Transition from Mott Insulator to Superconductor in GaNb_4Se_8 and GaTa_4Se_8 under High Pressure

摘要

Electronic conduction in GaM_4Se_8 (M = Nb, Ta) compounds with the fcc GaMo_4S_8-type structure originates from hopping of localized unpaired electrons (S = 1/2) among widely separated tetrahedral M_4 metal clusters. We show that under pressure these systems transform from Mott insulators to a metallic and superconducting state with T_C = 2.9 and 5.8 K at 13 and 11.5 GPa for GaNb_4Se_8 and GaTa_4Se_8, respectively. The occurrence of superconductivity is shown to be connected with a pressure-induced decrease of the MSe_6 octahedral distortion and simultaneous softening of the phonon associated with M-Se bonds.