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 t_ε in the presence of an anisotropic extracellular signal depends on the anisotropy degree ε through the power law t_ε α ε², and that in a cell of radius R there should exist a threshold value ε_th 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 11, '18

1pm - 2:00pm

Theory Seminar: Mark Ancliffe (Catholic University of Korea)

Thu, Jan 18, '18

1pm - 2:00pm

Hua Guo (University of New Mexico)

Thu, Jan 25, '18

1pm - 2:00pm

Antonio Rodriguez (Universidad Politécnica de Madrid)

Thu, Mar 1, '18

1pm - 2:00pm

Leonardo Banchi (Imperial)

Thu, Mar 8, '18

1pm - 2:00pm

Frank Schlawin (Oxford)

Thu, Mar 15, '18

1pm - 2:00pm

Laura Filion (Utrecht)

Thu, May 3, '18

1pm - 2:00pm

Angelika Knothe (Manchester)

Thu, May 10, '18

1pm - 2:00pm

Rodrigo Lima (Universidade Federal de Alagoas)

Thu, May 17, '18

1pm - 2:00pm

Graeme Henkelman (Utexas)

Thu, May 31, '18

1pm - 2:00pm

Thomas Ouldridge (Imperial)

Thu, June 8, '18

1pm - 2:00pm

Sebatrata Mukherjee (Heriot-Watt)

Thu, Oct 4,'18
1pm - 2:00pm

Theory Seminar: Mark Dennis (Birmingham), Scientific Properties Of Complex Knots

PS1.28

Thu, Oct 11,'18
1pm - 2:00pm

Theory Seminar: Thomas Machon (Bristol), Topology and broken translational symmetry: two liquid crystalline case studies and some general results

PS1.28

Thu, Oct 25,'18
1pm - 2:00pm

Theory Seminar: John Biggins (Cambridge), Large strain elasticity: geometry, instability and brains

PS1.28

Thu, Nov 1,'18
1pm - 2:00pm

Theory Seminar: Shigeyuki Komura (Tokyo), A three-sphere microswimmer in a structured fluid

PS1.28

Thu, Nov 8,'18
1pm - 2:00pm

Theory Seminar: Fabian Maucher (Durham), tba

PS1.28

Thu, Nov 15,'18
1pm - 2:00pm

Theory Seminar: Robert Jack (Cambridge), tba

PS1.28

Thu, Nov 22,'18
1pm - 2:00pm

Theory Seminar: Patricia Bassereau (Institut Curie)

PS1.28

Thu, Nov 29,'18
1pm - 2:00pm

Theory Seminar: Thorsten Wahl (Oxford), Tensor network approaches to many-body localisation

PS1.28

Thu, Dec 6,'18
1pm - 2:00pm

Theory Seminar: Gabriele Sosso (Warwick, Chemistry)

PS1.28

Thu, Dec 13,'18
1pm - 2:00pm

Theory Seminar: Manuel dos Santos Dias (Forschungszentrum Jülich), Spin interactions, excitations and fluctuations in magnetic adatoms and small clusters from first principles

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Thu, Jan 10,'19
1pm - 2:00pm

Rebecca Milot (Warwick), Optoelectronic Properties of Hybrid Metal Halide Perovskites

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Hybrid metal halide perovskites have shown extraordinary success as active layers in solar cells, with power conversion efficiencies rivalling existing silicon technologies. A benefit of perovskites is that they are comprised of low-cost, earth abundant materials, and perovskite thin films are easily synthesized with simple starting materials. Additionally, they exhibit exceptional optoelectronic properties, which include strong absorption across the entire visible spectrum, long charge-carrier lifetimes, and high charge-carrier mobilities. Optical-pump/THz-probe (OPTP) spectroscopy has proven to be an essential technique for studying the charge-carrier dynamics and charge-carrier mobility in many of these materials including lead-based, tin-based, two-dimensional, and mixed-halide/mixed-cation perovskites. These studies have determined that the charge-carrier mobility and charge-carrier recombination dynamics are strongly dependent on the chemical composition, defect density, band structure, and crystallinity.

Thu, Jan 24,'19
1pm - 2:00pm

Andreas Honecker (Paris Cergy), Thermodynamic properties of the two-dimensional Shastry-Sutherland model for SrCu2(BO3)2

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Thu, Jan 31,'19
1pm - 2:00pm

Piero Naldesi (Grenoble-Alpes), The quantum advantage of attractive bosons in a ring-shaped potential

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Thu, Feb 7,'19
1pm - 2:00pm

Markus Muller (Swansea), tba

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Thu, Feb 14,'19
1pm - 2:00pm

Anton Souslov (Bath), Chiral Active Metamaterials

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Thu, Feb 21,'19
1pm - 2:00pm

Frank Kruger (UCL), Suppression of topological Mott insulators and novel quantum criticality of interacting Dirac fermions on the honeycomb lattice

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