Impurity Moments Conceal Low-Energy Relaxation Of Quantum Spin Liquids

We scrutinize the magnetic properties of kappa-(BEDT-TTF)(2)Hg(SCN)(2)Cl through its first-order metal-insulator transition at T-CO = 30K by means of 1H nuclear magnetic resonance (NMR). While in the metallic state we find Fermi-liquid behavior with temperature-independent (T1T)(-1), the relaxation rate exhibits a pronounced enhancement when charge order sets in. The NMR spectra remain unchanged through the transition and we find no evidence for magnetic order down to 25 mK. Similar to the isostructural spin-liquid candidates kappa-(BEDT-TTF)(2)Cu-2(CN)(3) and kappa-(BEDT-TTF)(2)Ag-2(CN)(3), T-1(-1) 1 acquires a dominant maximum (here around 5 K). An examination of the field dependence identifies the low-temperature feature as a dynamic inhomogeneity contribution that is typically dominant over the intrinsic relaxation but is suppressed with increasing magnetic field.