Mechanisms of absolute negative photoconductivity in solids

The microscopic mechanisms governing the absolute negative photoconductivity in solids (j_ph = σ_ph, σ_ph < 0) discovered recently in concentrated ruby crystals are described. It is shown that in hopping models of photoconductivity the contribution to the current, connected with the field-induced asymmetry of processes of electron photoexcitation and recombination, dominates. It is predicted that the sign of σ_ph(ω) changes as a transition over an absorption resonance is performed.