Microwave-Enhanced hopping-conductivity; a non-Ohmic Effect

Hopping conductivity is enhanced when exposed to microwave fields (Phys. Rev. Lett., 102, 206601, 2009). Data taken on a variety of Anderson-localized systems are presented to illustrate the generality of the phenomenon. Specific features of these results lead us to conjecture that the effect is due to a field-enhanced hopping, which is the high frequency version of the non-Ohmic effect, well known in the dc transport regime. Experimental evidence in support of this scenario is presented and discussed. It is pointed out that existing models for non-Ohmic behavior in the hopping regime may, at best, offer a qualitative explanation to experiments. In particular, they cannot account for the extremely low values of the threshold fields that mark the onset of non-Ohmic behavior.Comment: 6 pages 8 figure

Optical excitation of Electron-Glasses

Electron-glasses can be readily driven far from equilibrium by a variety of means. Several mechanisms to excite the system and their relative merits are reviewed. In this study we focus on the process of exciting electron-glasses by interaction with near infrared radiation. The efficiency of this protocol varies considerably among different electron-glasses, but it only weakly depends on their resistance at liquid helium temperatures. A dramatic enhancement of the excitation efficiency is observed upon doping crystalline indium-oxide with Au. Some enhancement is observed also in samples doped with Pb but this enhancement fades away with time unlike the situation in the Au-doped samples. Several structural and analytical tools are used to characterize the changes in the materials that may be responsible for these effects. Possible routes by which high-frequency electromagnetic fields take the system far from equilibrium are discussed.Comment: 10 pages 12 figure