limits (metal and insulator) in a remarkable precise way when combined with DFT .

The ONETEP +DMFT implementation will be shortly discussed, and its applications illustrated by two examples: i) the interplay of Mott and Anderson localization within disordered Vanadium dioxide and ii) a typical biological molecular system, iron porphyrin, which plays an important biological function in human haemoglobin.


Thu, May 17, '12

Andrew Green, UCL

PS1.28

Quantum order-by-disorder near criticality and the secret of partial order in MnSi

The vicinity of quantum phase transitions has proven fertile ground in the search for new quantum phases. We propose a physically motivated and unifying description of phase reconstruction near metallic quantum-critical points using the idea of quantum order-by-disorder. Certain deformations of the Fermi surface associated with the onset of competing order enhance the phase space available for low-energy, particle-hole fluctuations and self-consistently lower the free energy. Applying the notion of quantum order-by-disorder to the itinerant helimagnet MnSi, we show that near to the quantum critical point, fluctuations lead to an increase of the spiral ordering wave vector and a reorientation away from ?the lattice favored directions. The magnetic ordering pattern in this fluctuation-driven phase is found to be in excellent agreement with the neutron scattering data in the partially ordered phase of MnSi.


Thu, May 24, '12

David Logan, Oxford

PS1.28

Electronic transport in carbon nanotube quantum dots

The talk will focus on aspects of electronic transport in CNT quantum dots -- theoretically, but set firmly in an experimental context. Particular emphasis will be given to zero-bias transport, and the evolution of conductance as a function of gate voltage, temperature and dot-lead tunnel couplings. The symmetry-breaking role of spin-orbit coupling will also be discussed; in particular its interplay with the two regimes of SU(4) Kondo physics towards the centres of the Coulomb blockade valleys, which has rather striking implications for experiment.

Thu, May 31, '12

Student Seminar Day

PS1.28

Sally Bridgwater, Galbadrakh Dagvadorj and Stepan Ruzicka


Wed, Jun 6, '12

IoP TCM Group Annual Meeting 2012

PLT + Concourse

Nigel Cooper, Cambridge, 'Synthetic gauge fields for ultracold atoms';
Mark Thompson, Bristol, 'Integrated quantum photonics';
Chris Pickard, UCL, 'Random Search: Exploring Extremely Dense Matter';
Maurice Rice, ETH, 'The Anomalous Properties of the Pseudogap Phase in Underdoped Cuprates - A Challenge to Theory'


Thu, Oct 11, '12

Martin Weigel, Coventry

PS1.28

Spin glasses with many components


Thu, Oct 18, '12

Alastair Kay, Oxford

PS1.28

The monogamy of quantum correlations

Quantum correlations are monogamous - the more entangled Alice and Bob are, the less entangled Alice can be with anyone else. Indeed, this is the fundamental concept behind most quantum cryptography schemes. However, how do we make such a statement quantitative? One route, which I will describe in the talk, is by understanding quantum cloning better; if you try and copy a quantum state, the better you make one copy, the worse the other copies have to be. I will also describe an important consequence of this monogamy of correlations - how the classical world (technically, local realism) emerges from the quantum one. The talk is based on the following papers: arXiv:1010.2016, arXiv:1208.5574.


Thu, Oct 25, '12

Tapash Chakraborty, Antwerp and Manitoba

PS1.28

Quantum Hall-Marks in Monolayer and Bilayer Graphene

In this talk, I shall discuss the properties of interacting electrons in monolayer graphene in a strong magnetic field. I shall demonstrate how the effect of the Coulomb interaction differs in a crucial way from that in a conventional two-dimensional electron system [PRL 97, 126801 (2006)]. I will also discuss briefly the experimental work reported on the fractional QHE in graphene. In the second half of the talk, I plan to discuss the physics of bilayer graphene in a strong magnetic field. I will explain how the physics of FQHE in this system differs dramatically from that in monolayer graphene and offers unique possibilities to probe the nature of incompressible/compressible states [PRL 105, 036801 (2010)]. I will also discuss (very briefly) the nature of the Pfaffian state in bilayer graphene [PRL 107, 186803 (2011)].


Thu, Nov 1, '12

POSTPONED Jorge Quintanilla, RAL & Kent

PS1.28

Nonunitary Triplet Paring in the Centrosymmetric Superconductor LaNiGa2

Wed, Nov 7, '12

George Rowlands' Big Birthday. A celebration of his work since 'Retirement'

PLT

Mark Dowsett 'From dress sense to Entropy via a point spread function'

Matthew Turner 'There is something fishy about these models'

George Rowlands 'Life of a physicist - stage 3'

Cake and Champagne on the Concourse

Steve Dixon 'Sound Mathematics - Experimental Ultrasonic Results Explained Through Modelling'

Nick Watkins (British Antarctic Survey) '50 Shades of George ... and how not to misunderestimate extremes'

Thu, Nov 15, '12

Rex Godby, York

PS1.28

Exact Density-Functional Potentials for Time-Dependent Quasiparticles


Thu, Nov 22, '12

Benjamin Beri, Cambridge

PS1.28

Topological Kondo Effect with Majorana Fermions


Thu, Nov 29, '12

Tony Arber, Warwick

PS1.28

QED-Plasmas in High Intensity Laser-Plasmas


Thu, Dec 6, '12

Steven Fitzgerald, Culham

PS1.28

Unstable Dislocations in Anisotropic Crystals

Fri, Dec 7, '12

Bohmian mechanics: Exploring AMO Physics with Trajectories

PS0.17a

Prof. Ángel S. Sanz Ortiz,

Instituto de Física Fundamental (CSIC), Madrid


Thu, Jan 24, '13
1pm - 2pm
Mark van Schilfgaarde, KCL
PS1.28

Quasiparticle Self-consistent GW Approximation as a Universal Framework for Electronic Structure

A new type of self-consistent scheme within the GW approximation is presented, which we call the quasiparticle self-consistent GW (QSGW ) approxi- mation. It is based on a kind of self-consistent perturbation theory, where the self-consistency is used to minimize the difference between the many-body and single-particle hamiltonians. QSGW describes optical properties in a wide range of materials rather well, including cases where the local-density and LDA-based GW approximations fail qualitatively. Self-consistency dramatically improves agreement with experiment, and is sometimes essential. QSGW avoids some formal and practical problems encountered in conventional self-consistent GW, which will be discussed. It handles both itinerant and correlated electrons on an equal footing, without any ambiguity about how a localized state is defined, or how double-counting terms should be subtracted. Weakly correlated materials such as Na and sp semiconductors are described with uniformly high accuracy. Discrepancies with experiment are small and systematic, and can be explained in terms of the approximations made.

Its consistently high accuracy make QSGW a versatile method that can reliably predict critical energy band properties of graphene, CuInSe2, CaFe2As2 and NiO in a unified framework. Many other properties attendant to the electronic structure can be calculated, such as magnetic excitations, the Auger recombination process, the transmission through a metal-semiconductor contact. In principle it can serve both as a framework to construct effective hamiltonians for many-body physics, and as an engine to build models for device design from first principles, with unprecedented reliability. How to do this in practice is a major challenge today. I will briefly present some discussion of each.

Thu, Jan 31, '13
1pm - 2pm
Robert Brady, Cambridge Computing Lab
PS1.28

Irrotational solutions to Euler's equation for a compressible fluid

This talk is about the patterns of flow in a fluid such as the air if it had no viscosity. It is illustrated with animations and movies, and should be accessible to those without prior knowledge. We show that a compressible inviscid fluid supports structures which are similar to smoke rings, but are irrotational. They obey the same equations of motion and diffraction as natural particles, which is illustrated in movies of an experimental analogue in two dimensions, due to Couder, which show tunnelling, double-slit diffraction, and quantised energy levels. Some of the structures are chiral. Opposite chiralities attract and like chiralities repel with a force which obeys Maxwell's equations, whose strength is characterised by a fine structure constant less than approximately 1/45.


Thu, Feb 7, '13
1pm - 2pm
Yong Mao, Nottingham
PS1.28

Structural Optimisation and Fractal Design

Hierarchical design is ubiquitous in nature. Material properties can be tailored by having structural features on many length scales. In our recent work, we demonstrate that through the use of hierarchical, self-similar design principles, advantageous structural properties can be obtained. We show that the scaling of the amount of material required for stability against the loading can be altered in a systematic manner. A particular structure is fabricated through rapid prototyping, and we obtain the optimal generation number (for our specific structure) for any given value of loading.

Mon, Feb 11, '13
2pm - 3pm
Daniele Sanvitto, Lecce
PLT

Quantum Fluids of Polariton Condensates: from fundamental phenomena to polariton logics

Polaritons are light-matter particles formed by a strong interaction between the electronic excited states in a semiconductor and the light field of a microcavity. Recently, polaritons have attracted particular attention for their capacity to undergo phase transition to a collective coherent state in a similar way to the standard Bose-Einstein condensation demonstrated in ultracold gases [1]. However, they offer strong advantages compared to their atomic counterparts, among which a much higher temperature of condensation, the ability to be easily manipulated/observed and the possibility of straightforwardly integrate with present semiconductor technology, which opens new ways for the use of condensate in novel futuristic devices.

In this talk we will review some of the most interesting and significant physical phenomena associated with polariton condensation. Indeed, due to their intrinsic dissipative nature, polariton condensates possess an incredibly rich phenomenology of quantum effects which span from the manifestation of superfluid flow [2,3] and quantized circular currents [4] to the exhibition of a complex and important dynamics of vortex formation and migration [5].

Furthermore, given their strong non-linearities and very high propagation velocities (1% the speed of light), polariton condensates are particularly attractive for their potential use as optical switches in integrated circuits. In particular we can see that it is possible to realize a polariton-based transistor for logic gates made out of polariton fluids [6].

[1] J. Kasprzak, M. Richard, S. Kundermann, et al., Nature 443, 409 (2006).

[2] A. Amo, D. Sanvitto, F. P. Laussy, et al. Nature 457, 291 (2009).

[3] A. Amo, J. Lefrère, S. Pigeon, et al., Nat. Phys. 5, 805 (2009).

[4] D. Sanvitto, F. M. Marchetti, M. H. Szymanska, et al., Nat. Phys. 6, 527 (2010).

[5] D. Sanvitto, S. Pigeon, A. Amo et al. Nat. Phot. 5, 610 (2011).

[6] D. Ballarini, M. De Giorgi, et al. arXiv:1201.4071 (2012).


Thu, Feb 14, '13
1pm - 2pm
Fabien Paillusson, Cambridge
PS1.28

Statistical properties of an assembly of vibrated granular systems

Akin to what is observed in equilibrium thermodynamics, granular systems display statistical properties because of the huge number of particles they involve. Although athermal, it is thought that some of these properties can be described reliably within an "equilibrium" statistical mechanics framework very similar to that of Gibbs ensembles and called Edwards' statistical mechanics. Focusing on simulations of vibrated granular systems, we will firstly look at an ensemble of them and probe the evolution of their statistics with time. This will allow us to question the type of approach --- ensemblist of frequentist --- that should be used and compared to Edwards' predictions. This questioning is inevitably related to some formulation of the ergodicity property of the system that we shall then test for our system. Finally, the compatibility with Edwards' framework will be discussed.


Thu, Feb 21, '13
1pm - 2pm
Jorge Quintanilla, Kent & RAL
PS1.28

Anomalous Nodal Quasiparticles in Superconductors

Thu, Feb 28, '13
1pm - 2pm
Juan Garrahan, Nottingham POSTPONED
PS1.28

Trajectory phase transitions in quantum non-equilibrium


Thu, Mar 7, '13
1pm - 2pm
Nick Hine, Cavendish
PS1.28

Semiconductor and Metal-Oxide Nanocrystal Simulations with Linear-Scaling PAW DFT

Nanocrystals enable tuning of material properties by varying attributes not available in bulk crystals, such as size, shape and surface termination, and such systems have innumerable applications in the field of energy materials, particularly in photovoltaics and photocatalysis. While whole nanocrystals are too large to be studied with traditional cubic-scaling first-principles methods, Linear-Scaling formulations of Density Functional Theory enable the study of systems of many thousands of atoms. This allows nanocrystal simulations to make contact with the realistic size regime of 5-10nm, thus overlapping with the feasible scale of experimental characterisation and control. I will discuss recent developments in the ONETEP LS-DFT code [1,2] that enable these large-scale, high-accuracy simulations, including the Projector Augmented Wave method, and recent applications to TiO2 nanocrystals, pressure-induced phase transformations in II-VI semiconductor nanocrystals, and wurtzite-structure III-V semiconductor nanorods [3,4]. I will discuss the origin of the large dipole moments which can be observed in such structures, and show how an effect akin to Fermi-level pinning can have a determining influence on the overall polarisation, explaining its variation with size, shape, surface chemistry and composition.

References

1 C. K. Skylaris, P. D. Haynes, A. A. Mostofi, and M. C. Payne, J. Chem. Phys. 122, 084119 (2005).


Thu, Mar 14, '13
1pm - 2pm
Andrea Fischer, Cambridge
PS1.28

Superfluid pairing in a quasi-2D gas of fermionic atoms

During the last decade, several groups have successfully created quasi-2D quantum-degenerate gases of fermionic atoms. These are of interest, since 2 is the limiting dimensionality of the Mermin-Wagner theorem and so the superfluidity is fundamentally different to that in 3D. In addition, these simple systems could provide insight into the much more complicated high-Tc superconductors. We consider a population balanced two-component Fermi gas, strongly confined in one direction by a harmonic oscillator potential. The dimensionality of the system depends on the ratio of the Fermi energy to the confinement energy. At low temperatures, only atoms with opposite "spins" interact at short range. These interactions can be tuned via the Feshbach resonance mechanism. For relatively weak interactions, atoms form large Bardeen-Cooper-Schrieffer (BCS)-type pairs. However, as the interaction strength increases, the pairs become smaller and turn into composite bosons, which undergo Bose-Einstein condensation (BEC). We study the evolution of pairing in the BCS-BEC crossover within a mean field approximation. The equations for the two band problem can be solved analytically to give the first order correction to the exactly 2D mean field expressions. It is also possible to extrapolate to infinitely many harmonic oscillator bands numerically. For large enough interaction strengths and Fermi energies, these higher bands contribute significantly. We believe that recent radio frequency spectroscopy experiments have already seen evidence of this.

