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

TL;DR

This study uses $^{133}$Cs NMR to investigate low energy excitations in Cs$_2$CuCl$_4$, revealing gapless or nearly gapless fermionic excitations in the short-range ordered region, with implications for understanding 2D spin liquids.

Contribution

It provides experimental evidence of gapless excitations in Cs$_2$CuCl$_4$ and highlights the importance of 2D interactions in its low-energy physics.

Findings

Low energy excitations are gapless or nearly gapless fermionic.

An upper bound of 0.037 meV on the excitation gap.

Quantitative differences between data and both 1D and 2D theoretical models.

Abstract

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 $\approx J/10$ 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.