NONEQUILIBRIUM TRANSPORT AND SLOW RELAXATION IN HOPPING CONDUCTIVITY

We present experimental results that demonstrate the nonergodic nature of charge transport in the insulating regime of indium oxide samples. These results include an anomalous field effect described in detail b y Ben-Chorin et al. and persistent photoconductivity created by exposu re to light. The similarity of the temporal dependence of the conducta nce after excitation due to a burst of light and that due to charging the sample by a nearby gate suggests that in both processes the electr onic system is excited and the time it takes the system to reach therm al equilibrium is much longer than the Maxwell time. We offer a simple theoretical model that ascribes all of these effects to nonequilibriu m transport phenomena peculiar to the hopping regime. It is argued tha t exciting a hopping system out of thermal equilibrium leads to a cond uctivity that is higher than in equilibrium. The excited state is long lived and similar in nature to that observed in the phenomenon of per sistent photoconductivity of various semiconductors measured at low te mperatures. The sluggish equilibration process of the electronic syste m is ascribed to the inhomogeneous nature of charge transport and to t he slow energy relaxation which are inherent features of disordered so lids.