Thu, May 2, '13
1pm - 2pm
Barry Narod (Narod Geophysics/UBC)
PS1.28

The origin of noise and of magnetic hysteresis in permalloy ring-core fluxgate sensors

6-81.3 Mo permalloy, developed in the 1960s for use in high performance ring-core fluxgate sensors, remains the state-of-the-art for permalloy-cored fluxgate magnetometers. The choices made at that time are reviewed in the context of what are now considered best practices for sensor design. The magnetic properties of 6-81.3, namely magnetocrystalline and magnetoelastic anisotropies and saturation induction are all optimum in the Fe-Ni-Mo system. Recent data suggest that the geometries of typical ring-cores are not optimum, and further suggest a direct relationship between magnetometer noise and domain wall energy. A model is presented for domain walls in magnetically soft, thin foil material, one that provides a basis for relating fluxgate noise power, domain wall energy and magnetostatic energy due to anisotropy. The model predicts the existence of a novel domain configuration for magnetic saturation that requires the presence of shallow, channel-like domains [channel domains], topologically distinct from typical thin foil stripe domains. Transitions between these two states require exothermic magnetic domain wall reconnections which can be identified with Barkhausen jumps, and are the basis for magnetometer noise. The channel domain concept also leads to explanations for remanence, for DC coercivity and for DC hysteresis in an otherwise ideal soft magnetic material. The open loop of DC hysteresis results from the continual cycling between ordinary stripe domain states and channel domain states.

Wed, May 8, '13
2pm - 3pm
Kristen Fichthorn, Penn State
PS1.28

Growth, Transformation, and Assembly of Nanoscale Materials: Insights from Simulation

Achieving the controlled synthesis of colloidal nanomaterials with selected shapes and sizes is an important goal for a variety of applications that can exploit their unique properties (e.g., optical, catalytic, magnetic, etc.). In the past decade, a number of promising solution-phase synthesis techniques have been developed to fabricate various nanostructures.? A deep, fundamental understanding of the phenomena that promote selective growth and assembly in these syntheses would enable tight control of nanostructure morphologies in next-generation techniques. I will discuss our efforts to understand how colloidal nanostructures assume selected shapes during their synthesis. Two ideas will be presented and explored using computer simulations and first-principles calculations based on density-functional theory. To highlight one of our research directions, I will discuss our efforts to understand the workings of structure-directing molecules, which facilitate the formation of selective nanoparticle shapes. I will also discuss a second set of efforts aimed at understanding the origins of oriented attachment, a mechanism whereby selective nanostructures are formed via nanoparticle aggregation.

Thu, May 9, '13
1pm - 2pm
Sergio Ulloa, FU Berlin
PS1.28

Magnetoelectric Spin Control in Nanostructures

Atomic spin-orbit interactions (SOIs) result in interesting dynamical properties on electronic nanostructures. These systems, accessible experimentally on metallic surfaces, semiconducting heterostructures, and carbon nanotubes, to name a few, allow the exploration of symplectic symmetries on a number of measurable quantities.

In this talk we will discuss how SOIs result in interesting magnetoelectric effects at the atomic scale when considering adatoms on surfaces. We will describe how quantum corrals made with magnetic atoms allow one to control the spectral properties of quantum systems located inside, via the application of moderate magnetic fields. The unique features of the electronic states in the corral allow for tunableKondo screening effects, among other things [1].

Similarly, we will discuss the ability to control the spin polarization of current (without magnetic fields) through carbon nanotubes wrapped helically with polar molecules, such as DNA [2].

These properties provide powerful alternative tools for probing spintronic properties at the atomic scale.

[1] A. T. Ngo, J. Rodriguez-Laguna, S. E. Ulloa, and E. H. Kim, Nano Letters 12, 13–16 (2012).

[2] G. S. Diniz, A. Latge, and S. E. Ulloa, Phys. Rev. Lett. 108, 126601 (2012).

Thu, May 16, '13
1pm - 2pm
Juan Garrahan, Nottingham
PS1.28

Trajectory phase transitions in quantum non-equilibrium

 

Thu, May 23, '13
1pm - 2pm
Gil Rutter and Matthew Bates, Warwick
PS1.28


Coarse-grained simulations of intrinsically disordered proteins and surfaces (Gil)

Organisms exhibit an ability to form nano-structured hard tissues through biomineralisation processes which are not well understood, but suggest a vast range of potential applications. It has emerged that an important part of the biomineralisation process is the use of intrinsically disordered proteins (IDPs), though the nature of their role is not understood. IDPs have an interaction energy between their residues which is lower than that of globular proteins, favouring faster shifts between bound and unbound states, temporary secondary structure and lending itself to intrinsic disorder.

IDPs are less easy to categorise and understand on the basis of secondary structure; lack of fixed secondary motifs mean that they are also harder to simulate accurately, and novel simulation ideas are beginning to emerge. This project first aims to develop a suitable approach to simulation of IDPs, which brings the advantages of coarse-graining and accelerated sampling techniques to bear on the problem, and second aims to begin to explore the question of the role of IDPs in the biomineralisation process.

_______________________________________

A Theoretical Study of the Electrocaloric Effect in Relaxor Ferroelectrics Using a Spin Model (Matthew)

The Electrocaloric effect (ECE) is the observed temperature change of a polarisable material under an electric field applied in adiabatic and reversible conditions at a phase change[1]. Relaxor ferroelectrics are capable of producing a strong ECE for small applied field [2]. Here I explore the properties of a spin model of a relaxor system, examining the effects of disorder on the system at the ferroelectric to paraelectric transition and describe how such a model may be used to predict the strength of the ECE within a relaxor.

[1] Valant et al. Phys. Rev. B 81, 214110 (2010), [2] Pirc et al. J. Appl. Phys. 110, 074113 (2011)

 

Thu, May 30, '13
1pm - 2pm
Francesco Gervasio, UCL
PS1.28

Conformational changes and allosteric control of protein-kinases

Protein kinases (PK) show a remarkable conformational dynamics that is tightly regulated in physiological conditions, often by allosteric signals. A shift of the conformational ensemble towards the active state, with the consequent hyper-activation of the PK, leads to a number of human diseases, including cancer. Thus, understanding how allosteric signals regulate the inactive to active equilibrium in PK could lead to the rational design of a new class of allosteric drugs. Historically, drug discovery programs have been dominated by efforts to develop antagonists that compete for binding with endogenous ligands at orthosteric sites. However, allosteric drugs might offer several therapeutic advantages over traditional orthosteric ligands, including greater safety and/or selectivity. Here, by combining state-of-the-art computer simulations with spectroscopy, chemical and molecular biology approaches we study in great details the role of conformational changes and oncogenic mutations in the allosteric control of pharmaceutically relevant kinases such as: cAbl, EGFR and FGFr.

1. S. Lovera, L. Sutto, R. Boubeva, L. Scapozza, N. Doelker and F. L. Gervasio* The different flexibility of c-Src and c-Abl kinases regulates the stability of the DFG-out inactive state J. Am. Chem. Soc., 134 2496^?2499, 2012; 2. F. Bono, et al. Inhibition of tumor angiogenesis and growth by a small-molecule multi-FGF receptor blocker with allosteric properties, Cancer Cell, 23, 477-488, 2013; 3. C. Herbert, et al. Molecular mechanism of SSR128129E, an extracellularly acting small molecule allosteric inhibitor of FGF receptor signaling, Cancer Cell, 23, 489-501, 2013; 4. L. Sutto and F. L. Gervasio The effect of oncogenic mutations on the conformational free energy landscape of the EGFR kinase. Proc Natnl Aca Sci USA, in press

 

Thu, Jun 6, '13
1pm - 2pm
Alex Rautu & Andrew Goldsborough, Warwick
PS1.28

Bending Elasticity Measurements for Fluctuating Vesicles (Alex)

The mechanical properties of lipid membranes have been extensively studied over the past few decades [1]. Their ability to bend under very low stress is one of the main mechanical properties of such soft materials. This softness is characterised by a very small value of the bending modulus (on the order of 10 kBT). As a result, a flaccid vesicle can attain many thermally allowed shapes at constant volume, which leads the thin-walled vesicles to fluctuate (the so-called flicker phenomenon) [1]. Measurements of these thermal excitations have been used to estimate the bending modulus of red blood cells and artificial vesicles [2][3][4]. Here, we re-examine this methodology and discuss some of its limitations; e.g., video-microscopy gives only partial information in the sense that it provides a two-dimensional view of the three-dimensionally fluctuating vesicle. In order to overcome this technical limitation, we develop two new possible methods for inferring mechanical information about membranes from the projected intensity of fluorescent quasi-spherical vesicles.

[1] U. Seifert, Adv. Phys. 46, 13 (1997), [2] J. F. Faucon et al., J. Phys. (Paris) 50, 2389 (1989), [3] J. Pécréaux et al., Eur. Phys. J. E 13, 277 (2004), [4] P. Méléard et al., Eur. Phys. J. E 34, 116 (2011)

_______________________________________

The Strong Disorder Renormalisation Group in the age of Tensor Networks (Andrew)

We have developed a tensor network method of performing the numerical strong disorder renormalisation group (SDRG) approach [1] to the random 1D spin-1/2 Heisenberg model. The numerical SDRG can be reformulated as a randomly branching binary tree tensor network (TTN). This knowledge then enables us to perform a variational update to the network to improve accuracy as well as efficiently calculate expectation values and entanglement entropy. Furthermore, I will discuss how the geometry of the network is related to the physical properties that the network can model.

[1] T. Hikihara, A. Furusaki, and M. Sigrist, Phys. Rev. B, vol. 60, p. 12116, 1999.

Mon, Jul 8, '13
Computation Meets Experiment: KKR Greens functions for calculations of spectroscopic, transport and magnetic properties
CS1.04 / PLT (please check website)

Runs from Monday, July 08 to Monday, July 15.

This will consist of 2 parts:

1) The 'Hands-on' Computational Tutorial: 8th - 12th July 2013 (CS1.04)

2) Research Meeting: 13th - 15th July 2013 (PLT)


Wed, Jul 24, '13
2pm - 3pm
Randy Kamien, UPenn
P5.23

Topology of Soft Materials

Soft matter, despite its name, is surprisingly robust. Under broad conditions liquid crystals, colloids, and copolymers assemble into complex, refined structures, often stable over large temperature ranges and capable of self-repair and healing. Their elasticity is soft and so it is relatively easy to create long-wavelength distortions, topological defects, and configurations frustrated by conflicting boundary conditions. I will describe how geometric and topological principles can be applied in these systems to tailor complex, tunable and robust self-assembled arrays of defects and colloids in a variety of liquid crystals, ranging from the hierarchical formation of focal conic domains from patterned micro-pillar templates, to novel colloidal assemblages built from Janus washers with hybrid boundary conditions.

 

Thu, Sep 19, '13
1pm - 2pm
Debabrata Panja, Utrecht

Bacteriophages: fantastic little machines for infecting bacteria

Bacteriophages, phages for short, are viruses of bacteria. In ~95% cases their genomic material is a double-stranded DNA, typically 15 microns long, and is packed within a proteinaceous capsid of dimension ~60 microns, at a density of ~500 mg/ml. After a brief introduction to phages, their dimensions, structures and infection-initiation phenomenology, I will argue, using in vitro experimental data, that the phages maintain a high osmotic pressure within their capsids. This osmotic pressure can be used, along with hydrodynamic input, to eject their genomic material in vitro. The actual infection dynamics - in vivo - is even more curious. For that the phages use the osmotic gradient that the bacteria need to maintain across their cell membranes for growth. The very mechanism that allows bacteria to grow, thus, also becomes their Achilles heel.

Debabrata Panja and Ian J. Molineux, "Dynamics of Bacteriophage Genome Ejection In Vitro and In Vivo", Phys. Biol. 7, 045006 (2010).

Ian J. Molineux and Debabrata Panja, "Popping the Cork: Mechanisms of Phage Genome Ejection", Nature Reviews Microbiology 11, 194-204 (2013).

Serge G. Lemay, Debabrata Panja and Ian J. Molineux, "Role of Osmotic and Hydrostatic Pressures in Bacteriophage Genome Ejection", Phys. Rev. E 87, 022714 (2013).

 

Thu, Oct 10, '13
1pm - 2pm
Mark Hindmarsh, Sussex
PS1.28

Gravitational waves from phase transitions in the early universe

The universe went through at least two phase transitions very early in its history, related to the physics of the strong and electroweak interactions. I will report on recent 3-dimensional numerical simulations showing how a phase transition in the early universe generates gravitational waves from the sound it makes. The characteristic pattern of gravitational radiation from an electroweak transition (about 10 picoseconds after the Big Bang) is potentially observable at a future space-based detector.

Thu, Oct 17, '13
1pm - 2pm
Gareth Conduit, Cambridge
PS1.28

Few-atom approach to many-body physics

We study the correlations between a few fermionic atoms in a tight confining potential. Exact diagonalization demonstrates that repulsive interactions between pseudo up and down-spin atoms drive ferromagnetic correlations. Attractive interactions produces a pairing state, exposing an inhomogeneous pairing density analogous to the Larkin-Ovchinnikov state. Finally, we present results from the ongoing experiments of the Jochim group at Heidelberg who have delivered the first experimental evidence of strongly correlated physics in a few-fermion system.

