Phase diagram of the two-dimensional quantum antiferromagnet in a magnetic field

The two-dimensional Heisenberg antiferromagnet experiences an effective easy-plane anisotropy when a magnetic field is applied, giving rise to Berezinskii-Kosterlitz-Thouless (BKT) critical behavior. Remarkably, the strength of the effective anisotropy, and consequently the critical BKT temperature, can be tuned by varying the field. By means of the pure-quantum self-consistent harmonic approximation, that reduces the quantum model to an effective classical system, the phase diagram is constructed from its known classical counterpart. By classical Monte Carlo simulations of the effective system the thermodynamic quantities can be calculated. Comparison is made with the behavior observed experimentally in the S=5/2 compound Mn(HCOO)(2)2H(2)O. (C) 2006 American Institute of Physics.