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

Abstract Motivated by recent studies of the cluster Mott insulator candidate compound Nb 3 Cl 8 , this study performs 93 Nb and 35 Cl nuclear magnetic resonance (NMR) measurements to investigate the electron correlations. Below the structural transition temperature T s ∼ 97 K, all satellites of the 93 Nb 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 1 T , increases upon cooling to T 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 3 Cl 8 , which are closely associated with Mottness.