Antiferromagnetic spin fluctuations and structural transition in cluster Mott insulator candidate Nb₃Cl₈ revealed by ⁹³Nb- and ³⁵Cl-NMR

Motivated by recent studies on the cluster mott insulator candidate compound Nb₃Cl₈, we perform ⁹³Nb and ³⁵Cl nuclear magnetic resonance (NMR) measurements to investigate its electron correlations. Below the structural transition temperature T_s ~ 97 K, all the satellites of the ⁹³Nb NMR spectra split into three distinct peaks, which suggest a symmetry lowering due to the structural transiton and could be attribute to the change of the Nb-Nb bond-lengths of Nb₃ clusters. The spin-lattice relaxation rate 1/T₁ devided by temperature T, 1/T₁T, increases upon cooling down to T_s for all Cl sites, whereas only the Knight shift K of Cl located at the center of Nb₃ clusters exhibits a similar temperature dependence as observed in magnetic susceptibility. These findings collectively demonstrate the existence of strong spin correlations between Nb atoms in Nb₃Cl₈, which should be closely associated with its Mottness.