High-frequency zero-phonon conductivity of disordered two-level atom systems

In the present paper, the Kubo equation of conductivity is applied to the calculation of the zero-phonon high-frequency conductivity of disordered two-level atom systems at low temperatures. A general expression is derived, making allowance for the contribution (to conductivity) of diagonal and nondiagonal (with respect to operators) nodes of the electron dipole moment. The important role of these contributions both in the single-level (single-band) and two-level (two-band) models of disordered systems is demonstrated. The frequency dependence of zero-phonon conductivity in electron intraband and interband transitions is studied for a continuous Gaussian density distribution of the atomic electron levels.