Bose-Einstein condensation of magnons inCs2CuCl4: A dilute gas limit near the saturation magnetic field

Abstract

Based on the realistic spin Hamiltonian for the frustrated quasi-two-dimensional spin-$1∕2$ antiferromagnet ${\mathrm{Cs}}_{2}{\mathrm{CuCl}}_{4}$, a three-dimensional spin ordering in the applied magnetic field $B$ near the saturation value ${B}_{c}$ is studied within the magnon Bose-Einstein condensation scenario. Using a hard-core boson formulation of the spin model, a strongly anisotropic magnon dispersion in ${\mathrm{Cs}}_{2}{\mathrm{CuCl}}_{4}$ is calculated. In the dilute magnon limit near ${B}_{c}$, the hard-core boson constraint results in an effective magnon interaction which is treated in the Hartree-Fock approximation. The critical temperature ${T}_{c}$ is calculated as a function of the magnetic field $B$ and compared with the phase boundary ${T}_{c}(B)$ experimentally determined in ${\mathrm{Cs}}_{2}{\mathrm{CuCl}}_{4}$ [Phys. Rev. Lett. 95, 127202 (2005)].