Thu, Oct 24, '13
1pm - 2pm
Vasily Kantsler, Warwick
PS1.28

Surface interactions in suspensions of swimming cells

Interactions between swimming cells and surfaces are essential to many microbiological processes, from bacterial biofilm formation to human fertilization. However, despite their fundamental importance, relatively little is known about the physical mechanisms that govern the scattering of flagellated or ciliated cells from solid surfaces. In the talk I will reveal recent advances in understanding of flagella interaction with surfaces, provide mechanisms for utilizing our knowledge about these interactions to control swimming of flagellated cells. In addition, I will describe our very recent results on sperm rheotaxis near surfaces. The key focus will be on the experimental results, supported by numerical simulation using minimal models.

Thu, Oct 31, '13
1pm - 2pm
Sven Dorosz, Luxembourg POSTPONED
PS1.28

Fluctuation Relations and Crystallization

Thu, Nov 7, '13
1pm - 2pm
Sebastiaan Vlaming, Dresden
PS1.28

Anomalous exciton localization in systems with heavy-tailed disorder

The optical and excitation transport properties of a wide variety of systems, such as molecular aggregates, photosynthetic complexes and organic photovoltaics, are determined by the collective properties of the relevant excitations, which are strongly influenced by interactions with their environment. In modeling the behavior of these collective excitations (excitons) in a disordered environment, one conventionally often considers model parameters as stochastic quantities with Gaussian distributions. However, we have shown that the limitation to Gaussian distributions is not necessarily the best choice, and that a generalization to the wider class of Lévy distributions leads to qualitatively different collective optical properties.

In this talk, I will discuss how the details of the considered disorder distributions influence the localization properties of excitons in supramolecular systems, and in turn their optical and energy transport properties. First, I will discuss how a generalization to Lévy disorder distributions affects the localization behavior and the absorption properties of the optically relevant exciton states. It will be shown that such a generalization leads to novel effects such as exchange broadening and an anomalous exciton localization. Moreover, I will show that Lévy distributions follow quite naturally from a simple microscopic model. In addition, the modified localization properties are shown to cause a qualitative change in exciton dynamics, leading to subdiffusive (i.e. less mobile than diffusive) behavior of the exciton transport.

 

Thu, Nov 14, '13
1pm - 2pm
Animesh Datta, Oxford

Sensing and imaging at the quantum limit

Quantum correlated probes have the potential of delivering enhanced precision in estimating individual parameters. Obtaining quantum enhancements in scenarios of wider appeal such as imaging require an understanding of the quantum limits of estimating several parameters across multiple modes simultaneously. The problem is made theoretically and well as practically interesting and non-trivial by the possible non-commutativity of the optimal measurements needed to attain the quantum limits for estimating individual parameters. We present developments on the quantum theory of estimating multiple parameters -- arising from both unitary dynamics as well as decoherence -- simultaneously in a few scenarios, and its ramifications in the imaging of real-world samples.

Thu, Nov 21, '13
1pm - 2pm
Mark Gilbert, Culham
PS1.28

Integrated assessment of material performance in a fusion reactor: Neutron transport, transmutation and activation, radiation damage, and helium embrittlement

Thu, Dec 5, '13
1pm - 2pm
Peter Brommer, Warwick
PS1.28

From graphene functionalisation to phase-change materials - current and proposed research

In the first half of this week's seminar, I will describe my current research before outlining a research project for which I am looking to obtain funding.

For many applications it is essential to modify the electronic properties of graphene in a controlled fashion. This can be achieved via oxygen and nitrogen functionalization in ultra-high vacuum, leading to a system in which electronic and structural properties can be systematically studied. We directly compare insights from DFT calculations on functionalized graphene systems (e.g. low-energy configurations, binding energies and effective band structures) to results from angle-resolved photoemission spectroscopy (ARPES) and low-voltage aberration-corrected transmission electron microscopy experiments.

Phase-change materials (PCMs) are promising candidates for widely used non-volatile RAM modules. I propose to create an atomistic model of a PCM to illuminate the largely unknown processes involved in fast recrystallisation. These results could then be employed to design a sustainable alternative to the currently used Ge-Se-Te compounds.

 

Thu, Jan 9, '14
1pm - 2pm
Mario Cuoco, Salerno

Spin-triplet superconductors: interface to ferromagnets and magnetic edge states

In this talk I will discuss two remarkable effects of spin-triplet superconductors (TSC): i) the spin-orbital coupling emerging at the interface with an itinerant ferromagnet (FM) [2], ii) the occurrence of magnetic Andreev states at their edge if the system allows for singlet pairing in a subdominant channel [3].

In a TSC-FM heterostructure, the orientation of the FM moment relative to the TSC vector order parameter is a crucial variable that controls the physical behavior. For a single-component p-wave TSC, we find that the variation of the gap controls the orientation of the FM's moment mainly via the change in condensation energy. When the interface is imperfect or spin active the scenario is different and other processes can play the decisive role in setting the magnetic profile [5].

Concerning the surface states, novel magnetic effects can occur if triplet and singlet pairing get mixed and have a non-uniform spatial profile. As a result, the Andreev bound states are spin-polarized, leading to a finite surface magnetization, spin current and surface charge currents that exhibit anomalous dependence on the magnetization [3].

[1] See for instance Y. Maeno, S. Kittaka, T. Nomura, S. Yonezawa, K. Ishida, J. Phys. Soc. Jpn. 81, 011009 (2012) and references therein.

[2] P. Gentile, M. Cuoco, A. Romano, C. Noce, D. Manske, P. M. R. Brydon, Phys. Rev. Lett. 111, 097003 (2013).

[3] A. Romano, P. Gentile, C. Noce, I. Vekhter, M. Cuoco, Phys. Rev. Lett. 110, 267002 (2013).

[4] P. M. R. Brydon, Phys. Rev. B 80, 224520 (2009); P. M. R. Brydon, Y. Asano, and C. Timm, Phys. Rev. B 83, 180504(R) (2011).

[5] D. Terrade, P. Gentile, M. Cuoco, and D. Manske, Phys. Rev. B 88, 054516 (2013).

 

Thu, Jan 16, '14
1pm - 2pm
Sven Friedemann, Cambridge
PS1.28

Electronic Structure investigations in Yb-based heavy fermion materials

In Lanthanide based metals new electronic quasiparticles emerge from the interaction of f electrons with conduction electrons. They are called heavy fermion materials because of the highly enhanced effective mass of these quasiparticles. Great opportunities arise from the good control and tunability of these materials. As such heavy fermion materials provide outstanding access to study competing ground states and the quantum criticality associated with the zero temperature phase transition. Quantum criticality leads to novel states of matter and is discussed to underlie high temperature superconductivity.

The prototypical materials YbRh2Si2 features a quantum critical points which requires descriptions that go beyond the conventional order-parameter notion. Here, electronic structure studies played a key role to identify the intriguing physics of YbRh2Si2. I will discuss Hall effect measurements which find a reconstruction of the Fermi surface in the zero temperature limit. Furthermore, energy-over-temperature scaling deduced from the Hall effect measurements indicate that the fluctuations between the two different Fermi surface configurations is underlying the finite temperature quantum critical behavior.

The newly discovered heavy-fermion material YbNi4P2 appears to be one of the first examples of a truly continuous quantum phase transition from a ferromagnetic state to a paramagnetic state. Remarkably, this is in contrast to theoretical predictions which exclude ferromagnetic quantum critical points in metallic systems. However, these theoretical considerations are valid for two and three dimensional materials only. I will present electronic structure calculations in conjunction with quantum oscillation measurements. These comprehensive electronic structure studies reveal a quasi-one-dimensional electronic structure which might be the key to understand the presence of a ferromagnetic quantum critical point.


Thu, Jan 23, '14
1pm - 2pm
Mark Fromhold, Nottingham
PS1.28
In this talk I will consider the mutual interaction between ultracold atom clouds and nearby quantum electronic structures. In particular I will consider the potential advantages of using quantum electronic components to trap, manipulate, and electrically image ultracold atoms. Conversely, I will also consider how the cold atom clouds can be used to provide functional imaging of the electronic systems.

I will present calculations which predict that current through quantum electronic components fabricated within a two-dimensional electron gas (2DEG) in semiconductor heterostructures [1] and graphene multilayers [2,3] can trap ultracold atoms ~200 nm away, with orders of magnitude less spatial and temporal noise than for metal trapping wires. This noise reduction, combined with low Casimir-Polder attraction [2], may enable the creation of hybrid atom chip structures, which exploit small changes in the conductance of quantum electronic devices to control the trapped atoms. For example, activating a single quantized conductance channel in a quantum point contact can split a Bose-Einstein (BEC) for atom interferometry [1,4]. In turn, the response of the BEC to the opening and closure of conduction channels offers a route to functional imaging of quantum devices and transport.

[1] G. Sinuco-León, B. Kaczmarek, P. Krüger, and T.M. Fromhold, Phys. Rev. A 83, 021401(R) (2011).

[2] T.E. Judd, R.G. Scott, A.M. Martin, B. Kaczmarek and T.M. Fromhold, New J. Phys. 13, 083020 (2011).

[3] L. Britnell et al., Nature Commun. 4, 1794 (2013).

[4] T.W.A. Montgomery, W. Li and T.M. Fromhold, Phys. Rev. Lett. 111, 105302 (2013).


Thu, Jan 30, '14
1pm - 2pm
Timo Betz, CNRS Curie
PS1.28

Learning from fluctuations: The mechanics of active and passive cellular assemblies

Understanding the intriguing complexity of living systems is one of the main driving forces of science. To gain insight we use biomimetic systems that reconstitute well defined cellular assemblies and compare these to the living system. Our main interests are the mechanical properties and the generation of forces, both mediated by the cytoskeleton and its interaction with the plasma membrane. Recent advances allow to mimic structures such as the actin cortex, sparse actin networks and actin bundles, and we use optical tweezers to quantify the mechanical properties of these structures and to compare them to living cells. While sparse actin networks and polymerizing actin bundles show rather passive behavior, we find clear signs of activity that drives the red blood cell membrane fluctuation. Our studies of the mechanical fluctuation and the mechanical response function reveals a violation of the dissipation fluctuation theorem, which is turn shows that the red blood cell membrane fluctuations should be described as a process that is out of the thermodynamic equilibrium.

Thu, Feb 6, '14
1pm - 2pm
Bortolo Mognetti, ULB
PS1.28

Living Cluster in Suspensions of Active Colloids

Active colloids are interesting in view of their unique collective behaviours and because of their relevance in modelling biological systems like the cytoskeleton or the bacterial motility. Recent experiments have shown that self-propelled particles assemble into finite aggregates that merge and split, but have a typical size that remains constant (living clusters). In this contribution we numerically investigate living clusters in suspensions of driven colloids. We use two different systems: self-propelled particles interacting via a generic attractive potential and colloids that can self-displace themselves towards each others by molecular motors. In both cases we rationalise the formation of living clusters in terms of a competition between active forces and thermodynamic equilibrium. When mixed with passive particles self-displacing colloids can also assemble into open aggregates.

Thu, Feb 13, '14
1pm - 2pm
Alex Chin, Cambridge POSTPONED

Thu, Mar 6, '14
1pm - 2pm
Gianluca Gregori, Oxford
PS1.28

Laboratory astrophysics with high power lasers

In the past decade, the advent of high power and high intensity laser system, such as the National Ignition Facility laser, extreme matter conditions and technological applications leading to inertial confinement fusion are becoming accessible. A new area of research has opened in which, using simple scaling relations, the astrophysical environment can be effectively reproduced in the laboratory. Here we report the results of such experiments aimed at investigating problems related to the large scale magnetization of the Universe and the crystallization of white dwarfs. We will then explore the use of such lasers in order to recreate the most extreme environments - as the ones occurring during supernova explosions and their connection to turbulence in the circumstellar medium.

Thu, Mar 13, '14
1pm - 2pm
Daniel Burgarth, Aberystwyth
PS1.28

Many-body physics from a control perspective

The Heisenberg and the XX model are the workhorses of statistical mechanics and many-body physics. In this talk, I will discuss some amazing properties of these well-studied systems that arise when one considers them from the perspective of quantum control. These not only give us new insights into the many-body dynamics but also have promising applications in quantum information processing.

Thu, Apr 24, '14
1pm - 2pm
Benjamin Trefz, Mainz
PS1.28

Computer Simulations of Active Particles

The study of active particles is an emerging field, which describes objects as diverse as self-propelled bacteria or janus-type colloidal particles propelled by catalytic reactions. These systems are intrinsically out of equilibrium.

In our molecular dynamics simulations we use a continuous Asakura-Oosawa [1] model as a reference system, for which the phase diagram is well-known. The AO model consists of two particle types, colloids and polymers, and exhibits phase transition due to depletion forces. The colloids are made active by adding a Vicsek-type activity [2, 3] to the particles: Each colloid is subject to an additional constant force applied in the direction of the mean velocity of neighbouring colloids. We demonstrate that the addition of this force facilitates phase separation [4].

[1] J. Zausch, P. Virnau, K. Binder, J. Horbach and R.L.C. Vink, J. Chem. Phys. 130, 064906 (2009); [2] T. Vicsek, A. Czirók, E. Ben-Jacob, I. Cohen and O. Shochet, Phys. Rev. Lett. 75, 1226 (1995); [3] A. Czirók and T. Vicsek, Physica A 281, 17 (2000); [4] S. K. Das, S. Egorov, B. Trefz, P. Virnau and K. Binder, arXiv:1308.5831 [cond-mat.stat-mech]


Thu, May 8, '14
1pm - 2pm
Jacopo Bertolotti, Exeter
Non-invasive imaging requires the ability to form sharp pictures even when an opaque material act as a screen between the object and the detector. Light scattering scrambles the spatial information of the object, thereby blurring the picture and making imaging impossible. The typical distance that light can traverse in a turbid medium before its direction is scrambled varies from tens of meters in fog, to a fraction of a millimeters in skin, to microns in paint. We have recently demonstrated "speckle scanning microscopy", a reference-free imaging method that can obtain an image of a fluorescent object behind a thin layer that scatters all incident light. I will discuss the working principles of this method, its experimental implementation and how it compares with other approaches to imaging in disordered media, like Optical Coherence Tomography (OCT) and Diffuse Tomography.

