Impurity Moments Conceal Low-Energy Relaxation of Quantum Spin Liquids

Abstract

We scrutinize the magnetic properties of $\hat{I}º$-(BEDT-TTF)${}_2$Hg(SCN)${}_2$Cl through its first-order metal-insulator transition at $T_{\mathrm{CO}}=30$ K by means of ${}^1$H nuclear magnetic resonance (NMR). While in the metal we find Fermi-liquid behavior with temperature-independent $(T_{1}T{)}^{-1}$, the relaxation rate exhibits a pronounced enhancement when charge order sets in. The NMR spectra remain unchanged through the transition and no magnetic order stabilizes down to 25 mK. Similar to the isostructural spin-liquid candidates $\hat{I}º$-(BEDT-TTF)${}_2$Cu${}_2$(CN)${}_3$ and $\hat{I}º$-(BEDT-TTF)${}_2$Ag${}_2$(CN)${}_3$, $T_1^{-1}$ acquires a dominant maximum (here around 5 K). Field-dependent experiments identify the low-temperature feature as a dynamic inhomogeneity contribution that is typically dominant over the intrinsic relaxation but gets suppressed with magnetic field.

6 pages, 4 figures; Supplemental Material: 3 pages, 2 figures, 1 table