TUNNELING STUDIES ON SODIUM TUNGSTEN BRONZES NEAR THE METAL-INSULATOR-TRANSITION

We present measurements of tunnelling conductances (at T = 4.2 K) of p oint contact and planar junctions formed on sodium tungsten bronze sin gle crystals as the carrier concentration is changed through the metal -insulator (MI) transition. We find that the junction conductance is a lpha\V\(l/2) on the metallic side, and that as the Mr transition is ap proached, the voltage dependence becomes deeper and changes to a linea r dependence (alpha\V\) in the critical region. We interpret this obse rved behaviour as being due to evolution of short-range screened Coulo mb interactions (giving rise to a square-root cusp in the density of s tates) in the disordered metallic phase into an Efros-Shklovskii-type Coulomb gap in the insulating phase. The possibility of breakdown of n ormal Fermi-liquid-like behaviour in the critical region is discussed.