Nonequilibrium transport and slow relaxation in hopping conductivity

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Abstract
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 by Ben-Chorin et al. and persistent photoconductivity created by exposure to light. The similarity of the temporal dependence of the conductance 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 electronic system is excited and the time it takes the system to reach thermal equilibrium is much longer than the Maxwell time. We offer a simple theoretical model that ascribes all of these effects to nonequilibrium transport phenomena peculiar to the hopping regime. It is argued that exciting a hopping system out of thermal equilibrium leads to a conductivity that is higher than in equilibrium. The excited state is long lived and similar in nature to that observed in the phenomenon of persistent photoconductivity of various semiconductors measured at low temperatures. The sluggish equilibration process of the electronic system is ascribed to the inhomogeneous nature of charge transport and to the slow energy relaxation which are inherent features of disordered solids.