Impurity moments conceal low-energy relaxation of quantum spin liquids
Andrej Pustogow, T. Le, H.-H. Wang, Yongkang Luo, Elena Gati, Harald Schubert, Michael Lang, S. E. Brown
Abstract
We scrutinize the magnetic properties of $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}\mathrm{Hg}{(\mathrm{SCN})}_{2}\mathrm{Cl}$ through its first-order metal-insulator transition at ${T}_{\mathrm{CO}}=30\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ by means of $^{1}\mathrm{H}$ nuclear magnetic resonance (NMR). While in the metallic state we find Fermi-liquid behavior with temperature-independent ${({T}_{1}T)}^{\ensuremath{-}1}$, 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 $\ensuremath{\kappa}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{Cu}}_{2}{(\mathrm{CN})}_{3}$ and $\ensuremath{\kappa}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{Ag}}_{2}{(\mathrm{CN})}_{3}$, ${T}_{1}^{\ensuremath{-}1}$ 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.