Meron Hall effect signatures of Berezinskii-Kosterlitz-Thouless transition

We have analyzed the meron Hall response and spin superfluidity, in a 2D easy-plane Heisenberg model, near the BKT transition. Below the BKT temperature, merons are expected to appear only in tightly bound pairs, suppressing vorticity current. Above the BKT temperature, the thermal contribution to the free energy of the system is sufficient to break these bonds, resulting in a nonzero free meron density. These unbound merons behave as free (topological) charge carriers which are driven in the transverse direction relative to an injected spin current, allowing for a significant vorticity Hall current. Using a combination of numerical methods including Monte Carlo, and spin dynamics simulations, we have calculated this Hall response for different values of anisotropy as a function of temperature. We give proposals how this Hall current may be used to detect the BKT transition in a 2D easy plane ferromagnetic insulators.