The London penetration depth of strongly coupled isotropic superconductors : low temperature behaviour

TL;DR

This paper investigates how strong electron-phonon coupling influences the low temperature behavior of the London penetration depth in isotropic superconductors, revealing that strong coupling can mimic power law dependencies.

Contribution

It provides a systematic analysis within strong coupling theory showing that phonon-induced scattering can produce power law-like temperature dependence in penetration depth.

Findings

Strong coupling effects can simulate power law dependence.

Low frequency phonons cause quasi elastic scattering.

Wide phonon spectra are necessary for strong coupling effects.

Abstract

We proceed to a systematic exploration of the low temperature dependence of the London penetration depth of isotropic superconductors within strong coupling theory in the clean limit. For a sizeable range of parameters, we find that strong coupling effects can reasonably simulate a power law dependence, sometimes with an excellent precision. In such cases it would be quite difficult to distinguish experimentally between a pure power law and the strong coupling result. Physically we have been able to ascribe this temperature dependence to low frequency phonons which produce a quasi elastic scattering for electrons. The presence of these low frequency phonons requires rather wide phonon spectra and their effectiveness in scattering implies fairly strong coupling.