The gap maximum of anisotropic superconductors

TL;DR

This paper demonstrates that weak coupling theory cannot explain the high maximum gap values observed in anisotropic high-temperature superconductors, supporting the need for strong coupling mechanisms like spin fluctuations.

Contribution

It shows that weak coupling models are incompatible with experimental gap values in anisotropic superconductors, advocating for strong coupling theories such as spin fluctuation mechanisms.

Findings

Weak coupling theory cannot account for high gap ratios in certain superconductors.

Experimental gap ratios suggest the presence of strong coupling effects.

Supports spin fluctuation as a plausible strong coupling mechanism.

Abstract

For a completely general anisotropic order parameter (including changes of sign), we show that weak coupling theory is incompatible with high values of the maximum \Delta_{M} of the zero temperature gap as compared to the critical temperature T_{c}, such as those found experimentally in Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta} where 2 \Delta_{M} / T_{c} \approx 7.5 . This gives evidence for strong coupling effects. In particular this comes as a major support for a spin fluctuation mechanism with strong coupling, if one assumes that only a repulsive pairing interaction is at work in high T_{c} superconductors.