Impurity Moments Conceal Low-Energy Relaxation of Quantum Spin Liquids

Abstract

We scrutinize the magnetic properties of $\kappa$-(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 $\kappa$-(BEDT-TTF)${}_2$Cu${}_2$(CN)${}_3$ and $\kappa$-(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.

arXiv comment6 pages, 4 figures; Supplemental Material: 3 pages, 2 figures, 1 table

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• Observation of a gapped phase in the one-dimensional $S=\frac{1}{2}$ Heisenberg antiferromagnetic chain Cu(Ampy)ClBr
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• ${}^{13}$C NMR observation of a nonmagnetic charge-ordered state in the organic conductor $\kappa$-(ET)${}_2$Hg(SCN)${}_2$Cl