SELF-TRAPPING OF ELECTRON PAIRS IN MATERIALS WITH TUNNELING STATES - ELECTRONS IN PERFECT COVALENT GLASSES

It is shown that in perfect covalent glasses (and some local structures) the self-trapping of negative-U electron (on hole) pairs is realized on selected ″flexible″ atomic configurations. The basic concepts of the theory of the phenomenon are presented. Criteria and some associated effects are analyzed. The selected configuration contains an appropriate atom (atomic group) in ″critical local potential″ with unusually small spring constant in the respective well. The phenomenon (self-trapping of electron or hole pair) can generate both Anderson negative-U centers in ″perfect″ glass (i. e. not associated with defects in glasses) and, possibly, a class of deep-level impurity centers in some crystals.