Conductance relaxation in the electron glass: Microwave versus infrared response

We study the time-dependent conductance of electron glasses excited by electromagnetic radiation at microwave and infrared frequencies. In either case, the conductance G is enhanced during exposure, but its time dependence after the radiation is turned off is qualitatively different depending on the frequency. For comparison, results of excitation produced by a gate voltage and temperature changes are also shown. The glassy nature of the system allows us to demonstrate that the microwave-enhanced conductance is not due to heating. These findings are discussed in terms of an energy Ec that characterizes the equilibrium charge distribution of the electron glass.