Termination of the Berezinskii-Kosterlitz-Thouless phase with a new critical universality in spin-crossover systems

Abstract

Two-dimensional systems with $U(1)$ symmetry exhibit a peculiar phase, i.e., the Berezinskii-Kosterlitz-Thouless (BKT) phase. In particular situations, the BKT phase exists as an intermediate-temperature phase. There have been scenarios for the phase transitions at the two end points of the intermediate BKT phase; i.e., the phase transition at the low-temperature end point is a BKT transition and that at the high-temperature end point is either a BKT transition or a first-order transition. The present study gives a novel scenario, i.e., a second-order transition with a new critical universality and a BKT transition. We found that this new phase transition is realized in spin-crossover systems on a triangular lattice with an antiferromagnetic short-range interaction. At the low-temperature transition the elastic interaction plays as a ferromagnetic infinite-range interaction and encourages the breaking of ${Z}_{2}$ symmetry between high-spin-rich and low-spin-rich states.