The Electron Glass in a Switchable Mirror: Relaxation, Aging and Universality*

At room temperature, the rare earth hydride YH$_{3-\delta}$ can be transformed reversibly from shiny metal to transparent insulator by simply changing the surrounding hydrogen pressure. At low temperature, it can be tuned with high resolution through the metal- insulator transition by illumination with ultraviolet light. The transition is dominated by strong electron-electron interactions, with transport in the insulator sensitive to both a Coulomb gap and persistent quantum fluctuations. Via a systematic variation of UV illumination time, photon flux, separation between electrons, and temperature, we demonstrate that polycrystalline YH$_{3-\delta}$ serves as a model system for studying the properties of the interacting electron glass. Prominent among its features are logarithmic relaxation, aging, and universal scaling of the conductivity.