Excitation spectra and ground state properties of the layered spin- 1 2 frustrated antiferromagnets Cs2 Cu Cl4 and Cs2 Cu Br4

We use series expansion methods to study ground- and excited-state properties in the helically ordered phase of spin-1/2 frustrated antiferromagnets on an anisotropic triangular lattice. We calculate the ground state energy, ordering wave vector, sublattice magnetization and one-magnon excitation spectrum for parameters relevant to Cs2CuCl4 and Cs2CuBr4. Both materials are modeled in terms of a Heisenberg model with spatially anisotropic exchange constants; for Cs2CuCl4 we also take into account the additional Dzyaloshinskii-Moriya (DM) interaction. We compare our results for Cs2CuCl4 with unpolarized neutron scattering experiments and find good agreement. In particular, the large quantum renormalizations of the one-magnon dispersion are well accounted for in our analysis, and inclusion of the DM interaction brings the theoretical predictions for the ordering wave vector and the magnon dispersion closer to the experimental results