Thu, May 15, '14
1pm - 2pm
Frank Pinski, Cincinnati
PS1.28

Theory of Fluctuations in a Thermodynamic System and the Limitations of the Onsager-Machlup Functional


The aim of this work is to describe diffusion-like paths that are explored by a particle moving in a potential while being in thermal equilibrium with its surroundings. To do this, we examine functionals that are similar to the Onsager-Machlup functional. The difference is that they are based on Hamiltonian or Hybrid Monte Carlo methods (HMC) instead of Brownian dynamics.
Then we focus our attention to the "double-ended" problem, where the Onsager-Machlup functional is interpreted as a "thermodynamic action" and is used to define a measure, which in turn can be employed to sample paths that are constrained to start and end at predetermined points
Even in simple systems, we find that the paths generated by sampling this measure are not consistent with the Boltzmann distribution. By using the new perspective described in the first part of the talk, I will identify why this is so.

Thu, May 22, '14
1pm - 2pm
Ahsan Nazir, Manchester
PS1.28

Environmental dynamics and the emergence of noncanonical equilibrium states in open quantum systems


Standard open quantum system methods eliminate all information on the environmental state in order to provide a tractable description of the system dynamics. By incorporating a collective coordinate of the environment into the system Hamiltonian, we develop a formalism that circumvents this limitation, allowing straightforward access to important properties of the environmental dynamics that are typically inaccessible using other methods. Focussing on the non-perturbative problem of a quantum system coupled to a low frequency environment, we reveal that the canonical thermal system steady-state predicted by standard perturbative methods is almost always incorrect. We show this to be due to the generation of long-lasting system-environment correlations that persist into the steady-state, leading also to non-Gaussian environmental states. We can, nevertheless, fully characterise the system-environment steady-state as a thermal state of the combined system-collective coordinate Hamiltonian. We outline how the resulting noncanonical system steady-state could be investigated in ongoing experiments to study deviations from canonical thermodynamics, with direct relevance to molecular and solid-state nanosystems.

Thu, May 29, '14
1pm - 2:30pm
Anja Humpert, Michael Ambler, Sam Brown
PS1.28

Modelling of topological defects entangled around nanoparticles of nematic liquid crystal colloids (Anja),

Machine Learning for Structure Prediction of Nanotube-Encapsulated Crystals (Sam)

Calculation of Interfacial Free Energies for Two-Phase Systems via Capillary Waves (Michael)


Thu, Oct 9,'14
1pm - 2:00pm
vdW Epitaxy
PS1.28

Series of short presentations on vdW Epitaxy (all welcome).

A series of presentations to stimulate discussion on modelling weak epitaxy in van der Waals heterostructures. To include contributions from:

Nicholas Hine
Gavin Bell
Neil Wilson
Gwilym Enstone


Thu, Oct 16,'14
1pm - 2:00pm
Jiannis Pachos, Leeds
PS1.28

A useful superconductor: 3D DIII TSC

I will present tight-binding models of 3D topological superconductors in class DIII that support a variety of winding numbers. I will show that gapless Majorana surface states emerge at their boundary in agreement with the bulk-boundary correspondence. At the presence of a Zeeman field the surface states become gapped and the boundary behaves as a 2D superconductor in class D. Importantly, the 2D and 3D winding numbers are in agreement signifying that the topological order of the boundary is induced by the order of the 3D bulk. Hence, the boundary of a 3D topological superconductor in class DIII can be used for the robust realisation of localised Majorana zero modes.


Thu, Oct 23,'14
1pm - 2:00pm
Joakim Stenhammar, Edinburgh
PS1.28

Phase behaviour of active Brownian particles

So-called active Brownian particles (ABPs) - i.e., self-propelled, non-aligning colloids whose swimming direction relaxes through thermal diffusion - constitutes a paradigmatic example of active matter, and can be seen as a minimal model of synthetic swimmers as well as motile bacteria. Recently, simulations of ABPs have demonstrated the existence
of a phase transition which strongly resembles that of a gas-liquid coexistence in a system of passive particles with attractive interactions. Since the direct interaction potential between ABPs is purely repulsive, this phase transition is exclusively driven by the
far-from-equilibrium microscopic dynamics of ABPs, and one would therefore not expect any generic similarities between this type of phase coexistence and those present in passive attractive systems.

In this seminar, I will discuss how a semi-thermodynamic mapping, in the form of a dynamic continuum equation for the time-evolution of the density field, can be derived directly from the microscopic ABP dynamics. A numerical solution of the equations yields quantitative agreement with domain topologies and phase-separation dynamics (growth
exponents) obtained from explicit, large-scale Brownian dynamics simulations of ABPs in two and three dimensions. While the model weakly violates detailed balance through a non-standard interfacial energy, the effects of this violation are found to be surprisingly small. This result thus suggests unexpected analogies between phase transitions in
active and passive systems, in spite of the far-from-equilibrium microscopic dynamics of the former.


Thu, Oct 30,'14
1pm - 2:00pm
Myrta Gruening, Queens Belfast
PS1.28

yambo: An ab initio tool for excited state calculations

A detailed description on how electronic systems interact with electro-magnetic radiation is the starting point for understanding numerous phenomena in Physics, Chemistry, Biology and for developing new technologies (e.g. photovoltaics cells). Ab-initio numerical simulations are increasingly used to support, interpret and guide experimental works. In particular, approaches based on Many-Body perturbation theory such as the GW approximation and the Bethe–Salpeter equation are becoming a standard tool in the calculations of quasiparticle energies (related to direct and inverse photoelectron measurements) and the macroscopic dielectric function (related to e.g optical absorption or electron-energy loss experiments). yambo [1] is an ab initio code for
calculating quasiparticle energies and optical properties of electronic systems within the framework of many-body perturbation theory and time-dependent density functional theory. Quasiparticle energies are
calculated within the GW approximation for the self-energy. Optical properties are evaluated either by solving the Bethe–Salpeter equation or by using the adiabatic local density approximation. yambo is a plane-wave code that, although particularly suited for calculations of periodic bulk systems, has been applied to a large variety of physical systems. yambo relies on efficient numerical techniques devised to treat systems with reduced dimensionality, or with a large number of degrees of freedom. The code has a user-friendly command-line based interface, flexible I/O procedures and is interfaced to several publicly available density functional ground-state codes.

After a quick review of the theoretical approaches I will present the basic features of Yambo as well as some more advanced ones and showcase typical applications. Finally I will give an overview of recent or in-progress developments (e.g. yambo for HPC, real-time implementation, etc...)

[1] A. Marini, C. Hogan, M.G. and D. Varsano Comp. Phys. Comm. 180, 1392 (2009)


Thu, Nov 6,'14
1pm - 2:00pm
Rhoda Hawkins, Sheffield
PS1.28

Active solids: stress reorganisation and response

Initially I will introduce the concept of active matter, that is matter driven out of equilibrium by an internal energy source. In particular I will then focus on active solids. I will present a microscopic model of a disordered viscoelastic active solid, i.e., an active material whose long time behaviour is elastic as opposed to viscous. It is composed of filaments, passive cross-links, and molecular motors powered by stored chemical energy, e.g., actomyosin powered by ATP. Our model allows us to study the collective behavior of contractile active elements and how their interaction with each other and the passive elastic elements determines the macroscopic mechanical properties of the active material. As a result of the (un)binding dynamics of the active elements, we find that this system provides a highly responsive material with a dynamic mechanical response strongly dependent on the amount of deformation.


Thu, Nov 13,'14
1pm - 2:00pm
Paola Verrucchi, Firenze
PS1.28

A description of the measurement process by the parametric representation with environmental coherent states

We propose a description of the measurement process based on the parametric representation with environmental coherent states [1], where the environment is the measurement apparatus. Referring to the Von Neumann scheme, we first show that the premeasurement step induces a dynamical evolution for the density of environmental coherent states. The analysis of such evolution allows us to establish a formal relation between the loss of quantum coherence and the distinguishability of the measurement outputs. Moreover, having made use of generalized coherent states for the apparatus, we can consider the consequences of its being macroscopic referring to the relation between classical and large-N limit of a quantum theory, as established by G.Yaffe in Ref.[2]. This finally leads us to a statistical description of the actual production of the output that inherently includes both the probabilistic character of the process, with the Born rule properly recovered, and a symmetry breaking that entails the overall quantum state reduction.

[1] Proceedings of the National Academy of Sciences 110, 6748-6753 (2013)
[2] Rev. Mod. Phys. 54, 407 (1982)


Thu, Nov 20,'14
1pm - 2:00pm
Paul Chaikin, NYU
PS1.28

Diffusion and Organization in Driven Particles Systems


Thu, Dec 4,'14
1pm - 2:00pm
Bryan Chen, Leiden
PS1.28

Topological soft matter: from linkages to kinks

Networks of rigid bars connected by joints, termed linkages, provide a minimal framework to design robotic arms and mechanical metamaterials built out of folding components. These linkages may admit motions that perform useful functions. Can these motions be made to be topologically robust? I will explain this question and illustrate our answer with a chain-like linkage that, according to linear elasticity, behaves like a topological mechanical insulator whose zero-energy modes are localized at the edge. Simple experiments we performed using prototypes of the chain vividly illustrate how this edge mode can in fact propagate unobstructed all the way to the opposite end. Indeed, the chain is a mechanical conductor, whose carriers are nonlinear solitary waves, not captured within linear elasticity. This chain can be regarded as the simplest example of a topological mechanical metamaterial whose protected excitations are solitons, moving domain walls between distinct topological mechanical phases. Live demonstrations on real toys will be performed. (Based on work with Nitin Upadhyaya and Vincenzo Vitelli).


Thu, Jan 15,'15
1pm - 2:00pm
Andrew James, UCL
PS1.28

Quantum quenches in 2D via arrays of coupled chains

Matrix product state (MPS) methods, while highly effective when applied to the study of quantum systems in 1D, stumble in higher dimensions due to the 'area law' growth of entanglement entropy. This growth of entanglement can be mitigated in 2D by studying anistropic systems composed of coupled integrable chains, because the required 'area' is reduced.

As a specific example I will describe the implementation of the time evolving block decimation algorithm to study quantum quenches in a system of coupled quantum Ising chains.


Thu, Jan 22,'15
1pm - 2:00pm
Aleks Reinhardt, Cambridge
PS1.28

Simulating the self-assembly of complex structures

Materials that can spontaneously self-assemble have been the subject of extensive recent research. It is possible to achieve a considerable degree of complexity using simple building blocks. For example, using computer simulations, we have found that 2D particles with five regularly arranged 'patches' spontaneously form dodecagonal quasicrystals in certain conditions. But whilst quasicrystals form spontaneously on cooling, it is not necessarily clear that they are also the thermodynamically stable phase. I will present a method to calculate
the free energy of the quasicrystal and use it to show that in our model system, the quasicrystal has the lowest free energy over a range of conditions and is stabilised by its greater configurational entropy over
the crystalline phases.
However, simple building blocks can only go so far and self-assembling truly 'complex' structures requires us to introduce more distinct building blocks into the system, which makes the problem of self-poisoning ever more difficult to counter. Recently, Ke and co-workers reported that DNA bricks successfully self-assembled into
structures containing not just a handful, but hundreds of distinct components [Science 338, 1117 (2012)]. However, it is not immediately obvious why such self-assembly should succeed where colloidal systems
have failed. In my talk, I will present our computational and theoretical work explaining how nucleation governs the self-assembly of these multi-component systems and the role this plays in the rational
design of the target structure.


Thu, Feb 5,'15
1pm - 2:00pm
James Kermode, Warwick
PS1.28

Multiscale modelling of materials chemomechanics: from stress corrosion cracking to catastrophic brittle fracture


Fracture is one of the most challenging ‘multi-scale’ problems to model: since crack propagation is driven by the concentration of a long-range stress field at an atomically sharp crack tip, an accurate description of the chemical processes occurring in the small crack tip region is essential, as is the inclusion of a very large model systems. Both these requirements can be met by combining a quantum mechanical description of the crack tip with a classical atomistic model that captures the long-range elastic behaviour of the surrounding crystal matrix. Examples of the application of these techniques to fracture problems include: low-speed dynamical fracture instabilities in silicon [1]; interactions between moving cracks and material defects such as dislocations or impurities [2]; the crossover from thermally activated to catastrophic fracture; very slow crack propagation via kink formation and migration; chemically activated fracture, where cracks advance under the concerted action of stress and corrosion by chemical species such as oxygen or water [3].

[1]. J. R. Kermode et al., Nature 455, 1224-1227 (2008).
[2]. J. R. Kermode, et al., Nat. Commun. 4, 2441 (2013).
[3]. A. Gleizer et al. Phys. Rev. Lett. 112 115501 (2014).

Thu, Feb 12,'15
1pm - 2:00pm
Enzo Orlandini, Padova
PS1.28

The knotted strands of life

Knots are part of our everyday life. In some cases they can be very useful as in climbing or sailing whereas in some others they can be a nuisance, as we experience each time we try to disentangle long extension cables or garden pipes.
Like extension cables, biological filaments such as DNA can be highly self-entangled and the presence of knots may have detrimental effects in several cellular process such as transcription, replication and recombination. Fortunately there exist enzymes such as topo-isomerases which control the topological state of the
DNA by cutting, disentangling and resealing DNA strands continuously.
On the other hand, within very small viruses, where there is space only for the DNA itself, knots inevitably accumulate because of the tight confinement. Yet their presence does not prevent the virus to infect the hosting cell by translocating its DNA through a small hole. So how could these viruses have highly knotted DNA and still be infective?
To gain insight into this problem we analysed the data on viral DNA packaging and knotting offered by beautiful experiments on bacteriophages.
In particular, by starting from the abundance of certain knot types (torus knots) and the shortage
of others (twist knots) we established that the aligning tendency of contacting DNA strands may play a significant role in leading the spatial organisation and knotting of the packaged DNA. By explicitly modelling this aligning interaction on a coarse grained model of ds DNA, we found that it favours ordered DNA spools
which, during the ejection process, experience a lower effective topological friction than more disordered entangled structures.
We also find that torus knots exit the bacteriophage more easily than complex knots or twist knots whose presence may slow down and stall ejection.


