Relaxation of nonequilibrium quasiparticles in a superconductor normal metal point contact

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 $\mathrm{\Delta }$ 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 $\mathrm{\Delta }$ 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.