$^{133}$Cs NMR investigation of 2D frustrated Heisenberg   antiferromagnet, Cs$_2$CuCl$_4$
  We report $^{133}$Cs nuclear magnetic resonance (NMR) measurements on the 2D frustrated Heisenberg antiferromagnet Cs$_2$CuCl$_4$ down to 2 K and up to 15 T. We show that $^{133}$Cs NMR is a good probe of the magnetic degrees of freedom in this material. Cu spin degrees of freedom are sensed through a strong anisotropic hyperfine coupling. The spin excitation gap opens above the critical saturation field. The gap value was determined from the activation energy of the nuclear spin-lattice relaxation rate in a magnetic field applied parallel to the Cu chains (b axis). The values of the g-factor and the saturation field are consistent with the neutron-scattering and magnetization results. The measurements of the spin-spin relaxation time are exploited to show that no structural changes occur down to the lowest temperatures investigated. 
M. -A. Vachon, W. Kundhikanjana, A. Straub, V. F. Mitrovi\'c, A. P.   Reyes, P. Kuhns, R. Coldea and Z. Tylczynski