Relaxation of nonequilibrium quasiparticles in a superconductor normal metal point contact

TL;DR

This study investigates how nonequilibrium quasiparticles relax in a superconductor-normal metal point contact, revealing new spectral features related to phonon interactions and the critical power for quasiparticle injection.

Contribution

It uncovers distinctive spectral features linked to quasiparticle injection power and phonon interactions, providing new insights into quasiparticle relaxation in superconducting point contacts.

Findings

Abrupt decrease in the superconducting gap at critical injection power.

Sharp, stabilized phonon peaks at specific energies in the spectrum.

Enhanced phonon spectroscopy in dirty contacts despite thermal effects.

Abstract

The point-contact spectra of tantalum in the superconducting state, with $Ta$, $Cu$, and $Au$ counterelectrodes, have been studied. We discovered some new distinctive features, whose position on the $eV$ axis is determined by the critical power required for the injection of nonequilibriumquasiparticles. At this level of power the band gap $\Delta $ decreases abruptly in the vicinity of the contact. A correction to the point-contact spectrum, with the sign opposite to that of the usual correction, arises in the region of phonon energies. The maxima in the $Ta$ spectrum become sharper and their position on the energy axis becomes stabilized near the values $e{{V}_{ph}}=7.0$, 11.3, 15.5, and 18 $meV$, which correspond to low phonon group velocities $\partial \omega /\partial q\simeq 0$ in $Ta$. This is confirmed by the existence of corresponding flattenings on the dispersion relations $\omega (q)$ of lattice vibrations. Slow phonons are created near the $N-S$ interface in quasiparticle recombination and relaxation processes and cause a decrease in $\Delta $ and an increase in the differential resistance in the vicinity of $e{{V}_{ph}}$. An excess quasiparticle charge is accumulated in the region of the contact, producing a correction to the resistance, which decreases as $eV$, $T$, and $H$ increase. These mechanisms are particularly effective in dirty contacts, thus permitting phonon spectroscopy in the superconducting state even when the current flow occurs in a nearly thermal mode.