Mechanism of Hopping Transport in Disordered Mott Insulators

By using a combination of detailed experimental studies and simple theoretical arguments, we identify a novel mechanism characterizing the hopping transport in the Mott insulating phase of ${\mathrm{C}\mathrm{a}}_{2-x}{\mathrm{S}\mathrm{r}}_x{\mathrm{R}\mathrm{u}\mathrm{O}}_4$ near the metal-insulator transition. The hopping exponent $\alpha$ shows a systematic evolution from a value of $\alpha =1/2$ deeper in the insulator to the conventional Mott value $\alpha =1/3$ closer to the transition. This behavior, which we argue to be a universal feature of disordered Mott systems close to the metal-insulator transition, is shown to reflect the gradual emergence of disorder-induced localized electronic states populating the Mott-Hubbard gap.