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

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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 endpoints of the intermediate BKT phase, i.e., the phase transition at the low-temperature endpoint is a BKT transition and that at the high-temperature endpoint 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 Z₂ symmetry between high-spin rich and low-spin rich states.