Dynamical properties of the three-dimensional XY model. II. Singularity structure of the frequency dependent susceptibility

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.