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

Abstract

We report the results of electrical conductivity [\ensuremath{\sigma}(T)] measurements in the temperature range 300>T>0.1 K on single crystals of tantalum-substituted sodium tungsten bronze (${\mathrm{Na}}_{\mathit{x}}$${\mathrm{Ta}}_{\mathit{y}}$${\mathrm{W}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{O}}_{3}$) with compositions near the metal-insulator transition (x-y\ensuremath{\approxeq}0.19). We find that over the entire temperature range investigated, strong interaction effects dominate the temperature dependences of \ensuremath{\sigma}(T). For samples that lie on the metallic side, the \ensuremath{\sigma}(T) values in the temperature range 4>T>0.3 K follow a power law in T with an exponent 1/3 and \ensuremath{\sigma}(T=0)\ensuremath{\approxeq}0.01${\mathrm{\ensuremath{\sigma}}}_{\mathrm{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 \ensuremath{\sigma}(T) of the insulating samples below 4 K either show correlated hopping behavior or follow a power law with \ensuremath{\sigma}(T=0)=0. At higher temperatures (T>10 K), the \ensuremath{\sigma}(T) of both types of samples show an almost linear dependence with T, which we suggest as arising from electron-phonon interactions.