Thu, Feb 19,'15
1pm - 2:00pm
Tim Rogers, Bath
PS1.28

Growth-induced breaking and unbreaking of ergodicity in spin dynamics


Thu, Mar 5,'15
1pm - 2:00pm
David O'Regan, Trinity Dublin

Frequency-dependent Hubbard U corrections: a viewpoint from density-functional theory

In contemporary first-principles atomistic simulation, the augmentation of approximate density functionals with spatially or energetically localised corrections derived from model Hamiltonians is a common approach to improving their accuracy in more strongly interacting systems. This augmentation may take place on the level of subspace-projected density-matrices, as in the widely-used density-functional theory + Hubbard U (DFT+U) method, or at the level of subspace-projected Green's functions, as in DFT + dynamical mean-field theory (DFT+DMFT). In the context of DFT+U, the Hubbard U parameter is usually interpreted either as a measure of the curvature of the total-energy with respect to subspace occupancies, deemed erroneous and due for cancellation, or as the static limit of the screened Coulomb interaction. In the context of DFT+DMFT, the latter interpretation prevails, but in both cases a generalisation to dynamical, or non-adiabatic interaction parameters U seems admissible. It remains a somewhat open question, however, how essential it is to incorporate dynamical interaction parameters, both in order to match experiment and on fundamental grounds.

Here, I will develop a viewpoint from density-functional theory, starting from the definition of the Hubbard U as an energy curvature and seeking connections with the dynamical Coulomb interaction computed using the constrained random phase approximation and sometimes used in DFT+DMFT. I will introduce a recently-developed, inexpensive and very simplistic approach to computing model dynamical Hubbard U parameters, dubbed DFT+U(ω), developed to explore these connections. This is based on a readily-available combination of static density-functional linear-response theory for the Hubbard U and methods for the dielectric function, such as time-dependent density-functional theory (TDDFT), in which case we can move beyond the random phase approximation. I will discuss different strategies for solving the resulting non-Hamiltonian models, using either a local GW approximation to the self-energy, for which I will show some preliminary results on SrVO_3, or TDDFT.


Thu, Apr 30,'15
1pm - 2:00pm
Halim Kusumaatmaja, Durham
PS1.28

Exploring the Free Energy Landscapes of Continuum Soft Matter Systems: Minima, Transition States, Pathways

In many soft matter systems, the stability of minimum free energy configurations and the transition pathways from one state to another often play a central role. A variety of methods are therefore developed for characterising the free energy landscapes of continuum, Landau-type free energy models. Using morphologies of lipid vesicles and a multistable liquid crystal device as examples, I show that the methods allow systematic study of not only the most relevant minimum energy configurations, but also competing transition pathways between any two minima, as well as their corresponding energy barriers and transition state configurations. A global view of the free energy landscapes can therefore be obtained and visualised. Different forms of free energy functionals and boundary conditions can be readily implemented, thus allowing these tools to be utilised for a broad range of problems.


Thu, May 7,'15
1pm - 2:00pm
Francisco Domínguez-Adame, Madrid
PS1.28

Electron transport properties of graphene quantum rings

Graphene is a material with a combination of many remarkable properties, in particular, large electron mobility and long spin-coherence length. These features spurred the interest in graphene as a material of choice for the design of new electronic devices. In this talk I will review some recent proposals for
exploiting unique properties of graphene quantum rings, such as proximity exchange splitting of electronic state and Fano-like resonances, in spintronics and thermoelectric devices.


Thu, May 14,'15
1pm - 2:00pm
2nd Year PhD student talks
PS1.28


Poppy Asman (Warwick) - Modelling dissipative transport in the quantum Hall regime

The thermoelectric effect is an interesting phenomenon in physics and is related to the entropy per charge of an excitation. Its study can then lead to the understanding of the statistics of quantum Hall states. I will provide an overview of a model of transport in the quantum Hall regime in terms of an equivalent random resistor network, and some results on the dependence of the total response on the distribution of resistors. I will outline progress on how this model can take account of edge state contributions for the thermoelectric response.

David Turban (Cambridge) - Singlet fission in pentacene dimers

Singlet fission (SF) is a multi exciton generation process which could be harnessed to improve the efficiency of photovoltaic devices. Recently, fast and efficient SF has been observed in molecular dimers derived from the pentacene molecule. We employ constrained density functional theory to explore the electronic states participating in fission. The SF mechanism is discussed with a focus on symmetry constraints peculiar to the dimer systems under consideration. We find that solvent-induced symmetry breaking plays a crucial role in the SF process.


Thu, May 21,'15
1pm - 2:00pm
Sam Azadi, Imperial
PS1.28

Metallic solid hydrogen: The "holy grail" of high-pressure physics

In this talk I’ll present comprehensive results for the high-pressure phase diagram of solid hydrogen.
I mainly focus on the energetically most favorable molecular and atomic crystal structures. I briefly describe semi-local and hybrid density functional theory (DFT) as well as diffusion quantum Monte Carlo (DMC) methods, which have employed to obtain the ground-state static and dynamic phase diagram.
The closure of the band-gap with increasing pressure is investigated utilizing quasi-particle many-body calculations within the GW approximation. I show that the molecular-to-atomic phase transition happens at
the experimentally accessible pressure range.

1. Physical Review Letters 112 (16), 165501 (2014)
2. Physical Review B 90, 014110 (2014)
3. Physical Review B 88 (1), 014115 (2013)
4. New Journal of Physics 15, 113005 (2013)
5. JETP letters 95 (9), 449-453 (2012)


Thu, May 28,'15
1pm - 2:00pm
Madan Rao, NCBS India
PS1.28

Active clustering and implications for information processing on the cell surface

There is growing evidence that cells can locally control their membrane composition by active, energy-consuming processes. I will discuss the theory of active clustering of cell surface molecules and its implications for optimisation of information processing.


Thu, Jun 4,'15
12am - 12:00am
TCM Meeting
L4 & Physics Concourse

PROGRAMME: 11:00 Johannes Lischner, Imperial , Theoretical Spectroscopy beyond GW 11:50 Gene Mele, UPenn & Loughborough, Twist and Texture in Multilayer Graphene 13:40 Zoran Hadzibabic, Cambridge, Uniform Bose Gases 14:40 Posters 16:10 Matthew Turner, Warwick, Swarming


Thu, Oct 15,'15
1pm - 2:00pm
Alberto Rodriguez, Freiburg
PS1.28

Matter-wave scattering from interacting ultracold bosons in optical lattices

We study matter-wave scattering from ultracold bosons in a one-dimensional optical lattice described by a Bose-Hubbard Hamiltonian. The phase transition from the superfluid (SF) state to the Mott insulator (MI) is clearly displayed in the decay of the inelastic scattering cross-section for increasing onsite interaction U/J [1]. We obtain analytical expressions for the cross-section from a Bogoliubov expansion, valid in the regime of small condensate depletion, and from a strong-coupling expansion, valid in the regime of large interactions U/J. This allows for the description of the inelastic cross-section’s decay in the entire range of the relevant system parameters, excluding the vicinity of the critical point of the MI-SF phase transition. In the weak-interaction regime, the cross section is found to decay linearly, with a slope that is independent of the bosonic density and the system size [2]. In the strong-interaction regime, the decay is quadratic and vanishes only as U/J → ∞, resulting in a non-vanishing inelastic cross section throughout the entire Mott phase [3]. To support our analytical results, we present numerical studies obtained from exact diagonalization methods.

[1] S. Sanders, F. Mintert, E. Heller, PRL 105, 035301 (2010)
[2] K. Mayer, A. Rodriguez, A. Buchleitner, PRA 90, 023629 (2014)
[3] K. Mayer, A. Rodriguez, A. Buchleitner, PRA 91, 053633 (2015)

Wed, Oct 21,'15
1pm - 2:00pm
Artem Abanov, Texas A&M
PS1.28

Joint Condensed Matter and Theory Group seminar

I will discuss ways to manipulate various magnetization textures by current. I will start by introducing magnetization statics and dynamics. I will show how the magnetic domain walls can be moved by a time dependent resonant current. I will also discuss the ways to measure different properties of the magnetization textures in nano wires and nano dots, and the ways to produce nontrivial textures with the current. The effects of the topologically nontrivial textures in magnetization on current will also be shown. The manipulation of textures in antiferromagnets will be discussed.

Thu, Oct 29,'15
1pm - 2:00pm
Nikos Fytas, Coventry
PS1.28

Universality in disordered systems: the case of the random-field Ising model

Universality in disordered systems has been severely questioned, especially the last 20 years where the development of sophisticated algorithms and the access to supercomputing resources provided us with numerical data of unprecedented accuracy. Playing devil’s advocate however, in this talk I will present an unexpected instance of universality in disordered systems, well-hidden under the presence of strong scaling corrections, using as a platform the random-field Ising model in three-dimensions

Thu, Nov 5,'15
1pm - 2:00pm
Alex Rautu, Warwick
PS1.28

The Role of Optical Projection on Vesicle Fluctuations

The optical spectroscopy of thermally induced shape fluctuations of giant unilamellar vesicles (GUVs) has been widely used as a method to extract mechanical information about fluid membranes [1]. Working with the model system of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) lipids, we re-examine this methodology and discuss how the projection of fluctuations within the focal depth of the microscope may affect the inferred value of the bending modulus (and the surface tension). Within a Gaussian approximation we derive an analytical expression for a mode spectrum that varies with the ratio of the focal depth to the vesicle size. A comparison of our model with the existing approach [2-4] (that compares experiments with the equatorial fluctuations, without averaging over the focal depth) shows a significant and systematic decrease in the inferred value of the bending modulus. The new procedure is found to be in good agreement with the values measured through X-ray scattering and other micromechanical manipulation techniques [5].

[1] U. Seifert, Adv. Phys. 46, 13 (1997)
[2] J. F. Faucon et al., J. Phys. (Paris) 50, 2389 (1989)
[3] J. Pécréaux et al., Eur. Phys. J. E 13, 277 (2004)
[4] P. Méléard et al., Eur. Phys. J. E 34, 116 (2011)
[5] J. F. Nagle, Farad. Discuss. 161, 11 (2013)


Thu, Nov 12,'15
1pm - 2:00pm
Dmitry Kovrizhin, Cambridge
PS1.28

Dynamics of fractionalization in quantum spin liquids

I will present the theory of dynamical spin response for the Kitaev honeycomb model, discussing exact results for the structure factor — which shows signatures of spin fractionalization into emergent quasiparticles: Majorana fermions and fluxes of Z2 gauge field — in gapped and gapless, Abelian and non-Abelian quantum spin-liquid (QSL) phases.

[1] J. Knolle, D.L. Kovrizhin, J.T. Chalker, R. Moessner, Phys. Rev. Lett. 112, 207203 (2014)
[2] J. Knolle, D.L. Kovrizhin, J.T. Chalker, R. Moessner, Phys. Rev. B 92, 115127 (2015)
[3] A. Smith, J. Knolle, D.L. Kovrizhin, J.T. Chalker, R. Moessner, arXiv:1508.05324
[4] A. Banerjee, et.al., arXiv:1504.08037


Thu, Nov 19,'15
1pm - 2:00pm
Thomas Swinburne, Imperial
PS1.28

Using Zwanzig's projection technique to understand the stochastic dynamics of crystal defects

The mechanical response and microstructural evolution of a crystal is in large part dictated by the motion of the crystal defects (vacancies, dislocations, impurities) it contains. At finite temperature defect motion is stochastic and viscous due to a strong coupling with thermal phonons, but existing theories based on phonon scattering often show large disagreements with the results from classical atomistic simulations, failing completely for nanoscale defects such as self-interstitial clusters.

We have shown that these failures stem from treating defects and phonons as canonical particles in a harmonic system. In our approach [1], defect motion is a general structural transformation described by an affine parameter isomorphic to the defect position. We have used Zwanzig's projection technique[2] to derive a stochastic equation of motion for the defect with the defect-phonon coupling emerging as a Green-Kubo relation to the defect force, which can be evaluated statically or dynamically. The form of the friction kernel is closely related to previous microscopic heat bath models.

In my talk I will discuss some properties of this new stochastic equation of motion and explain why phonon scattering theories fail to predict the defect-phonon coupling.


Thu, Nov 26,'15
1pm - 2:00pm
Karoline Wiesner, Bristol
PS1.28

The mathematics of information in complex systems

A hallmark of complex systems are the presence of order and randomness. The interplay between them allows for robust function. A mathematical framework for uncovering structure and randomness
is found in information theory. As one example I will illustrate how information measures help uncover the mechanism of glass formation. No background in information theory required to attend.


Dunleavy, Andrew J., Karoline Wiesner, Ryoichi Yamamoto, and C. Patrick Royall. “Mutual Information Reveals Multiple Structural Relaxation Mechanisms in a Model Glass Former.” Nature Communications 6 (2015).
Ladyman, James, James Lambert, and Karoline Wiesner. “What Is a Complex System?” European Journal for Philosophy of Science 3, no. 1 (2013): 33–67.


Thu, Dec 3,'15
1pm - 2:00pm
Brendon Lovett, St Andrews

Coupled quantum systems in structured environments: Master equations and applications to quantum-enhanced solar energy harvesting

Coherently coupled nanostructures can provide a rich playground for developing more efficient devices that exploit quantum mechanical properties, and for fundamental studies of open quantum systems. In this presentation, I will first show how to tailor the optical properties of molecules by using particular geometries of coupled chromophores. I will discuss the illustrative examples of engineered ‘superabsorption’ [1], and the notion of optical ratchet states [2]. In the second part of the talk, I will discuss the general problem of deriving time-local master equations for two coupled systems interacting with a bosonic environment [3].

