DC CONDUCTIVITY OF SEMICONDUCTING GLASSES : THE ROLE OF NEGATIVE-U CENTERS, AND PRESSURE-INDUCED PHENOMENA

The ohmic dc conductivity, as a representative example of the electronic transport coefficients for semiconducting glasses, is theoretically characterized within the framework of the recent theory of negative-$U$ centers. The latter are self-trapped singlet electron (hole) pairs, strongly bound and localized at ambient pressure. Pressure-induced delocalization of the negative-$U$ centers and their single-particle excitations in semiconducting glasses is predicted at high pressures $pg~{p}_{c},$ the critical pressure being ${p}_{c}\ensuremath{\approx}{10}^{5}$ bar. Thereby, the temperature and pressure dependencies of the dc conductivity, as well as its scale, are basically different at $p\ensuremath{\ll}{p}_{c}$ (in particular at ambient pressure) or at $pg{p}_{c}.$ The standard electron-hole pairs determine the thermally activated conductivity at $pl{p}_{c},$ whereas the delocalized negative-$U$ centers give rise to the quasimetallic conductivity at high pressures and not too high temperatures.