The Frequency Dependent Conductivity of Electron Glasses

TL;DR

This paper investigates the frequency-dependent conductivity of electron glasses, including doped silicon and niobium-silicon alloys, revealing different regimes and proposing a phase diagram for their electrodynamic response.

Contribution

It provides the first comprehensive phase diagram describing the quantum, Fermi, and Coulomb glass regimes in disordered insulating systems.

Findings

Identification of quantum critical, Fermi glass, and Coulomb glass regimes.

Development of a phase diagram for electron glass electrodynamics.

Experimental characterization using microwave and millimeter wave spectroscopy.

Abstract

Results of DC and frequency dependent conductivity in the quantum limit, i.e. hw > kT, for a broad range of dopant concentrations in nominally uncompensated, crystalline phosphorous doped silicon and amorphous niobium-silicon alloys are reported. These materials fall under the general category of disordered insulating systems, which are referred to as electron glasses. Using microwave resonant cavities and quasi-optical millimeter wave spectroscopy we are able to study the frequency dependent response on the insulating side of the metal-insulator transition. We identify a quantum critical regime, a Fermi glass regime and a Coulomb glass regime. Our phenomenological results lead to a phase diagram description, or taxonomy, of the electrodynamic response of electron glass systems.