The Dicke model of superradiance describes a signature quantum effect: N atoms collectively emit light at a rate proportional to N-squared [4,5]. By using environment engineering [4] together with a ring of coupled chromophores, I will show that this phenomenon can be inverted to make a quantum-enhanced absorption device [1]. Potential applications of this effect include photon detection, enhanced light energy harvesting and light-based power transmission.

In my second example, I will discuss how to make optically excited states that can absorb more photons, but do not re-emit [2]. Natural and artificial light harvesting systems often operate in a regime where the flux of photons is relatively low. Besides absorbing as many photons as possible it is therefore paramount to prevent excitons from annihilating via photon re-emission until they have undergone an irreversible conversion process. Again using a coupled ring system, I will introduce a class of states we call ratchets: excited states capable of absorbing but not emitting light. This allows our antennae to absorb further photons whilst retaining the excitations from those that have already been captured. Simulations for a ring of four sites reveal a peak power enhancement of 35% under ambient conditions owing to a combination of ratcheting and the prevention of emission through dark state population. In the slow extraction limit the achievable current enhancement exceeds hundreds of percent.

The final section of my talk will be devoted to the structure of master equations for coupled systems in general [3]. Specifically, for two boson modes with similar frequencies we will show that secularisation - the usual trick used in order to make sure a resulting master equation is in Lindblad form - does not lead to agreement with an exact solution. On the other hand, Lindblad form is required to guarantee that a density operator stays completely positive as time evolves [6]. Nonetheless, it is possible to derive a time-local form for the master equation that is accurate but not in Lindblad form, by relaxing the requirement of secularization. We find that these considerations have profound consequences for the existence of long time correlations between the two coupled systems.

[1] K. D. B. Higgins, S. C. Benjamin, T. M. Stace, G. J. Milburn, B. W. Lovett and E. M. Gauger, Nature Communications 5 4705 (2014).
[2] K. D. B. Higgins, B. W. Lovett and E. M. Gauger, arXiv:1504.05849 (2015).
[3] P.R. Eastham, P. Kirton, H. M. Cammack, B. W. Lovett, J. Keeling arxiv.org/abs/1508.04744 (2015)
[4] R. H. Dicke, Phys. Rev. 93 99 (1954).
[5] M. Gross and S. Haroche, Physics Reports 93 301 (1982).
[6] H. P. Breuer and F. Petruccione, The Theory of Open Quantum
Systems, OUP (2002)

Thu, Dec 10,'15
1pm - 2:00pm
Michael Hartmann, Heriot Watt
PS1.28

Strongly Interacting Photons: Coherence, Correlations and Propagation

Light consists of photons, mass-less particles that do not interact with one another. Recent technological developments however give rise to structures with strong interactions between light and matter in multiple nodes of a network. These devices may enable us to drive photons into novel strongly correlated quantum many-body regimes. Interestingly, these may by studied in non-equilibrium scenarios where inevitable photon losses are constantly compensated by input drives. They thus give rise to an intriguing class of quantum many-body systems where instead of ground or thermal states one is interested in the still largely unexplored stationary states of their driven and dissipative dynamics.

In this talk, I will present some of our recent approaches to this physics that explore photon-photon correlations in chains of nonlinear resonators with coherent or incoherent pumping.

1) J. Ruiz-Rivas, E. del Valle, C. Gies, P. Gartner, and M. J. Hartmann, Spontaneous, collective coherence in driven, dissipative cavity arrays, Phys. Rev. A 90, 033808 (2014).
2) J. Jin, D. Rossini, R. Fazio, M. Leib, and M. J. Hartmann, Photon solid phases in driven arrays of non-linearly coupled cavities, Phys. Rev. Lett. 110, 163605 (2013)
3) M. J. Hartmann, Polariton crystallization in driven arrays of lossy nonlinear resonators, Phys. Rev. Lett. 104, 113601 (2010)
4) L. Neumeier, M. Leib, and M. J. Hartmann, Single Photon Transistor in Circuit Quantum Electrodynamics, Phys. Rev. Lett. 111, 063601 (2013).


Thu, Jan 21,'16
1pm - 2:00pm
Hugues Chate, CEA
PS1.28

Active matter: An introduction and some recent advances

In this talk, I will introduce the new, fast-growing, interdisciplinary field of active matter and present some recent important advances.

Active matter is the term now used by physicists to designate out-of-equilibrium systems in which energy is spent in the bulk, locally, to produce persistent motion/displacement. Examples abound, not just within living systems (bird flocks, fish schools, collective motion of cells, etc.) but also, increasingly, in man-made, well- controlled, non-living systems such as micro- and nano- swimmers, active colloids, in vitro mixtures of biofilaments and motor proteins, etc.

I will show some striking experimental/observational examples and then proceed to give an account of our current understanding of some of the simplest models, which consist of self-propelled particles locally aligning their velocities. In this context, the fluid in which the particles move is neglected, and one speaks of ”dry active matter”. I will argue that these models do have experimental relevance, in addition to being important per se, much as the Ising model is important in statistical mechanics. I will show that a wealth of new physics arises, which calls for further theoretical studies.

Thu, Jan 28,'16
1pm - 2:00pm
Keith McKenna, York
PS1.28

Understanding defects in metal-oxide materials

We are surrounded by metal-oxide materials, from everyday devices such as phones and computers, to the many natural minerals which make up our planet. Metal-oxides are finding an incredibly diverse range of technological applications in areas such as electronics, energy generation, catalysis and medicine. For many of these applications atomic-scale defects in the oxide materials (such as vacancies, impurities and grain boundaries) control performance. Materials modelling can provide invaluable insights into the role of defects which are often challenging to unravel by experiment alone. In this talk I will present a number of examples from our recent research including the effect of grain boundaries on electron mobility in nanocrystalline TiO2, interdiffusion in CoFeB/MgO thin films and antiphase boundary defects in the magnetic oxide Fe3O4.

Thu, Feb 11,'16
1pm - 2:00pm
Erik Gauger, Heriot Watt
PS1.28

Flow of information and energy in quantum networks

Understanding and harnessing the efficient and robust flow of information and energy in quantum networks is an important challenge for practical quantum technologies.
In the first part of this talk I will consider a wire of nitrogen impurities connecting two distant NV- centre qubits. This setup has been suggested as the fundamental building block for an quantum computing architecture [1]. Using realistic parameters and models of environmental decoherence, I will argue that such wires can indeed serve as channels for quantum information, albeit in a different way than originally proposed [2].
As the second part of the talk, I shall present a simple and intuitive explanation for the intriguing observation that optimally efficient networks are not purely quantum, but are assisted by some interaction with a ‘noisy’ classical environment. By considering the systemʼs dynamics in both the site-basis and the momentum-basis, I will argue that the effect of classical noise is to sustain a broad momentum distribution, countering the depletion of high mobility terms which occurs as energy exits from the network. I will also discuss how insights from this picture can unlock further improvements in performance when a global driving field specifically targets noise at the low mobility components [3].
Time permitting, I will finish with a discussion of how quantum interference enhances energy flow through asymmetric, noisy two site networks.

[1] Yao, Jiang, Gorshkov, Gong, Zhai, Duan, Lukin, Phys. Rev. Lett 106 040505 (2011); Yao, Jiang, Gorshkov, Maurer, Giedke, Cirac, Lukin, Nat. Comm. 3 800 (2012)
[2] Ping, Lovett, Benjamin, Gauger, Phys. Rev. Lett. 110 100503 (2013)
[3] Li, Caruso, Gauger, and Benjamin, New Journal of Physics 17, 013057 (2015)
[4] Fruchtman, Gomez-Bombarelli, Lovett, and Gauger. arXiv:1511.06302 (2015)

Thu, Feb 18,'16
1pm - 2:00pm
Francesco Ginelli, Aberdeen
PS1.28

Leading birds by the beak: On the response of flocks to external perturbations

Flocking – the collective motion of many active particles – is a ubiquitous emergent phenomenon that occurs
in many living and synthetic systems over a wide range of scales. Examples range from mammal herds,
fish schools and bird flocks to bacteria colonies and cellular migrations, down to sub-cellular molecular motors and biopolymers. While our knowledge of collective motion in unperturbed, isolated, systems greatly advanced in recent years, little is known concerning the response of moving groups to external perturbations. This is an important question in statistical physics: symmetry breaking systems are often characterized by their response to a small external field, and studying response can also help answer the question of whether a generalized fluctuation dissipation relation of some sort holds in flocks. Ethologists, on the other hand, are interested
in response to external threats and more generally in the biological significance of group response mechanisms. Finally, understanding response is essential for controlling flocking systems, either biological or artificial.

In this talk, I will first discuss the asymptotic response to small external fields, extending a classic equilibrium
field-theoretic results to far-from-equilibrium polar ordered active fluids (“flocks”). Next, I will consider a more realistic set-up, in which finite flocks interact with the external world via the flock own boundary. In this problem, we consider a finite, dynamic perturbation that only affects the flock boundary, and characterize the information inflow towards the flock bulk.

Thu, Feb 25,'16
1pm - 2:00pm
Tyler Shendruk, Oxford
PS1.28

A Mesoscopic Particle-based Method for Nematohydrodynamics

Research on topological microfluidic transport, the dynamics of self-assembly in liquid crystals and so-called hypercomplex fluids requires versatile and numerically efficient mesoscopic algorithms. I will describe a multi-particle collision dynamics (MPCD) based algorithm for simulating fluctuating nematohydrodynamics, the flow of liquid crystals. This nematic-MPCD method successfully reproduces the features of a nematic liquid crystal, including an isotropic-nematic phase transition, intrinsic elastic coefficients, tumbling and shear alignment regimes, and defect dynamics. Though simple, it represents a promising tool for modelling defect dynamics within porous media, the interactions of colloids, self-propelled particles and dispersed carbon fibres within liquid crystal media. I demonstrate of the method can be extended to simulate active fluids, which represent an exciting path for studying intrinsically out-of-equilibrium phenomena with direct ramifications for biological systems. Active MPCD simulations exhibit the hallmarks of active nematic fluids, including the formation of lines of kinks in the orientation field and the onset of mesoscale trubulence via the unzipping of these lines through the creation of topological defects.


Thu, Mar 10,'16
1pm - 2:00pm
Gerardo Adesso, Nottingham
PS1.28

Robustness of coherence: An operational and observable measure of quantum coherence

Quantifying coherence is an essential endeavour for both quantum foundations and quantum technologies. Here the robustness of coherence is defined and proven a full monotone in the context of the recently introduced resource theories of quantum coherence. The measure is shown to be observable, as it can be recast as the expectation value of a coherence witness operator for any quantum state. The robustness of coherence is evaluated analytically on relevant classes of states, and an efficient semidefinite program that computes it on general states is given. If time permits, the framework will be extended to any symmetry group, defining the robustness of asymmetry and discussing its general properties.


Thu, Mar 17,'16
1pm - 2:00pm
Antonio Garcia-Garcia, Cambridge
PS1.28

Smaller is different and more: Low dimensional superconductivity for new physics and applications

Recent experimental advances in superconducting interfaces, heterostructures and nano-grains are revolutionizing the field of low dimensional superconductivity. In contrast with bulk high temperature superconductors, the superior experimental control in these engineered materials offers an ideal playground to unveil novel forms of quantum matter and also to achieve more robust superconductivity. In this talk I give a pedagogical introduction to this emerging field of research that includes some of my contributions to the theory of nano-structured, out of equilibrium and topological superconductors. I also review some of the research questions with the potential to set the agenda of the field in the coming years.


Thu, May 5,'16
1pm - 2:00pm
Edgar Engel, Cambridge
PS1.28

Calculating anharmonic vibrational properties of water ice

Quantum nuclear vibrations are important in many materials where they influence properties such as thermodynamic stability, thermal expansion, phase stability, isotope effects, etc. Vibrational anharmonicity often plays a key role in determining these properties not only at high temperatures and/or pressures, but also at ambient pressure and low temperatures. Water ice provides a good example of this. I will briefly introduce some recently developed methods for calculating anharmonic vibrations in (crystalline) solids and the resultant vibrational corrections to properties such as the electronic band gap. Results for hexagonal and cubic ice will be presented. In particular, I will explain why hexagonal rather than cubic ice is the thermodynamically stable form of ice at ambient pressure. I will then discuss how quantum nuclear vibrations change the electronic band
gap. I will conclude with a brief overview of further applications.


Thu, May 12,'16
1pm - 2:00pm
Aires Ferreira, York
PS1.28

Critical delocalization of chiral zero energy modes in disordered graphene

Graphene subjected to chiral disorder is believed to host zero energy modes resilient to localisation, as dictated the renormalisation group analysis of the underlying effective field theory [1]. For “C-z” chiral disorder—such as vacancies and bond disorder—a line of fixed points with conductivity ~e2/h is predicted. Such an unconventional quantum transport regime is found at variance with recent numerical works, however, which report the localisation of all states, including the zero energy modes [2]. In this talk, I introduce an exact expansion of response functions in terms of Chebyshev polynomials, whose implementation in large memory machines allows to tackle non-interacting systems with in excess of 109 atoms and fine meV resolutions [3,4]. Its application to the honeycomb lattice with random dilute vacancy defects (orthogonal chiral class, BDI) reveals a remarkably robust metallic state at the band’s centre. The Kubo conductivity of zero energy modes is found to match graphene’s universal ballistic conductivity—4e2/(pi h) —within 1% accuracy, regardless of the vacancies’ concentration [4]. These results testify to the power of the new Chebyshev polynomial method, and provide strong evidence that the field-theoretical picture is valid well beyond its “controlled” weak-coupling regime.

