The rare earth hydride YH3−δ​ can be tuned through the
metal-insulator transition both by changing δ and 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, Coulomb gap depth, and temperature, we
demonstrate that polycrystalline YH3−δ​ 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