Earlier we reported an observation at low temperatures of activation conductivity with small activation energies in strongly doped uncompensated layers of p-GaAs/AlGaAs quantum wells. We attributed it to Anderson delocalization of electronic states i
n the vicinity of the maximum of the narrow impurity band. A possibility of such delocalization at relatively small impurity concentration is related to the small width of the impurity band characterized by weak disorder. In this case the carriers were activated from the bandtail while its presence was related to weak background compensation. Here we study an effect of the extrinsic compensation and of the impurity concentration on this virtual Anderson transition. It was shown that an increase of compensation initially does not affect the Anderson transition, however at strong compensations the transition is suppressed due to increase of disorder. In its turn, an increase of the dopant concentration initially leads to a suppression of the transition due an increase of disorder, the latter resulting from a partial overlap of the Hubbard bands. However at larger concentration the conductivity becomes to be metallic due to Mott transition.
We reconsider the theory of magnetoresistance in hopping semiconductors. First, we have shown that the random potential of the background impurities affects significantly preexponential factor of the tunneling amplitude which becomes to be a short-ra
nge one in contrast to the long-range one for purely Coulomb hopping centers. This factor to some extent suppresses the negative interference magnetoresistance and can lead to its decrease with temperature decrease which is in agreement with earlier experimental observations. We have also extended the theoretical models of positive spin magnetoresistance, in particular, related to a presence of doubly occupied states (corresponding to the upper Hubbard band) to the case of acceptor states in 2D structures. We have shown that this mechanism can dominate over classical wave-shrinkage magnetoresistance at low temperatures. Our results are in semi-quantitative agreement with experimental data.
We suggest that negative magnetoresistance in small magnetic fields at temperatures lower than 3 K reported in the paper under discussion may be related to superconducting transition in In leads (with Tc = 3.4 K).
We propose a mechanism of energy relaxation for carriers confined in a non-polar quantum dot surrounded by an amorphous polar environment. The carrier transitions are due to their interaction with the oscillating electric field induced by the local v
ibrations in the surrounding amorphous medium. We demonstrate that this mechanism controls energy relaxation for electrons in Si nanocrystals embedded in a SiO$_2$ matrix, where conventional mechanisms of electron-phonon interaction are not efficient.
