MATHEMATICAL PHYSICS WEBINAR
RUTGERS UNIVERSITY
__________________________________________
Sriram Ramaswamy - Dept of Physics, Indian Institute of Science, Bangalore and ICTS-TIFR, Bangalore
Wednesday, June 25th, 2025
Zoom opens: 10:30AM DST
Seminar Begins: 10:45AM DST
Flocking and defect cloaking in nonreciprocal XY models
We examine "active" planar models (Dadhichi et al. 2020) in which the spin vectors point in real space, and the interaction of a spin with a neighbour depends on the degree to which it points towards that neighbour. The aligning torques in these models are nonpotential, and — off-lattice — their coarse-grained dynamics is that of a density-free flocking field theory (Toner 2012). There is neither motility or matter transport, but the order parameter advects itself as if it was a velocity. My talk will show that on- and off-lattice versions of this class of models differ fundamentally: the anisotropy of the lattice models survives to the largest scales, so their universality class is that of an active clock model, with no quasi-long-range ordered phase for any lattice coordination number. We show analytically that spin waves in the lattice model must acquire a "mass", for which our simulations find evidence at the largest scales. We show that a pair of vortices in these models are shielded from each other in a class of configurations, deferring their annihilation and allowing time for the nucleation of further defects. The result, we show numerically, is the destruction of the polarised phase via an aster apocalypse resembling that in Besse et al. 2022. This work was done with Pankaj Popli and Ananyo Maitra and is available at arXiv:2503.06480.