Transition from Mott insulator to superconductor in GaNb$_{4}$Se$_{8}$ and GaTa$_{4}$Se$_{8}$ under high pressure

TL;DR

This study demonstrates that applying high pressure to GaNb4Se8 and GaTa4Se8 transforms them from Mott insulators into superconductors, with critical temperatures up to 5.8K, linked to structural and phonon softening effects.

Contribution

It reveals the pressure-induced transition from Mott insulators to superconductors in GaM4Se8 compounds and elucidates the structural and phononic mechanisms involved.

Findings

Superconductivity appears at 13GPa for GaNb4Se8 and 11.5GPa for GaTa4Se8.

Critical temperatures are 2.9K and 5.8K respectively.

Superconductivity correlates with reduced octahedral distortion and phonon softening.

Abstract

Electronic conduction in GaM$_{4}$Se$_{8}$ (M=Nb;Ta) compounds with the fcc GaMo$_{4}$S$_{8}$-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 5.8K at 13 and 11.5GPa for GaNb$_{4}$Se$_{8}$ and GaTa$_{4}$Se$_{8}$, 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 MSe-bonds.