[1] F. Evers and A.D. Mirlin, Rev. Mod. Phys. 80, 1355 (2008); P.M. Ostrovsky et al., Phys. Rev. Lett. 105, 266803 (2010); S. Gattenlohner et al., Phys. Rev. Lett. 112, 026802 (2014)
[2] G.T. de Laissardiere and D. Mayou, Phys. Rev. Lett. 111, 146601 (2013); A. Cresti et al., Phys. Rev. Lett. 110, 196601 (2013); Z. Fan et al., Phys. Rev. B 89, 245422 (2014)
[3] A. Ferreira, unpublished
[4] A. Ferreira, and E. Mucciolo, Phys. Rev. Lett. 115, 106601 (2015)

Thu, May 19,'16
1pm - 2:00pm
2nd Year PhD student talks I
PS1.28

Antonino Savojardo & Edoardo Carnio

Antonino Savojardo - Rogue wave generation due to inelastic quasi-soliton collisions in optical fibres

Edoardo Carnio - Ab initio metal-insulator transition in doped silicon


Thu, May 26,'16
1pm - 2:00pm
2nd Year PhD student talks II + Chris Patrick
PS1.28

Eduardo Mendive-Tapia & Chris Patrick

Eduardo Mendive-Tapia - Four-ion Magnetic Coupling in the Heavy Rare Earth Elements


Chris Patrick - Modelling anharmonicity within the harmonic approximation - self-consistent phonons and CsSnI3


Wed, Jun 1,'16
12am - 12:00am
TCM Meeting
L3, Physics Concourse

PROGRAMME: 11:00 Hai-Qing Lin, Beijing 11:50 Sania Jevtik, Imperial, Disentangling entangled quantum states 13:40 Mike Cates, Cambridge, What is the pressure of an active suspension? 14:40 Posters 16:10 Hannah Price, Trento, Synthetic Gauge Fields in Synthetic Dimensions with Ultracold Atoms and Integrated Photonics


Thu, Oct 13,'16
1pm - 2:00pm
Theory Seminar: Pier-Emmanuel Tremblay (Warwick)
PS1.28

3D Model Atmospheres of White Dwarfs


Thu, Oct 20,'16
1pm - 2:00pm
Theory Seminar: Alexander Taylor (Bristol)
PS1.28

Geometry and topology of vortices in random quantum eigenfunctions Disordered complex 3D scalar wave fields typically contain a dense tangle of nodal lines (quantized vortices), which are important in diverse physical wave systems including turbulent superfluids, optical volume speckle, the quantum eigenfunctions of chaotic 3D cavities, and liquid crystal phases. Based on extensive numerical simulations these nodal tangles are known to have fractal properties on large scales, although more subtle topological quantities such as the probability of knotted or linked vortices are sensitive to the details of the model. We numerically generate many examples of wave chaos in three random systems at fixed energy (3D cube with periodic boundary conditions, 3-sphere and 3D harmonic oscillator), analysing aspects of their statistical geometry and identifying the knot types of the vortex curves which appear. Knots tend to occur with high probability even at comparatively low energies, and the statistics of knot complexity vary significantly amongst the three systems. Furthermore, the different symmetries and boundary conditions of these systems strongly affect the knotted conformations that can occur, and we discuss how this relates to the statistics of knotting with mode count in different systems.


Thu, Oct 27,'16
1pm - 2:00pm
Theory Seminar: Simon Connell (Leeds)
PS1.28

TBA


Thu, Nov 3,'16
1pm - 2:00pm
Theory Seminar: Haixing Miao (Birmingham)
PS1.28

Quantum limits of laser interferometric gravitational-wave detectors Current ground-based gravitational-wave detectors, e.g., Advanced LIGO, are kilometre scale Michelson-type laser interferometers with kilogram mirror-endowed test masses. Even though they are macroscopic in size, quantum mechanics plays an important role in determining their sensitivity. In particular, quantum fluctuation of the optical field not only sets the measurement imprecision in terms of shot noise, but also induces quantum back action noise that perturbs the motion of test masses. The trade-off between these two types of quantum noise gives rise to the so-called Standard Quantum Limit (SQL). For the first part of this talk, we will walk through different approaches to surpassing the SQL, which leads to a more stringent sensitivity limit---the Fundamental Quantum Limit (FQL). For the second part, we will present current understanding of the FQL and its implications for enhancing detector sensitivity. The discussions here are not limited to gravitational-wave detectors, and can be applied to general linear quantum measurement devices. References: arXiv:1305.3957 for the first part and arXiv:1608.00766 for the second.


Thu, Nov 10,'16
1pm - 2:00pm
Theory Seminar: Yutaka Shikano (Institute for Molecular Science, Japan)
PS1.28

Observation of Aharonov-Bohm effect with quantum tunneling Quantum tunneling is one such phenomenon that is essential for a number of devices that are now taken for granted. However, our understanding of quantum tunneling dynamics is far from complete, and there are still a number of theoretical and experimental challenges. The dynamics of the quantum tunneling process can be investigated if we can create a large tunneling region. We have achieved this using a linear Paul trap and a quantum tunneling rotor, which has resulted in the successful observation of the Aharonov–Bohm effect in tunneling particles. Also, this result shows that the spatially separated phonon can be interfered. This work is collaborated with Atshushi Noguchi, Kenji Toyoda, and Shinji Urabe. Nature Communications 5, 3868 (2014)


Thu, Nov 17,'16
1pm - 2:00pm
Theory Seminar: Johannes Knolle (Cambridge)
PS1.28

Quantum oscillations without a Fermi surface and the anomalous de Haas-van Alphen effect The de Haas-van Alphen effect (dHvAe), describing oscillations of the magnetization as a function of magnetic field, is commonly assumed to be a definite sign for the presence of a Fermi surface (FS). Indeed, the effect forms the basis of a well-established experimental procedure for accurately measuring FS topology and geometry of metallic systems, with parameters commonly extracted by fitting to the Lifshitz-Kosevich (LK) theory based on Fermi liquid theory. Here we show that, in contrast to this canonical situation, there can be quantum oscillations even for band insulators of certain types. We provide simple analytic formulas describing the temperature dependence of the quantum oscillations in this setting, showing strong deviations from LK theory. We draw connections to recent experiments an SmB6.


Thu, Nov 24,'16
1pm - 2:00pm
Theory Seminar: Ivan Coluzza (Vienna): Design of patchy polymers: biomimetic self-knotting chains
PS1.28

We present a novel theoretical framework within which we are able to design new experimentally realisable materials with tuneable self-assembling properties. Our work takes inspiration from the results obtained with our recently developed protein coarse graining procedure, namely the “Caterpillar” model [1,2]. Based on these results we postulated the “minimum valence principle" (MVP). According to the MVP in order for a generalised bead-spring system to be designable and foldable, it is sufficient for the chain to have a sequence of different isotropic interactions combined with directional interactions that further constrain the configurational space. Based on this principle we introduced an optimal set of modular sub-units, and the definition of a design procedure necessary to choose a string of the units that once bonded into a chain will spontaneously fold to a specific target structure [3-5]. We show that such structures can be highly non-symmetrical and posses interesting topological properties fully controllable by the sequence of beads along the chain. Biomimetic patchy polymers represent a considerable step forward in the synthesis of novel materials, because they are based on a limited alphabet of particles that can be reused and assembled, practically, in an infinite number of combinations. Artificial modular self assembling systems such as this one are not available at the moment and the one we propose is the first of this kind. [1] Coluzza, I. (2011). A coarse-grained approach to protein design: learning from design to understand folding. PloS one, 6(7), e20853. doi:10.1371/journal.pone.0020853 [2] Coluzza, I. (2013). Transferable coarse-grained potential for de novo protein folding and design. Submitted. [3] Coluzza, I., & Dellago, C. (2012). The configurational space of colloidal patchy polymers with heterogeneous sequences. Journal of Physics: Condensed Matter, 24(28), 284111. doi:10.1088/0953-8984/24/28/284111 [4] Coluzza, I., van Oostrum, P. D. J., Capone, B., Reimhult, E., & Dellago, C. (2012). Design and folding of colloidal patchy polymers. Soft Matter. doi:10.1039/c2sm26967h [5] Coluzza, I., van Oostrum, P. D. J., Capone, B., Reimhult, E., & Dellago, C. (2013). Sequence Controlled Self-Knotting Colloidal Patchy Polymers. Physical Review Letters, 110(7), 075501. doi:10.1103/PhysRevLett.110.075501


Thu, Dec 1,'16
1pm - 2:00pm
Theory Seminar: Stephen Clark (Bath) -- "Enhanced super-exchange pairing in a periodically driven Hubbard model"
PS1.28

Controlling the structural and electronic properties of solids with THz lasers has opened up tantalizing prospect in ultrafast materials science. In contrast to optical frequencies it enables mode-selective driving of vibrational excitations relevant for the establishment of various broken-symmetry states. In particular so-called light-induced superconductivity has been observed in several materials ranging from cuprates to alkali-doped fullerenes. Motivated by experiments on driven infrared active molecular vibrations in organic materials, I will discuss the effect of a finite frequency ω modulation of on-site energies in the Hubbard model with a checkerboard spatial periodicity. In particular we focus on the strong-coupling limit U >> t of the doped Hubbard model where the effective t-J Hamiltonian is applicable and super-exchange pairing can occur. Through a Floquet analysis, in the physically relevant regime where U >> ω and ω >> t, J, we show that this driving causes a substantial suppression of the electronic hopping t, while leaving the bare super-exchange interaction J unchanged. This suggests that electrons can be slowed down enough in the out-of-equilibrium state to allow the normally subordinate super-exchange interaction to become dominant, and thus favour nearest-neighbour pairing. I will show that this leads to a compelling new pathway to engineering light-induced superconductivity in strongly correlated quantum materials.


Thu, Dec 8,'16
1pm - 2:00pm
Theory Seminar: Arijeet Pal (Oxford): Many body localisation: one, two, three,…, infinity
PS1.28

Equilibrium statistical physics holds true for an ergodic system which loses all local information of its initial condition under time evolution. In the last decade, a flurry of theoretical work has shown that ergodicity can be broken in an isolated, quantum many-body system even at high energies in the presence of disorder, a phenomena known as many-body localisation (MBL). The recent experimental observation of MBL in ultra-cold atoms has raised a plethora of intriguing questions. In this talk I will throw some light on the effect of dimensionality on the properties of MBL. In one dimension, the strongly localized regime is described in terms of quasi-local integrals of motion, also known as l-bits. Based on this picture we develop an efficient tensor network method to evaluate the entire spectrum of fully many-body localised systems. I will also present the non-ergodic properties of eigenstates of infinite range quantum spin glass models governed by localisation on the infinite dimensional hypercube. On going away from the limiting cases of one and infinite dimensions, I will develop a refined phenomenology of MBL in terms of l*-bits which are only approximately conserved and discuss their experimental consequences.


Thu, Jan 19,'17
1pm - 2:00pm
Theory Seminar: Zlatko Papic (Leeds): Quantum integrability from the entanglement spectrum
PS1.28

Quantum many-body systems are challenging to study because of their exponentially large Hilbert spaces, but at the same time they represent an arena for exciting new physics which results from interactions between particles. For theoretical purposes, it is convenient to know if such systems can be expressed in a "simple" ways in terms of some nearly-free quasiparticles, or more generally if one can construct a large set of operators that approximately commute with the system’s Hamiltonian. In this talk I will discuss two ways of approaching these questions using the "entanglement spectrum". In the first part, I will show that strongly disordered systems in the many-body localized phase have a universal power-law structure in their entanglement spectra. This is a consequence of their local integrability, and distinguishes such states from typical ground states of gapped systems. In the second part, I will introduce a notion of “interaction distance” and show that the entanglement spectrum can be used to quantify “how far” an interacting ground state is from a free (Gaussian) state. I will discuss some examples of quantum spin chains and outline a few future directions. [1] M. Serbyn, A. Michailidis, D. Abanin, Z. Papic, arXiv:1605.05737. [2] C. J. Turner, K. Meichanetzidis, Z. Papic, and J. K. Pachos, arXiv:1607.02679.


Thu, Feb 23,'17
1pm - 2:00pm
Theoretical Physics Seminar: Dave Foster (Bristol), Skyrmions and Nuclear States
PS1.28

The Skyrme model is a nonlinear model of nuclear physics, which can be derived from fundamental physics. Its topological excitations model nuclei and are called Skyrmions. In this talk I shall introduce the Skyrme model, and a piece of research where we modelled key nuclei states as spinning Skyrmions (Nuclear Physics B 899 (2015) 513–526).


Thu, Mar 2,'17
1pm - 2:00pm
Theoretical Physics Seminar: Neophytos Neophytou (Warwick), Understanding and designing the thermoelectric properties of nanomaterials using atomistic and quantum transport simulations
PS1.28

tba


Thu, Mar 9,'17
1pm - 2:00pm
Theoretical Physics Seminar: Mark Howard (Sheffield), Application of a resource theory for magic states to fault-tolerant quantum computing
PS1.28

Motivated by their necessity for most fault-tolerant quantum computation schemes, we formulate a resource theory for magic states. We first show that robustness of magic is a well-behaved magic monotone that operationally quantifies the classical simulation overhead for a Gottesman-Knill type scheme using ancillary magic states. Our framework subsequently finds immediate application in the task of synthesizing non-Clifford gates using magic states. When magic states are interspersed with Clifford gates, Pauli measurements and stabilizer ancillas - the most general synthesis scenario - then the class of synthesizable unitaries is hard to characterize. Our techniques can place non-trivial lower bounds on the number of magic states required for implementing a given target unitary. Guided by these results we have found new and optimal examples of such synthesis. [arXiv: 1609.07488]


Thu, Mar 16,'17
1pm - 2:00pm
Theoretical Physics Seminar: Patrick Warren (Unilever), The Amontons-Coulomb percolation transition: how a staple yarn transmits tension and why our clothes don't fall apart
PS1.28

In his celebrated 1638 Dialogues Concerning Two New Sciences, Galileo identified a fascinating problem in the mechanics of ropes. His fictitious discussant Salviati asks "How are fibres, each not more than two or three cubits in length, so tightly bound together in the case of a rope one hundred cubits long that great force is required to break it?" He then proceeds to explain that " [...] in the case of the rope the very act of twisting causes the threads to bind one another in such a way that when the rope is stretched with a great force the fibres break rather than separate from each other." With the benefit of hindsight, one might say Galileo recognized that the mechanical integrity of ropes (and by implication staple yarns and woven fabrics) is down to frictional contacts between fibres. But beyond this general observation, and despite our everyday familiarity with these issues, one can argue that Galileo's physics problem has remained unresolved for nearly four hundred years. Here it is proposed that the mechanical integrity of such fibre assemblies is actually a consequence of a generic tensile stress percolation transition, which appears under the Amontons-Coulomb friction laws for long enough fibres and with enough entanglement. This is demonstrated in abstract yarn models in which the friction laws are formulated as a linear programming (LP) problem. In these models the percolation transition is manifest as the onset of LP feasibility, wherein the yarn can in principle support an unbounded tensile load without slippage even though the fibre ends remain tension-free.


