Phase diagram of the antiferromagnetic XXZ model in the presence of an external magnetic field

The anisotropic s=1/2 antiferromagnetic Heisenberg chain in the presence of an external magnetic field is studied by using the standard quantum renormalization group (RG). We obtain the critical line of the transition from the partially magnetized (PM) phase to the saturated ferromagnetic phase. The crossover exponent between the PM phase and antiferromagnetic Ising phase is evaluated. Our results show that the anisotropy (Delta) term is relevant and causes crossover. These results indicate that the standard RG approach yields fairly good values for the critical points and their exponents. The magnetization curve, correlation functions, and the ground state energy per site are obtained and compared with the known exact results. [S0163-1829(98)09141-3].