Spectroscopy of electron-phonon interaction of superconducting point contacts: experimental aspects

TL;DR

This paper discusses experimental methods for extracting electron-phonon interaction functions from the nonlinearities in the current-voltage characteristics of superconducting point contacts, considering both inelastic and elastic scattering effects.

Contribution

It introduces an approach that accounts for both inelastic and elastic processes affecting the electron-phonon interaction signals in superconducting point contacts.

Findings

The method works for ballistic contacts, where second derivatives of I-V are proportional to EPI functions.

In inhomogeneous contacts, the second derivatives show weak or no phonon singularities.

The approach systematically explains the influence of inelastic and elastic scattering on EPI measurements.

Abstract

The recovering procedure of the electron-phonon interaction (EPI) functions from the additional nonlinearities of the current-voltage curve ($I-V$ curve) of point contacts associated with an excess current is considered. The approach proposed takes into account both inelastic scattering, which causes suppression of the excess current in the reabsorption of nonequilibrium phonons by electrons undergoing Andreev reflection (Andreev electrons), and elastic processes associated with the electron-phonon renormalization of the energy spectrum in a superconductor. The results obtained are systematically expounded for both the ballistic contacts, wherein the second derivatives of the $I-V$ curve in the normal state are proportional to the EPI functions, and inhomogeneous contacts (with dirty constrictions and clean banks), whose second derivatives in the normal state are either free of phonon singularities or weakly pronounced.