A. Yu. Vasiliev, A. E. Tarkhov, L. I. Menshikov, P. O. Fedichev, Uwe R. Fischer
We investigate the nature of the phase transition occurring in a planar XY-model spin system with dipole-dipole interactions. It is demonstrated that a Berezinskii-Kosterlitz-Thouless (BKT) type of phase transition always occurs at a finite temperature, separating the ordered (ferro) and the disordered (para) phases. The low-temperature phase corresponds to an ordered state with thermal fluctuations, composed of a "gas" of bound vortex-antivortex pairs, which would, when considered isolated, be characterized by a constant vortex-antivortex attraction force which is due to the dipolar interaction term in the Hamiltonian. Using a topological charge model, we show that small bound vortex pairs are easily polarized, and screen the vortex-antivortex interaction in sufficiently large pairs. Screening changes the linear attraction potential of vortices to a logarithmic one, and leads to the familiar pair dissociation mechanism of the BKT type. The topological charge model is confirmed by numerical calculations, in which we demonstrate that the transition temperature slightly increases when compared with the standard BKT result for short-range interactions.
View original:
http://arxiv.org/abs/1303.4915
No comments:
Post a Comment