Commensurate and incommensurate ground states ofCs2CuCl4in a magnetic field

We present calculations of the magnetic ground state of ${\mathrm{Cs}}_2\mathrm{Cu}{\mathrm{Cl}}_4$ in an applied magnetic field, with the aim of understanding the commensurately ordered state that has been discovered in recent experiments. This layered material is a realization of a Heisenberg antiferromagnet on an anisotropic triangular lattice. Its behavior in a magnetic field depends on field orientation, because of weak Dzyaloshinskii-Moriya interactions. We study the system by mapping the spin-$1∕2$ Heisenberg Hamiltonian onto a Bose gas with hardcore repulsion. This Bose gas is dilute, and calculations are controlled, close to the saturation field. We find a zero-temperature transition between incommensurate and commensurate phases as longitudinal field strength is varied, but only incommensurate order in a transverse field. Results for both field orientations are consistent with experiment.