TUNNELING SPECTROSCOPY NEAR THE METAL-INSULATOR-TRANSITION

Barely insulating disordered solids have the electronic states localiz ed. Analyzing the density of states (DOS) by taking into account Coulo mb correlations results in a particularity both on the metallic and on the insulating side of the disorder-driven metal-insulator transition (MIT): Coulomb interactions deplete the DOS near the Fermi energy E-F . Efros and Shklovskii [1] pointed out that for solids on the insulati ng side of the MIT at T = 0. long-range Coulomb interactions lead to t he formation of a Coulomb gap. The DOS decreases then with a power law when the energy approaches EF. TO investigate the density of states a t E-F of n-type germanium we tried tunneling spectroscopy using mechan ically-controllable break junctions [2] down to 100 mK. The tunneling conductance was found to depend strongly on energy and temperature. We analyze these experiments together with the bulk electrical resistivi ty.