Inelastic electron-phonon scattering and excess current in superconducting point contacts with a short coherence length

TL;DR

This paper investigates inelastic electron-phonon scattering and excess current in superconducting point contacts with short coherence lengths, analyzing how magnetic fields and contact size affect the spectra and EPI function recovery.

Contribution

It provides new insights into the effects of magnetic fields and contact dimensions on EPI spectra and excess current in superconducting point contacts with short coherence lengths.

Findings

Peak positions in dV/dI match EPI spectra in zero and near-critical fields.

In low fields, peaks shift to lower energies; in intermediate fields, they split.

Negative phonon contributions significantly suppress high-frequency spectral peaks.

Abstract

Nonlinear electrical effects in superconducting S-c-S contacts, including the spectroscopy of electron-phonon interactions (EPI) in these systems, and the recovery of the EPI function from experimental data are discussed. The effect of a magnetic field on the current-voltage characteristics(I-V curves) and their derivatives for ErNi_2B_2C point contacts (PC) with $d\ge \xi $ (where $d$ is the diameter of the PC and $\xi $ is the coherence length) is studied. It is found that in zero magnetic fields and in near-critical fields, when the size of the superconducting gap can be neglected, the position of the peaks in $dV/dI$ coincides with the peaks in the Yanson EPI spectra. In low fields the peaks are shifted toward lower energies and in intermediate fields, the peaks split. For PC with diameters greater than or on the order of the coherence length, the relative size of the negative phonon contribution to the excess current is considerably greater than in ballistic contacts. This leads to substantial suppression of the high-frequency peaks in the spectra for the superconducting state. In order to recover the EPI function from these spectra it is necessary to correct their intensities at high energies. For "dirty" NbSe_2 and Nb point-contacts with $d\ge \xi $, which have no phonon features in the second derivative of the I-V curve in the normal state, the EPI can be reconstructed from the superconducting state.