Unless otherwise specified, all talks are held starting at 1.15pm in the Theory Library on the 4th floor of Physics East.

Everyone is encouraged to ask questions during the talk. You are welcome to leave when you need to, otherwise the talk will usually wrap up by 2.30pm, at which point there will be biscuits.

Thursday 25th April 2024: Martin Speight (University of Leeds)

Soliton crystals

Topological solitons are smooth spatially localized lump-like solutions of nonlinear field theories that can move around and interact in a particle-like manner. Examples are superconducting vortices, magnetic monopoles, domain walls and skyrmions. In condensed matter contexts they are usually observed in the laboratory in the form of spatially periodic arrays, analogous to crystals. The Abrikosov vortex lattice in type II superconductors is a prominent and influential example. Theoretical studies of soliton crystals almost always impose some plausible period lattice geometry (cubic, square or triangular) a priori, and optimize only over the size of the unit cell. I will argue that this assumption is, in general, ill-founded and that one should really optimize the system over the space of fields _and_ period lattices. The latter variation has an elegant reformulation in terms of the stress tensor of the field theory. When implemented numerically, one finds that much more exotic period lattices are possible than those conventionally imposed. The ideas will be illustrated by concentrating on skyrmion crystals in 2 and 3 spatial dimensions.

Joint work with Derek Harland and Paul Leask (also at Leeds).

Thursday 2nd May 2024: Alexander Mietke (University of Oxford)

Title and Abstract: TBC

Thursday 9th May 2024: Orazio Scarlatella (University of Cambridge)

Strongly-coupled atomic arrays: a dynamical mean-field theory study

Subwavelength arrays of quantum emitters have emerged as an interesting platform displaying prominent collective effects. In this talk I will discuss the steady-states of such arrays under coherent driving, realizing an open quantum many-body problem with long range interactions and dissipation. I will discuss a Dynamical Mean Field Theory approach to the problem, for which few theoretical methods are available. I will show that the combination of dipolar interactions and regular geometry have a dramatic effect on the spectrum of emitted light in the strong-drive regime: the famous Mollow triplet characterizing the emission of a single atom develops a structured broadening with flat sidebands, distinguishing the emission of atomic arrays from that of disordered atomic clouds or of non-interacting emitters. For moderate drive strengths instead, I will show that the steady-state is influenced by the existence of guided modes in the single-particle regime that are completely decoupled from dissipation. 

Thursday 16th May 2024: Multu Cukurova (University College London)

Title and Abstract: TBC