Invited Speakers

• The ubiquity of Kekule spiral order in magic-angle twisted bilayer graphene
  Nick Bultinck (University of Oxford)
  

  I will introduce a new type of symmetry-breaking order called "Incommensurate Kekule Spiral (IKS)" order, and argue that it is ubiquitous in the experimentally observed phase diagram of magic-angle twisted bilayer graphene.
  
  

• Phase transitions and fluctuations caused by nonreciprocity in classical many-body systems
  Sarah Loos (University of Cambridge)
  

  Reciprocity is a hallmark of any system in thermal equilibrium, but it is ubiquitously broken in systems far from equilibrium. A growing body of research demonstrates the significance of nonreciprocal interactions for classical many-body systems. In this talk, I will discuss how they can fundamentally affect the emergent phase transitions and collective fluctuations. Using a two-dimensional XY model, where each spin interacts only with its nearest neighbours within its ‘vision cone,’ we show how the inherent nonreciprocity can lead to the emergence of true long-range order [1]. Strikingly, defects propagate in a directional manner, thereby breaking the parity and time-reversal symmetry of the spin dynamics. Secondly, I turn to field-theoretical models for nonreciprocally coupled fluid components. Here, the nonreciprocity can cause the emergence of stable travelling waves through PT symmetry-breaking phase transitions. Near such transitions, fluctuations not only inflate, as is known from equilibrium criticality, but also develop an asymptotically increasing time-reversal asymmetry and associated surging entropy production [2]. For a nonreciprocal Cahn-Hilliard model, we show that this manifests itself in actively propelled interfaces whose dynamics can be mapped to the motion of a single microswimmer [3].

  [1] Loos, Klapp, Martynec, Long-Range Order and Directional Defect Propagation in the Nonreciprocal Model with Vision Cone Interactions, Phys. Rev. Lett. 130, 198301 (2023).
  [2] Suchanek, Kroy, Loos, Time-reversal and PT symmetry breaking in non-Hermitian field theories, ArXiv:2305.05633 (2023).
  [3] Suchanek, Kroy, Loos, Irreversible mesoscale fluctuations herald the emergence of dynamical phases, ArXiv:2303.16701 (2023).
  
  

• Symmetry, negative thermal expansion and superconductivity in layered perovskite oxides
  Arash Mostofi (Imperial College London)
  

  Layered perovskite oxides exhibit a wide range of functional properties including ferroelectricity, magnetoresistance, negative thermal expansion and superconductivity. In this talk I’ll focus on Ruddlesden-Popper layered perovskites with chemical formula A₂BO₄. I will give two examples of how an approach that combines first-principles electronic structure calculations and structural symmetry analysis with experimental characterisation is able to provide insights into the properties associated with materials in this class. The first example is negative thermal expansion in Ca₂MnO₄ and Ca₂GeO₄; and the second is the LTO-LTT phase transition in hole-doped lanthanum cuprates, which coincides with the suppression of superconductivity in favour of the stabilisation of stripe order.
  
  

• Wetting transition of active Brownian particles
  Nigel Wilding (University of Bristol)
  

  This talk describes recent simulation studies of a wetting transition in a minimal model of active matter.  To set the scene and establish terminology, we begin by reviewing how surface phase transitions manifest in assembles of passive (ie equilibrium) particles at an attractive impenetrable substrate. We then address the case of a system of active Brownian particles in contact with a purely repulsive potential barrier that mimics a thin permeable membrane.  Under conditions of bulk motility-induced phase separation into a gas-like and a liquid-like phase, we find that the interaction strength ε of the barrier controls the affinity of the liquid for the barrier region. We uncover clear signatures of a wetting phase transition as ε is varied.  We study this transition both in terms of how it relates to the location of the dense phase with respect to the barrier and in terms of the observed contact angle between the barrier and the surface of a liquid drop that is confined in a wedge-shaped geometry. In common with its equilibrium counterpart, the character of the wetting transition depends on the system dimensionality: a continuous transition with enhanced density fluctuations and gas bubbles is uncovered in 2d while 3d systems exhibit a sharp transition absent of large correlations.