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

We report the results of electrical conductivity [σ(T)] measurements in the temperature range 300>T>0.1 K on single crystals of tantalum-substituted sodium tungsten bronze (${\mathrm{Na}}_{\mathit{x}}$ ${\mathrm{Ta}}_{\mathit{y}}$ ${\mathrm{W}}_{1\mathrm{-}\mathit{y}}$ ${\mathrm{O}}_3$) with compositions near the metal-insulator transition (x-y≊0.19). We find that over the entire temperature range investigated, strong interaction effects dominate the temperature dependences of σ(T). For samples that lie on the metallic side, the σ(T) values in the temperature range 4>T>0.3 K follow a power law in T with an exponent 1/3 and σ(T=0)≊0.01${\mathrm{\sigma }}_{\mathrm{Mott}}$. 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 σ(T) of the insulating samples below 4 K either show correlated hopping behavior or follow a power law with σ(T=0)=0. At higher temperatures (T>10 K), the σ(T) of both types of samples show an almost linear dependence with T, which we suggest as arising from electron-phonon interactions.