Polaronic Effect Of A Metal Layer On Variable Range Hopping

We investigate the effect of a metal plate on the variable range hopping (VRH) conductivity of a two-dimensional electron-glass (EG) system. The VRH conductivity is known to have a stretched exponential dependence on temperature, with an exponent p that depends on the shape of the EG's single-particle density of states (DOS). For constant DOS p = 1/3 and for linear DOS p = 1/2, also known as Mott's and Efros-Shklovskii's VRH, respectively. The presence of the plate causes two effects on the EG system, static and dynamic. The well-known static effect accounts for the additional screening of the Coulomb repulsion in the EG and for the partial filling of the Coulomb gap in the DOS. This in turn causes an increase of the conductivity at very low temperatures. Here, we investigate the complementary dynamical effect, which is related to the polaronic phenomena. Our main result is the dynamical suppression of the standard phonon-assisted hopping and, thus, suppression of the conductivity in a much wider range of temperatures as compared to the low-temperature static effect. The relation to experiments is discussed.