The gap maximum of anisotropic superconductors

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.