Electron Glass in a three-dimensional system

We report on non-equilibrium transport features observed in experiments using three-dimensional amorphous indium-oxide films. It is demonstrated that all the features that characterize intrinsic electron-glasses which heretofore were seen in two-dimensional samples are also observed in field-effect measurements of systems that exhibit three-dimensional variable-range-hopping. In particular, a memory-dip is observed in samples configured with gate. The memory-dip width and magnitude support models that associate the phenomenon with the Coulomb-gap. The memory-dip and the glassy effects disappear once the quenched disorder in the system is reduced and the system becomes diffusive. This happens when the Ioffe-Regel dimensional parameter k_{F}l exceeds 0.3 which is the critical value for the metal-to-insulator transition in all versions of the amorphous indium-oxides [Phys. Rev. B 86, 165101 (2012)]. This confirms that being in the Anderson localized phase is a pre-requisite for observing the memory-dip and the associated glassy effects. The results of the gating experiments suggest that the out-of equilibrium effect caused by inserted charge extend over spatial scales considerably larger than the screening length.