Field-Dependent Instability of the Candidate Quantum Spin Liquid in EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ as Revealed by NMR

In recent years, the two-dimensional spin-$1/2$ triangular lattice of the organic salt EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ has emerged as a candidate for the realization of a quantum spin liquid. Furthermore, thermal conductivity and nuclear magnetic resonance (NMR) experiments unveiled the presence of a low-temperature instability in the spin liquid state, the opening of a spin gap. We performed a detailed $^{13}$C NMR study on this material at low temperatures \mbox{($30$mK$\leq T\leq 1.5$ K)} and for a wide range of external magnetic field values \mbox{($B_0=0.6-9$T)}. In finite fields, a clear break in the temperature derivative of the spin-lattice relaxation is observed at a temperature $T_m(B_0)$, with $T_m$ following the empirical form $T_m(B_0)\sim \left| B_0-B_c\right| ^{1/2}$. Moreover, a uniform broadening of the NMR line for finite fields suggests the presence of a small field-induced staggered magnetization. We discuss these results in the context of possible instabilities, and existing thermodynamic data.