Spin dynamics of the quasi-two-dimensional spin-12quantum magnetCs2CuCl4

We study dynamical properties of the anisotropic triangular quantum antiferromagnet ${\mathrm{Cs}}_2{\mathrm{CuCl}}_4$. Inelastic neutron scattering measurements have established that the dynamical spin correlations cannot be understood within a linear spin wave analysis. We go beyond linear spin wave theory by taking interactions between magnons into account in a $1∕S$ expansion. We determine the dynamical structure factor and carry out extensive comparisons with experimental data. We find that compared to linear spin wave theory, a significant fraction of the scattering intensity is shifted to higher energies and strong scattering continua are present. However, the $1∕S$ expansion fails to account for the experimentally observed large quantum renormalization of the exchange energies.