Elementary excitations in the spin liquid phase of Cs$_2$CuCl$_4$ as revealed by $^{133}$Cs NMR spin-lattice relaxation rate measurements.

We present $^{133}$Cs NMR spin-lattice relaxation rate measurements in the spin liquid phase of Cs$_2$CuCl$_4$ as a function of temperature and external magnetic field. At fixed temperature, we found that the rate increases with increasing field, reaching a maximum at the phase boundary of the spin liquid state. The results are compared with the calculated NMR rates in different proposed spin liquid states. The implications of the results on determining the fermionic or bosonic nature of the elementary excitations in the spin liquid phase will be discussed.