Finite-temperature phase transitions in quasi-two-dimensional spin-1 Bose gases

Recently, the Berezinskii-Kosterlitz-Thouless transition was found to be mediated by half-quantum vortices (HQVs) in two-dimensional (2D) antiferromagnetic Bose gases [Phys. Rev. Lett. 97, 120406 (2006)]. We study the thermal activation of HQVs in the experimentally relevant trapped quasi-2D systems and find that the crossover temperature is shifted upwards if skyrmions are allowed. Above the defect binding temperatures we observe transitions corresponding to the onset of a coherent condensate and a quasi-condensate and discuss the absence of a fragmented condensate.