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

We present the results of a calculation of the spin structure factor for the two-dimensional antiferromagnet on the triangular lattice, with strong directional anisotropy in the nearest-neighbour exchange couplings. The corresponding Heisenberg Hamiltonian describes the physics following from neutron scattering measurements in the frustrated quantum magnet $Cs_2 Cu Cl_4$, [R. Coldea, et. al., Phys. Rev. B, {\bf 68}, 134424, (2003)]. Since the experimental data reveal the presence of a small but finite on-site magnetic moment $S_z$, the calculations are performed using the Holstein-Primakoff representation for spins. The results for the structure factor, computed up to the order in $1/S$ that takes into account interactions between magnons, are compared with experiment.