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 Nb₃Cl₈, this study performs ⁹³Nb and ³⁵Cl nuclear magnetic resonance (NMR) measurements to investigate the electron correlations. Below the structural transition temperature T_s ∼ 97 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 Nb₃ clusters. The spin-lattice relaxation rate 1/T₁ divided by the temperature T, 1/T₁T, increases upon cooling to T_s for all Cl sites, whereas only the Knight shift K of Cl located at the center of the Nb₃ 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 Nb₃Cl₈, which are closely associated with Mottness.