Ordering and Defect Cloaking in Nonreciprocal Lattice XY Models.

We present a detailed analytical and numerical examination, on square and triangular lattices, of the nonreciprocal planar spin model introduced in Dadhichi et al. [Phys. Rev. E 101, 052601 (2020)PRESCM2470-004510.1103/PhysRevE.101.052601]. We show that the effect of lattice anisotropy should persist at large scales, leading to a "mass" for the angle field of the spins, and behavior not in the "Malthusian Toner-Tu" [Phys. Rev. Lett. 108, 088102 (2012)PRLTAO0031-900710.1103/PhysRevLett.108.088102] universality class. Numerically, we find evidence of this mass at large values of our nonreciprocity parameter; for smaller values, we find power-law scaling of long-wavelength equal-time correlators in the polar-ordered phase of our lattice model over the system sizes and wave number range explored. Focussing on topological defects, we show numerically that defect interactions are highly anisotropic with respect to the mean ordering direction. In particular, the constituents of a ±1 pair 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 polarized phase via an aster apocalypse reminiscent of that found by Besse et al. [Phys. Rev. Lett. 129, 268003 (2022)PRLTAO0031-900710.1103/PhysRevLett.129.268003] for flocks without number conservation.