Thermally driven Fermi glass states in warm dense matter: Effects on terahertz and direct-current conductivities

There is a growing interest in the electrical conductivity of warm dense matter from terahertz-frequency alternating current to direct current. Herein, using first-principles molecular dynamics simulations, we show that ionic thermal motion in warm dense matter drives thermal fluctuations in the electronic valence band that produce localized states in Lifshitz tails on the top and bottom of the bands. We predict Fermi glass states when these localized states extend and fill the gap between valence and conduction bands. This significantly affects the ultralow-frequency and direct current conductivity because of the very small but nonzero energy gaps between these localized states. An order parameter is proposed to describe the degree of glassiness of an electron energy band using the local density-of-state distribution. To take into account thermal hopping, we introduce electron energy-level broadening as a thermal correction term in the Kubo-Greenwood equation. The calculated terahertz conductivities of warm dense helium and argon show the differences between the Fermi glass and normal metal states.