The understanding of the linear response dc conductivity, described in length in Chapter 5, is fundamental for the treatment of the glassy properties of electron glasses. This chapter presents a few other transport properties, which are more loosely connected to the heart of this book. There is a great volume of literature on much of the material – experimental, theoretical, and computational. Such material is presented here rather briefly, focusing on properties relevant to the glassy properties and including references to more detailed discussions for the interested reader.

High field conductivity

Nonlinear effects in the conductivity are especially important in electron glasses. Interactions usually increase nonlinearities and can also establish an effective temperature for the electronic system higher than the phonon bath temperature. At the low temperatures where hopping systems are studied, the thermal coupling between the electrons and the phonons is not large enough to dissipate all the electrical power provided to the system even for relatively small values of the applied electric field.

Large electric fields – the “activationless” regime

There are many experimental studies of nonlinear effects on systems showingVRH, but there is no proper theory addressing the problem, except for extremely large values of the electric field. For electric fields larger than FT = kT/eξ, the socalled ‘activationless’ regime, the electric field plays a somewhat similar role to that played by the temperature in the linear regime. Shklovskii (1973) derived the field dependence of the conductivity in this regime.