Tunnelling studies on sodium tungsten bronzes near the metal-insulator transition

We present measurements of tunnelling conductances (at T=4.2 K) of point contact and planar junctions formed on sodium tungsten bronze single crystals as the carrier concentration is changed through the metal-insulator (MI) transition. We find that the junction conductance is varies as mod V mod ^1/2 on the metallic side, and that as the MI transition is approached, the voltage dependence becomes deeper and changes to a linear dependence ( varies as mod V mod ) in the critical region. We interpret this observed behaviour as being due to evolution of short-range screened Coulomb interactions (giving rise to a square-root cusp in the density of states) in the disordered metallic phase into an Efros-Shklovskii-type Coulomb gap in the insulating phase. The possibility of breakdown of normal Fermi-liquid-like behaviour in the critical region is discussed.