Infrared Absorption by Elementary Excitations of the One-DimensionalXYSystem

Far-infrared absorption experiments on Pr${\mathrm{Cl}}_3$ at low temperatures show a broad temperature-dependent absorption peaked at 3.0 ${\mathrm{cm}}^{-1\mathrm{}}$ which we attribute to cooperative spin excitations. Pr${\mathrm{Cl}}_3$ is a one-dimensional $\mathrm{XY}$ system and a calculation of the mean energy of zero-wave-vector excitations for this model has been carried out. Both the calculated mean energy and the peak of the absorption occur at $1.5J$, where $J$ is the nearest-neighbor interaction parameter.