Low-temperature electrical conductivity of Ta-compensated sodium bronze near the metal-insulator transition.

Abstract

We report the results of electrical conductivity ({sigma}({ital T})) measurements in the temperature range 300{gt}{ital T}{gt}0.1 K on single crystals of tantalum-substituted sodium tungsten bronze (Na{sub {ital x}}Ta{sub {ital y}}W{sub 1{minus}{ital y}}O{sub 3}) with compositions near the metal-insulator transition ({ital x}{minus}{ital y}{approx}0.19). We find that over the entire temperature range investigated, strong interaction effects dominate the temperature dependences of {sigma}({ital T}). For samples that lie on the metallic side, the {sigma}({ital T}) values in the temperature range 4{gt}{ital T}{gt}0.3 K follow a power law in {ital T} with an exponent 1/3 and {sigma}({ital T}=0){approx}0.01{sigma}{sub Mott}. This arises due to interaction effects in the critical regime, where the coherence length is very large due to the proximity to the metal-insulator transition. These samples below 0.3 K show signatures of a weak superconducting transition. The {sigma}({ital T}) of the insulating samples below 4 K either show correlated hopping behavior or follow a power law with {sigma}({ital T}=0)=0. At higher temperatures ({ital T}{gt}10 K), the {sigma}({ital T}) of both types of samples show an almost linear dependence with {ital T}, which we suggest as arising from electron-phonon interactions.