Physica A, Statistical and Theoretical Physics vol:84 issue:1 pages:197-204
The SAW-approximation is calculated for the quantum-mechanical XY-model for spin 1/2 in a parallel magnetic field. For this purpose we derive an exact expression for the internal weight factors of the polygon graphs.
In this approximation we obtain an analytic expression for the phase-separation line in the (H,T)-phase. The critical exponents are the same as for classical models.