# Theory Group Lunchtime Seminars

Scheduled seminars are listed below.

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Thu, Oct 4, '18
1pm - 2pm
###### Theory Seminar: Mark Dennis (Birmingham), Measuring Geometric Phase without Interferometry
PS1.28

A simple noninterferometric approach for probing the geometric phase of a structured Gaussian beam is proposed. Both the Gouy and Pancharatnam-Berry phases can be determined from the intensity distribution following a mode transformation if a part of the beam is covered at the initial plane. Moreover, the trajectories described by the centroid of the resulting intensity distributions following these transformations resemble those of ray optics, revealing an optical analogue of Ehrenfest’s theorem associated with changes in the geometric phase.

Thu, Oct 11, '18
1pm - 2pm
###### Theory Seminar: Thomas Machon (Bristol), Topology and broken translational symmetry: two liquid crystalline case studies and some general results
PS1.28

We study the topology of smectic defects in two and three dimensions. We give a topological classification of smectic point defects and disclination lines in three dimensions. In addition we describe the combination rules for smectic point defects in two and three dimensions, showing how the broken translational symmetry of the smectic confers a path dependence on the result of defect addition.

Thu, Oct 25, '18
1pm - 2pm
PS1.28
Thu, Nov 1, '18
1pm - 2pm
###### Theory Seminar: Shigeyuki Komura (Tokyo), A three-sphere microswimmer in a structured fluid
PS1.28

We discuss the locomotion of a three-sphere microswimmer in a viscoelastic structured fluid characterized by typical length and time scales [1]. We derive a general expression to link the average swimming velocity to the sphere mobilities. In this relationship, a viscous contribution exists when the time-reversal symmetry is broken, whereas an elastic contribution is present when the structural symmetry of the microswimmer is broken. As an example of a structured fluid, we consider a polymer gel, which is described by a two-fluid" model. We demonstrate in detail that the competition between the swimmer size and the polymer mesh size gives rise to the rich dynamics of a three-sphere microswimmer.

[1] K. Yasuda, R. Okamoto, and S. Komura, EPL 123, 34002 (2018).

Thu, Nov 8, '18
1pm - 2pm
PS1.28
Thu, Nov 15, '18
1pm - 2pm
PS1.28
Thu, Nov 22, '18
1pm - 2pm
PS1.28
Thu, Nov 29, '18
1pm - 2pm
###### Theory Seminar: Thorsten Wahl (Oxford), Tensor network approaches to many-body localisation
PS1.28

We propose a tensor network encoding the set of all eigenstates of a fully many-body localized system in one dimension. Our construction, conceptually based on the ansatz introduced in Phys. Rev. B 94, 041116(R) (2016), is built from two layers of unitary matrices which act on blocks of contiguous sites. We argue that this yields an exponential reduction in computational time and memory requirement as compared to all previous approaches for finding a representation of the complete eigenspectrum of large many-body localized systems with a given accuracy. Concretely, we optimize the unitaries by minimizing the magnitude of the commutator of the approximate integrals of motion and the Hamiltonian, which can be done in a local fashion. This further reduces the computational complexity of the tensor networks arising in the minimization process compared to previous work. We test the accuracy of our method by comparing the approximate energy spectrum to exact diagonalization results for the random-field Heisenberg model on 16 sites. We find that the technique is highly accurate deep in the localized regime and maintains a surprising degree of accuracy in predicting certain local quantities even in the vicinity of the predicted dynamical phase transition. To demonstrate the power of our technique, we study a system of 72 sites, and we are able to see clear signatures of the phase transition. Our work opens a new avenue to study properties of the many-body localization transition in large systems.