Antiferromagnetic Spin Fluctuations and Structural Transition in Cluster Mott Insulator Candidate Nb₃Cl₈ Revealed by ⁹³Nb- and ³⁵Cl-NMR

Motivated by recent studies of the cluster Mott insulator candidate compound Nb_3Cl_8, this study performs ^93Nb and ^35Cl nuclear magnetic resonance (NMR) measurements to investigate the electron correlations. Below the structural transition temperature T_\rm s\sim97 K, all satellites of the ^93Nb NMR spectra split into three distinct peaks, which suggests symmetry lowering due to the structural transition and could be attributed to the change in the Nb-Nb bond-lengths of the Nb_3 clusters. The spin-lattice relaxation rate 1/T_1 divided by the temperature T, 1/T_1T, increases upon cooling to T_\rm s for all Cl sites, whereas only the Knight shift K of Cl located at the center of the Nb_3 clusters exhibits a temperature dependence similar to that observed in magnetic susceptibility. These findings collectively demonstrate the existence of strong spin correlations between the Nb atoms in Nb_3Cl_8, which are closely associated with Mottness.