Hugues Chaté - CEA Saclay, France
Wednesday, July 23rd , 2025
Zoom opens: 10:30AM DST
Seminar begins: 10:45AM DST
Active crystals and the XY model with persistent noise
Two-dimensional crystals made of active particles can sustain very strong large scale spontaneous deformations without melting, in a clear departure from the bounds set by the equilibrium KTHNY theory on the decay of correlations. This can be traced back to the persistence in time of the active forces exerted by each particle. Indeed, a passive 2D crystal immersed in a bath of active, persistent, particles also displays fast-decaying correlations without melting. I will briefly recall these recent results.
I will then follow the above ideas to the end of their logic and study an XY model where the spins are subjected to some persistent noise. I will show that this model can resist spinwave fluctuations stronger than in equilibrium and remain quasi-ordered. The BKT transition is shifted along the line of fixed points, with some scaling exponents varying continuously with the persistence time of the noise. It is thus likely that the melting of 2D active crystals, when continuous, follows the KTHNY scenario but only qualitatively.