Anisotropy and Magnetic Field Effects on the Entanglement of a Two Qubit HeisenbergXYChain

We investigate the entanglement of a two-qubit anisotropic Heisenberg $\mathrm{XY}$ chain in thermal equilibrium at temperature $T$ in the presence of an external magnetic field $B$ along the $z$ axis. By means of the combined influences of anisotropic interactions and a magnetic field $B$, one is able to produce entanglement for any finite $T$, by adjusting the magnetic field strength. This contrasts with the isotropic interaction or the $B=0\mathrm{}$ cases, for which there is no entanglement above a critical temperature $T_c$ that is independent of the external $B$ field.