Thermodynamic behavior of the XXZ Heisenberg s = 1/2 chain around the factorizing magnetic field.

We have investigated the zero-and finite-temperature behaviors of the anisotropic antiferromagnetic Heisenberg XXZ spin-1/2 chain in the presence of a transverse magnetic field (h). The attention is concentrated on an interval of magnetic field between the factorizing field (h(f)) and the critical one (h(c)). The model presents a spin-flop phase for 0 < h < h(f) with an energy scale which is defined by the long range antiferromagnetic order while it undergoes an entanglement phase transition at h = h(f). The entanglement estimators clearly show that the entanglement is lost exactly at h = h(f), which justifies different quantum correlations on both sides of the factorizing field. As a consequence of zero entanglement (at h = h(f)) the ground state is known exactly as a product of single-particle states which is the starting point for initiating a spin wave theory. The linear spin wave theory is implemented to obtain the specific heat and thermal entanglement of the model in the interested region. A double-peak structure is found in the specific heat around h = h(f), which manifests the existence of two energy scales in the system as a result of two competing orders before the critical point. These results are confirmed by the low temperature Lanczos data which we have computed.