Dynamical Properties of the Three-Dimensional XY Model. II. Singularity Structure of the Frequency Dependent Susceptibility

Abstract

Twenty coefficients are presented in the exact high frequency, high temperature expansion of the perpendicular susceptibility of the spin-1/2 XY model on the f.c.c. lattice. Padé approximant analysis reveals a weak high frequency, temperature independent singularity at ω ≈ (14 ± 2)J/h which we identify with a "quasi-atomic" resonance at ω = 12J/h. We also find a strong singularity at a lower frequency which decreases with decreasing temperature, and for which we have not yet found a satisfying explanation.