Thu, May 25,'17
1pm - 2:00pm
Theory Seminar: 2nd year PhD talks, Christopher Lakey and Oliver Dyer, 1300 in PS1.28
PS1.28

Oliver Dyer: An introduction to Wavelet Monte Carlo dynamics The inclusion of long-range hydrodynamic interactions (HIs) in simulations of soft-matter systems leads to large computational costs that make the simulation of large systems impractical, motivating the search for more efficient algorithms. In this talk I introduce Wavelet Monte Carlo dynamics (WMCD), a new algorithm that includes hydrodynamics in the distributions of Monte Carlo moves such that a WMCD code does not need to calculate HIs explicitly. Together with an overview of the algorithm itself, I present results showing how WMCD compares to established algorithms and confirming its validity as a hydrodynamic simulator.


Thu, Jun 1,'17
1pm - 2:00pm
Theory Seminar: Elisabetta Matsumoto (Georgia Tech), Non-Euclidean Virtual Reality, 1300 in PS1.28 (joint with the Geometric Topology seminar)
PS1.28

The properties of euclidean space seem natural and obvious to us, to the point that it took mathematicians over two thousand years to see an alternative to Euclid’s parallel postulate. The eventual discovery of hyperbolic geometry in the 19th century shook our assumptions, revealing just how strongly our native experience of the world blinded us from consistent alternatives, even in a field that many see as purely theoretical. Non-euclidean spaces are still seen as unintuitive and exotic, but with direct immersive experiences we can get a better intuitive feel for them. The latest wave of virtual reality hardware, in particular the HTC Vive, tracks both the orientation and the position of the headset within a room-sized volume, allowing for such an experience. We use this nacent technology to explore the three-dimensional geometries of the Thurston/Perelman geometrization theorem. This talk focuses on our simulations of H³ and H²×E. Joint work with: Vi Hart, Andrea Hawksley, and Henry Segerman


Tue, Jun 6,'17
12am - 12:00am
TCM Meeting
L3, Chemistry Concourse

PROGRAM 11:00 Rebecca Nicholls, Oxford, Enhancing Materials Design 11:50 Nicholas Bristowe, Kent, Magnetoelectric Ferroics 13:40 Tapio Ala-Nissila, Loughborough, Polymer Driven Translocation 14:30 Posters 16:10 Andrew James, UCL, 2D Many-Body Systems


Thu, Jun 8,'17
1pm - 2:00pm
Theory Seminar: 2nd year PhD talks, Dominic Branford and Nelson Yeung, 1300 in PS1.28
PS1.28

TBA


Thu, Jun 15,'17
1pm - 2:00pm
Theory Seminar: Buddhapriya Chakrabarti (Sheffield), A tale of two problems: surface segregation in polymer mixtures, and self-assembly of orientable objects on curved flexible manifolds, 1300 in PS1.28
PS1.28

I shall be describing two of my current interests in soft matter, small molecule migration in complex matrices and shape minimisation of assembled liquid crystals (that break orientational order) on curved flexible manifolds with free boundaries and edges. The first problem arises in several products of daily use and lead to reduced functional properties, Controlling surface segregation therefore can lead to design of products with well tailored properties. Fundamental polymer physics issues arising in this context will be addressed and a new phenomenological free energy functional that incorporates elastic degrees of freedom in surface segregating systems of gels discussed. Next I will discuss a variational formulation that we have developed for simultaneously minimising elastic free energy and shape for liquid crystals on flexible surfaces. Frustration arising in curved geometries naturally leads to the formation of defects. Our formulation allows us to correctly evaluate and predict the existence of a defect phase that is a mixture of a disclination and a screw dislocation in a class of Smectics. Earlier attempts at obtaining the shape equations in systems having shape-orientational order coupling have been erroneous.


Thu, Oct 5,'17
1pm - 2:00pm
Theory Seminar: John Molina (Kyoto), Dynamics of Active Particles: From swimmers to crawling cells, 1300 in PS1.28
PS1.28

Active systems, composed of “particles” that consume local energy to perform work, have attracted a great deal of attention due to their relevance in Physics, Biology, Medicine, and Engineering. Examples of these systems can be found at vastly different length scales: from the nano-scale, with kinesin motors transporting cargo inside of cells, to the micro-scales of cells crawling around to close wounds or bacteria swimming in viscous media, and finally, to the macro-scales at which fish, birds, and humans move about. In our work, we have focused on studying the dynamics of micro-meter sized active particles, including both swimmers (e.g., bacteria) and crawlers (e.g., epidermal cells). While we have a fairly complete understanding of the propulsion mechanism used by such particles, the non-trivial coupling between the particle and its environment gives rise to complex dynamical behaviors that have yet to be fully explained. In other words, we know how a single bacteria or cell is able to move, but we cannot always predict what will happen when many of these particles come together. Given the difficulty of performing controlled experiments on these type of systems, computer simulations have become one of the preferred approaches for studying the properties of these active systems. We will introduce the basic computational models that allow us to study the dynamics of interacting swimmers, including the full hydrodynamic interactions, as well as the collective motion of crawling cells on 2D substrates. In the first part of the presentation, we will discuss the collective motion of particles swimming in a viscous medium. We will show that the type of swimming, determined by whether the propulsion is generated at the front (e.g., a puller like the Chlamydomonas algae) or at the back (e..g,, a pusher such as spermatozoids or most bacteria), has a crucial effect on the hydrodynamic interactions between swimmers, and thus, on the collective motion that can be observed[1-4]. In the second part of our talk, we will consider the dynamics of cells crawling on 2D substrates. Here, we will focus on the response of the cell to a periodic stretching of the substrate, which is known to result in a preferential alignment that is cell specific[5], and on the role of cell-cell interactions on the large scale collective motion of cell colonies[6]. References: [1] Molina, Nakayama, and Yamamoto, Soft Matter 9, 4923 (2013) [2] Molina and Yamamoto, Mol. Phys. 112, 1389 (2014) [3] Oyama, Molina, and Yamamoto, Phys. Rev. E 93, 043114 (2016) [4] Delfau, Molina, and Sano, Europhys. Lett. 114, 24001 (2016) [5] Okimura, Ueda, Sakumura, and Iwadate, Cell Adhes. Migr. 0, 1 (2016) [6] Schnyder, Molina, Tanaka, and Yamamoto, Sci. Rep. 7, 5163 (2017)


Thu, Oct 12,'17
1pm - 2:00pm
Theory Seminar: David Jennings (Oxford), Thermodynamics, symmetry principles and quantum information, 1300 in PS1.28
PS1.28

The concept of irreversibility lies at the heart of physics and can often be a subtle thing to pin down. In recent years it has acquired new guises that are motivated by information-theoretic aims. For example in the case of quantum entanglement, intrinsically non-classical correlations may be utilised to achieve tasks such as quantum teleportation or quantum computing. However, the use of this entanglement results in its consumption, and a form of irreversibility that can be quantified and studied in a precise manner. Here I will describe recent work that arises from both the study of entanglement and also the development of symmetry principles beyond Noether's theorem. The approach allows us to extend thermodynamic concepts into arbitrarily non-classical regimes and leads to a range of new insights: it shows that quantum systems display a form of disorder at the nanoscale very different from that at macroscopic scales; it allows us to rigorously quantify the effects of quantum coherence in thermodynamic processes; it also provides a quantum information toolkit to extend gauge symmetries in many-body physics beyond Lagrangian formulations. The discussion will be an introduction to these concepts, and so no specialist knowledge of the area is assumed.


Thu, Oct 19,'17
1pm - 2:00pm
Theory Seminar: Alex Chin, tba, 1300 in PS1.28
PS1.28

tba


Thu, Nov 2,'17
1pm - 2:00pm
Theory Seminar: Ana-Suncana Smith (Erlangen), From membranes, to tissues, to active motion, 1300 in PS1.28
PS1.28

Membranes are ubiquitous in living cells but many questions remain outstanding. These include how to characterize a membrane's material properties and their interactions with the environment. Living membranes are generically out of equilibrium and I will discuss how active fluctutations may be involved in the control of the formation of protein assemblies. These structures have important implications on the organization of cells within tissues and the control of their shape. I will outline basic physical principles which may contribute used to understand the epithelium. In the last part of the talk, I will outline the role of elasticity in the behavior of active swimmers, from a single entity to a swarm.


Thu, Nov 9,'17
1pm - 2:00pm
Theory Seminar: Ognyan Oreshkov (Oxford/Bruxelles), Indefinite causal order in quantum mechanics, 1300 in PS1.28
PS1.28

According to quantum theory, physical variables in general do not have definite values unless measured. Yet, the time and causal order of events are assumed definite. A natural question is whether the latter reflects a fundamental physical restriction or it is an artefact of our formulation of the theory. Is it possible that, in suitable circumstance, the causal order of events can be indefinite similarly to other physical variables, how would this be described formally, and what testable consequences would it entail. To investigate these questions, we recently introduced a theoretical framework for correlations between separate quantum experiments that does not assume a causal structure from the outset, but only the validity of standard quantum theory locally. This framework unifies all correlations between local quantum experiments in space-time via a mathematical object called the ‘process matrix’, which generalises the standard density matrix. Remarkably, the framework also reveals the in-principle possibility for a new kind of correlations incompatible with any definite causal structure. In this talk, I review these results and discuss recent progress in understanding whether such acausal scenarios could have a physical realisation within standard quantum mechanics.


Thu, Nov 16,'17
1pm - 2:00pm
Theory Seminar: Sergii Strelchuk (Cambridge), Optimal Port-based Teleportation in Arbitrary Dimension, 1300 in PS1.28
PS1.28

Quantum teleportation is one of the earliest and most widely used primitives in Quantum Information Science which performs an arbitrary quantum state transfer between two spatially separated systems. It involves pre-sharing an entangled resource state and consists of three simple stages. The first stage involves a joint measurement of the teleported subsystem together with the share of the resource state on the sender’s side. In the second step, a classical measurement outcome is communicated to the receiver. The last step consists of applying a requisite correction operation which recovers the transmitted quantum state. Port-based teleportation (PBT) is a unique set of teleportation protocols in that they do not require unitary correction. We study PBT protocols and fully characterize their performance for an arbitrary dimensions and number of ports. We find optimal probability of success and the fidelity of teleportation for all probabilistic and deterministic PBT schemes. In the latter case, surprisingly, the answer depends only on a largest eigenvalue of a certain easy to construct matrix which encodes the relationship between a set of Young diagrams and emerges as the the optimal solution to the relevant semidefinite program. To derive our results, we develop new mathematical tools to study the symmetries of the operators that arise in PBT protocols and belong to the algebra of partially transposed permutation operators. These tools can be used to characterize quantum systems with partial symmetries. Quantum states occurring in the PBT protocol are one such example. Systems with partial symmetries are widespread but in contrast to their permutational-invariant counterparts very little is known about how to efficiently estimate their properties.


Thu, Nov 23,'17
1pm - 2:00pm
Theory Seminar: Elsen Tjhung (Cambridge), Time reversal symmetry breaking in scalar field theory, 1300 in PS1.28
PS1.28

Active matter is a class of non-equilibrium systems where energy is injected to the system continuously by the constituent particles themselves. Many examples of active matter are biological in nature, for example, bird flocks, bacterial suspensions and biological tissues. In the case of bacterial suspensions, the fluid solvent is continuously stirred by the swimming motion of the bacteria, driving it out-of-equilibrium. Active matter is an interesting class of non-equilibrium systems because it often displays large-scale time reversal symmetry breakdown at steady state. For example, when we put an asymmetric gear into a bath full of bacteria, the gear will start to rotate in one direction at steady state. This is a manifestation of large-scale time reversal symmetry breaking because if we reverse the arrow of time, the gear will rotate in the other direction. In this talk, I will present a simple scalar field theory which can capture such large-scale time reversal symmetry breaking.


Thu, Nov 30,'17
1pm - 2:00pm
Theory Seminar: Stephen Powell (Nottingham), Non-equilibrium classical dynamics and quantum phases of dimer models, 1300 in PS1.28
PS1.28

Dimer models arise as effective descriptions in a variety of physical contexts, and provide paradigmatic examples of systems subject to strong local constraints. Their statistical mechanics exhibits unusual phenomena such as algebraic correlations and deconfinement of monomer excitations. I will first describe the classical non-equilibrium dynamics of the dimer model, where signatures of strong correlations are visible in both global and local observables, and can be understood in terms of one-dimensional strings of high mobility. I will then show how the classical dynamics can be used to study the corresponding quantum problem, and helps to resolve an outstanding puzzle about the structure of the phase diagram.


Thu, Dec 7,'17
1pm - 2:00pm
Theory Seminar: Paul Goddard (Warwick), Determining the Fermi surface of high-temperature superconductors and other low-dimensional materials, 1300 in PS1.28
PS1.28

I will discuss recent magnetotransport data on an underdoped high-temperature superconductor. To assist with the discussion I will first describe how one goes about mapping the Fermi surface of quasi-two-dimensional materials using high magnetic field measurements, focussing particularly on the technique of angle-dependent magnetoresistance. This will be illustrated using the results of earlier experiments on an organic superconductor, for which a full determination of the Fermi surface was possible. I then will contrast this with the more challenging measurements performed on YBa2Cu3O6+x and explain what conclusions can be drawn in this case.