AC AND NONLINEAR CONDUCTION IN V-4 CLUSTER COMPOUNDS GAV4S8 AND GA0.98ZN0.02V4S8 - EVIDENCE FOR A WIGNER GLASS

The conduction in pure and 2% Zn substituted GaV4S8 has been studied by means of moderate as well as high DC fields and AC fields upto 1 MHz. sigma(omega, Tau) shows a typical temperature scaling, characteristic of hopping conduction in disordered solids. At low temperatures, these compounds show non-linear I-V characteristics and a negative differential resistance in rather moderate fields. The results are analysed using Mott's variable range hopping relations and are found to give consistent parameters like localisation length, density of states (DOS) at the Fermi level and conduction bandwidth except for an unreasonably large value of the phonon assisted transition rates. The result that the conductivity exhibits all the elements of a variable range hopping is interpreted to support the thesis that a Wigner glass state exists among the electrons localised on the V-4 clusters.