TUNNELING SPECTRA OF HIGH-TEMPERATURE SUPERCONDUCTORS

The frequency dependence of the tunneling spectrum α2F(ω) is analyzed in terms of electron pairing induced by exchange of acoustic plasmons in addition to the usual phonon-exchange mechanism. Analytic expressions are obtained for the electron self-energy, the tunneling function α2F, and the electron pairing coupling λ which determines the superconducting properties. The resulting theory is applied to recent tunneling data of Nb3Sn in order to examine the anomalous discrepancies with the phonon density of states found by neutron scattering experiments. The results demonstrate how the high-temperature (Tc∼20°K) superconducting properties of A-15 compounds are enhanced by acoustic-plasmon contributions, and thus they reconcile these high transition temperatures with the relatively small values of the phonon part of λ estimated from several independent experiments. © 1979 The American Physical Society.