Quasi-1D s=1/2 antiferromagnet Cs 2 CuCl 4 in a magnetic field

Magnetic excitations of the quasi-1D $S\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/2$ Heisenberg antiferromagnet (HAF) ${\mathrm{Cs}}_{2}{\mathrm{CuCl}}_{4}$ have been measured as a function of magnetic field using neutron scattering. For $Tl0.62\mathrm{K}$ and $B\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0\mathrm{T}$ the weak interchain coupling produces 3D incommensurate ordering. Fields greater than ${B}_{C}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.66\mathrm{T}$, but less than the field $(\ensuremath{\simeq}8\mathrm{T})$ 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 M\"uller et al. for the 1D $S\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/2$ 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.