Maximum information from the superconducting gap measurements on high- Tc materials

Abstract

Exploiting the possibility, for all the equilibrium properties, of approximating the Eliashberg function by a set of two Einstein peaks, we systematically study the spectral and coupling dependence of the ratio Δ/ T c. We give general results for the spectral and coupling dependence of the gap ratio, which can be easily exploited for extracting the maximun information from any experimental gap measurement. In the regimes potentially relevant for high- T c superconductivity the gap ratio is strongly spectral dependent and our study is fruitful. In cuprates, our study combined with an analysis of the incompatibility of the infrared gap measurements with the other gap measurements, leads to the conclusion that the coupling is quite strong and it is concentrated in a region of frequencies of the order 25–50 meV. The previous incompatibility can be considered as evidence for the relevance of our framework in cuprates. The large gap values reported from tunneling experiments in fullerides, combined with Raman spectroscopy data, indicate that at least 90% of the coupling in these materials is concentrated in a low-energy region ≈ meV, the rest being concentrated in a high-energy region 180–200 meV.