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 $p>~p_c,$ the critical pressure being $p_c\approx {10}^5$ bar. Thereby, the temperature and pressure dependencies of the dc conductivity, as well as its scale, are basically different at $p\ll p_c$ (in particular at ambient pressure) or at $p>\mathrm{}{p}_c.$ The standard electron-hole pairs determine the thermally activated conductivity at $p<\mathrm{}{p}_c,$ whereas the delocalized negative-$U$ centers give rise to the quasimetallic conductivity at high pressures and not too high temperatures.