Mechanism for superconductivity at disordered interfaces

The origin of the enhancement of the superconducting transition temperature caused by the presence of disordered interfaces is studied. An enhancement mechanism involving the tunneling of the conduction electrons from the metal into the local bond state coupled to the two-level state systems in the interface region, exhibiting negative U/sub eff/, is considered. The atomic arrangement processes contributing to negative U/sub eff/ at metal-semiconductor and metal-metal disordered interfaces are studied. We use the Fermi liquid expansion technique to treat negative U/sub eff/, and confine ourselves to the V/sub k/ contribution which involves the inelastic scattering of the electrons via U/sub eff/. The calculation of this contribution to T/sub c/ is based on the method of Sakurai, but modified by taking into account the pseudo-magnon operator in the tunneling matrix V/sub k/. It is found that in the limit of Gamma much greater than vertical bar U/sub eff/ vertical bar, our results explain the increase of T/sub c/ observed in eutectic alloys, whereas in the small limit of Gamma, the contribution of the inelastic scattering to T/sub c/ is negligible. We also see in this model that the reduction of the tunneling matrix elements by orthogonality blocking can be avoided