Thermodynamic properties of the XXZ model in a transverse field

We have numerically studied the thermodynamic properties of the spin 1/2 XXZ chain in the presence of a transverse (noncommuting) magnetic field. The thermal, field dependence of specific heat and correlation functions for chains up to 20 sites has been calculated. The area where the specific heat decays exponentially is considered as a measure of the energy gap. We have also obtained the exchange interaction between chains in a bulk material using the random phase approximation and derived the phase diagram of the three-dimensional material with this approximation. The behavior of the structure factor at different momenta verifies the antiferromagnetic long-range order in the y direction for the three-dimensional case. Moreover, we have concluded that the low temperature Lanczos results [ M. Aichhorn et al. Phys. Rev. B 67 161103 (2003)] are more accurate for low temperatures and closer to the full diagonalization ones than the results of finite temperature Lanczos method [ J. Jaklic and P. Prelovsek Phys. Rev. B 49 5065 (1994)].