Why have no manifestations of the excitonic mechanism been detected in Ginzburg sandwiches?

Abstract

Using spatially inhomogeneous Éliashberg equations in the local-interaction limit, an exact solution of the problem of the superconducting transition temperature in a Ginzburg sandwich (a superconducting film coated with a dielectric layer containing Bose-type excitations, i.e., excitons) in the first order in a/L (where a is the interatomic distance and L is the film thickness) has been obtained. The result has been found to be independent of the exciton frequency. The excitonic mechanism appears only in second order in a/L since both components of the Cooper pair should enter a layer of thickness ∼a in order to interact through the exchange of excitons. Numerical estimates indicate that manifestations of the excitonic mechanism are practically undetectable in systems with L≫a. Calculations for the model with a narrow-gap and a wide-gap dielectric have been performed and compared to experimental data.