Quasi-1DS=1/2AntiferromagnetCs2CuCl4in a Magnetic Field

Magnetic excitations of the quasi-1D $S=1\mathrm{}/2$ Heisenberg antiferromagnet (HAF) ${\mathrm{Cs}}_2{\mathrm{CuCl}}_4$ have been measured as a function of magnetic field using neutron scattering. For $T<\mathrm{}0.62\mathrm{}\mathrm{K}$ and $B=0\mathrm{}\mathrm{T}$ the weak interchain coupling produces 3D incommensurate ordering. Fields greater than $B_C=1.66\mathrm{}\mathrm{T}$, but less than the field $\left(\simeq 8\mathrm{T}\right)$ required to fully align the spins, are observed to decouple the chains, and the system enters a disordered intermediate-field phase (IFP). The IFP excitations are in agreement with the predictions of Müller et al. for the 1D $S=1\mathrm{}/2$ HAF, and Talstra and Haldane for the related $1/r^2$ chain (the Haldane-Shastry model). This behavior is inconsistent with linear spin-wave theory.