# Recovery of the electron-phonon interaction function in superconducting tantalum ballistic contacts

## TL;DR

This paper investigates how electron-phonon interactions in superconducting tantalum point contacts can be reconstructed from their current-voltage characteristics, revealing the influence of nonequilibrium phonons and superconducting transition effects.

## Contribution

It introduces a detailed method for recovering the electron-phonon interaction spectral function from superconducting point contact spectra, accounting for nonequilibrium phonon effects.

## Key findings

- Superconducting state causes sharpening of phonon peaks in EPI spectra.
- Nonequilibrium phonons significantly influence the superconducting spectral component.
- The high-energy EPI spectrum remains relatively unchanged during the transition.

## Abstract

The experimentally observed nonlinearities of the current-voltage characteristics (CVCs) of tantalum-based point homo- and hetero- contacts in both normal and superconducting states related to electron-phonon interaction (EPI) were analyzed. It was taken into account that additional nonlinearity of CVCs arising upon contact transition to the superconducting state (superconducting spectral component) is formed not only near the constriction in the region roughly equal to the contact diameter (as is the case for the normal state, and as predicted theoretically for the superconducting state), but also in a markedly larger region that is about the size of the coherence length. In this case, a considerable role in the formation of this superconducting component is played by nonequilibrium phonons with low group velocity, which account for the experimentally observed sharpening of the phonon peaks in the EPI spectra (the second derivatives of the CVCs) during the superconducting transition of the contacts, instead of the theoretically expected peak broadening (spreading), and for the increase in the superconducting contribution to the point contact spectrum in the low and medium energy regions. The high-energy part of the EPI spectrum changes much less significantly during the superconducting transition, which is attributable to suppression of the excess contact current by nonequilibrium quasi-particles. A detailed procedure was proposed for the recovery of the EPI spectral function from the point contact spectrum contribution (the second derivative of the CVC) that arises during the superconducting transition of one or both contacting metals.