A Monte Carlo study of crossover behaviour and solitons in an anisotropic Heisenberg chain

The authors have examined the spin correlations and configurations of a classical Heisenberg antiferromagnetic chain with weak easy-plane anisotropy by means of a Monte Carlo simulation. The results show the expected crossover from XY to Heisenberg behaviour as the temperature is raised. Direct examination of the spin configurations shows the presence of out-of-easy-plane solitons as predicted by Mikeska (1980). However this soliton picture appears to be viable only over a rather narrow temperature range within the crossover region. It is found that the out-of-easy-plane correlation length does not have a maximum at non-zero temperature, in disagreement with transfer-matrix analysis of similar problems.