The BKT transition and its dynamics in a spin fluid

We study the effect of particle mobility on phase transitions in a spin fluid in two dimensions. The presence of a phase transition of the BKT universality class is shown in an off-lattice model of particles with purely repulsive interaction employing computer simulations. A critical spin wave region 0 < T < T-BKT is found with a nonuniversal exponent eta(T) that follows the shape suggested by BKT theory, including a critical value consistent with eta(BKT) = 1/4. One can observe a transition from power-law decay to exponential decay in the static correlation functions at the transition temperature T-BKT, which is supported by finite-size scaling analysis. A critical temperature T-BKT = 0.17(1) is suggested. Investigations into the dynamic aspects of the phase transition are carried out. The short-time behavior of the incoherent spin autocorrelation function agrees with the Nelson-Fisher prediction, whereas the long-time behavior differs from the finite-size scaling known for the static XY model. Analysis of coherent spin wave dynamics shows that the spin wave peak is a propagating mode that can be reasonably well fitted by hydrodynamic theory. The mobility of the particles strongly enhances damping of the spin waves, but the model still lies within the dynamic universality class of the standard XY model.

Automated summary

We investigate the impact of particle mobility on phase transitions in a two-dimensional spin fluid. By using computer simulations, we demonstrate the existence of a BKT universality class phase transition in an off-lattice model of repulsive particles. The results show a critical spin wave region with a nonuniversal exponent eta(T) that follows the shape predicted by BKT theory. The transition from power-law decay to exponential decay at the critical temperature T-BKT is supported by finite-size scaling analysis.