Spin Structure Factor of the Frustrated Quantum Magnet $Cs_2 Cu Cl_4$.

The ground state properties and neutron structure factor for the two-dimensional antiferromagnet on the triangular lattice, with unidirectional anisotropy in the nearest-neighbor exchange couplings and a weak Dzyaloshinskii-Moriya (DM) interaction, are studied. This Hamiltonian has been used to interpret neutron scattering measurements on the spin $1∕2$ spiral spin-density-wave system, ${\mathrm{Cs}}_{2}{\mathrm{CuCl}}_{4}$ [R. Coldea et al., Phys. Rev. B 68, 134424 (2003)]. Calculations are performed using a $1∕S$ expansion, taking into account interactions between spin waves. The ground state energy, the shift of the ordering wave vector, $\mathbf{Q}$, and the local magnetization are all calculated to order $1∕{S}^{2}$. The neutron structure factor, obtained using anharmonic spin-wave Green's functions to order $1∕S$, is shown to be in reasonable agreement with published neutron data, provided that slightly different parameters are used for the exchange and DM interactions than those inferred from measurements in high magnetic field.