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

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.