Flow equation approach to the linear response theory of superconductors

TL;DR

This paper employs the flow equation method to analyze the linear response of superconductors, reproducing BCS results and exploring pairing fluctuations, suggesting preformed pairs cause diamagnetism above Tc.

Contribution

It introduces a non-perturbative flow equation approach to study superconducting response functions, including pairing fluctuations, extending beyond standard BCS theory.

Findings

Reproduces the BCS response kernel using flow equations

Shows pairing fluctuations can explain diamagnetism above Tc

Suggests preformed pairs contribute to superconducting properties

Abstract

We apply the flow equation method for studying the current-current response function of electron systems with the pairing instability. To illustrate the specific scheme in which the flow equation procedure determines the two-particle Green's functions we reproduce the standard response kernel of the BCS superconductor. We next generalize this non-perturbative treatment considering the pairing field fluctuations. Our study indicates that the residual diamagnetic behavior detected above the transition temperature in the cuprate superconductors can originate from the noncondensed preformed pairs.