Mean-field theory of first-order quantum superconductor-insulator transition

Abstract

Recent experimental studies on strongly disordered indium oxide films have revealed an unusual first-order quantum phase transition between the superconducting and insulating states (SIT). This transition is characterized by a discontinuous jump from non-zero to zero values of superfluid stiffness at the critical point, contradicting the conventional “scaling scenario” typically associated with SIT. In this paper, we present a theoretical framework for understanding this first-order transition. Our approach is based on the concept of competition between two fundamentally distinct ground states that arise from electron pairs initially localized by strong disorder: the superconducting state and the Coulomb glass insulator. These ground states are distinguished by two crucially different order parameters, suggesting a natural expectation of a discontinuous transition between them at T=0T=0. This transition occurs when the magnitudes of the superconducting gap \DeltaΔ and the Coulomb gap E_CEC become comparable. Additionally, we extend our analysis to low non-zero temperatures and provide a mean-field “phase diagram” in the plane of (T/\Delta,E_C/\Delta)(T/Δ,EC/Δ). Our results reveal the existence of a natural upper bound for the kinetic inductance of strongly disordered superconductors.