Nature of the low energy excitations in the short range ordered region of Cs${}_2$CuCl${}_4$ as revealed by ${}^{133}$Cs NMR

We report nuclear magnetic resonance measurements of the spin-1/2 anisotropic triangular lattice antiferromagnet Cs${}_2$CuCl${}_4$ as a function of temperature and applied magnetic field. The observed temperature and magnetic field dependence of the NMR relaxation rate suggests that low energy excitations in the short-range ordered region stabilized over a wide range of intermediate fields and temperatures of the phase diagram are gapless or nearly gapless fermionic excitations. An upper bound on the size of the gap of 0.037 meV $\approx J\mathrm{/}10$ is established. The magnetization and NMR relaxation rate can be qualitatively described either by a quasi-1D picture of weakly coupled chains, or by mean-field theories of specific 2D spin liquids; however, quantitative differences exist between data and theory in both cases. This comparison indicates that 2D interactions are quantitatively important in describing the low-energy physics.