Superconductivity from incoherent Cooper pairs in the strong-coupling regime

TL;DR

This paper presents a theoretical scenario where strong attractive interactions lead to incoherent local Cooper pairs forming a pseudogap state, which transitions to superconductivity through phase coherence at low temperatures.

Contribution

It introduces a mean-field theory describing the transition from incoherent preformed pairs to a globally coherent superconducting state in strong-coupling regimes.

Findings

Identification of a pseudogap caused by phase fluctuations

Description of a phase transition to superconductivity at low temperatures

Applicability to both parabolic and flat-band systems

Abstract

We propose a scenario for superconductivity at strong electron-electron attractive interaction, in the case when the increase of the interaction strength promotes the nucleation of the local Cooper pairs and forms a state with a spatially phase incoherent Cooper pair order parameter. We show that this state can be characterized by a pseudogap which is determined by the electron scattering by phase fluctuations. At low temperatures, however, long-range correlations between the regions with different phases become important and establish global phase coherence and hence superconductivity in the system. We develop a mean-field theory to describe a phase transition between the preformed Cooper pair and superconducting states. This scenario of superconductivity applies not only to conductors with parabolic bands but also to the flat-band systems in which flat and dispersive bands coexist and are responsible for the formation of Cooper pairs as well as their phase synchronization.