Abstract: The Berezinskii-Kosterlitz-Thouless (BKT) transition is a famous example of a phase transition driven by topological defects, occurring in two dimensional systems such as the XY model. Motivated by recent experiments, for example on Rydberg atom arrays, in which spins not only interact with their nearest neighbours but over long distances, I will discuss whether the BKT transition survives in the presence of long-range couplings decaying as 1/r^(2+sigma). Such long-range tails couple vortices to spin waves, which renders many well known approaches to the short range model inapplicable. I will show how a simple Landau-Peierls-type argument for vortex pairs, supported by renormalization group calculations, captures this coupling and suggests that the BKT transition persists for arbitrary sigma. I will then compare this picture with recent related works on the model, including Monte Carlo studies and other analytical approaches, and conclude with the open questions that remain.

About the speaker: Luis Walther is a PhD student in the group of Roderich Moessner at the Max Planck Institute for the physics of complex systems in Dresden, Germany. He received his Master's degree from the Technical University of Munich, where he worked in the group of Johannes Knolle. His research focuses on defect driven phases and phase transitions in systems with long-range interactions. Additionally, more recently he has been studying spectral functions of quantum spin liquids, looking for signs of fractionalisation.

Inviter: 孙孝奇 xqsun@iphy.ac.cn