Quantum Coulomb glasses and electron assisted hopping

In Anderson insulators where the single particle localization length is much larger than the mean distance between electrons, Coulomb interactions drive the electrons into a strongly correlated quantum glass phase. In the limit of large localization length, the resulting quantum Coulomb glass can be studied analytically. The theory predicts many almost degenerate quantum states with a spectrum of gapless collective excitations in each of them. The latter can acts as a bath with which individual electrons can exchange energy. This is a crucial ingredient for activated transport, the collective modes of the quantum glass providing a natural mechanism for electron-assisted hopping conductance. In particular, for 2D systems we predict a weakly temperature dependent pre-exponential factor of order $e^2/h$ for variable range hopping, as has been reported in many recent experiments.