Relaxation of nonequilibrium quasiparticles in a superconductor normal metal point contact

Abstract

The point-contact spectra of tantalum in the superconducting state, with T a, 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 ∆ 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 T a spectrum become sharper and their position on the energy axis becomes stabilized near the values eV ph = 7.0, 11.3, 15.5, and 18 meV , which correspond to low phonon group velocities ∂ω/∂q 0 in T a. This is confirmed by the existence of corresponding flattenings on the dispersion relations ω(q) of lattice vibrations. Slow phonons are created near the N −S interface in quasiparticle recombination and relaxation processes and cause a decrease in ∆ and an increase in the differential resistance in the vicinity of eV 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.

Spatially inhomogeneous discrete states of a superconductor upon injection of nonequilibrium quasiparticles from a point contact with a normal metal

“…For the sake of comparison, it is worthwhile to consider the results of point-contact investigations in T a, [7] whose EPI function is well known. As a rule, the S−c−N contacts with noble metals reveal only one nonequilibrium singularity.…”

“…It is shown that the formation of this structure has a nonthermal origin and is associated with an essentially nonequilibrium state of the superconductor in the contact region. In the same way as the emergence of a nonequilibrium singularity on the IVC of tantalum point-contacts upon a decrease in temperature is associated usually with the characteristic energies hω ph of the phonon spectrum [7], the jumps in the excess current and differential resistance on the IVC appearing upon a temperature decrease of superconducting point contacts with a small ξ 0 are also fixed near certain characteristic energies eV ph corresponding to the characteristic energies of the phonon spectrum [8] or other excitations with which the conduction electrons effectively interact [9]. On the other hand, the multiplicity of the jumps in the differential resistance may lead to oscillating dV /dI (V ) dependences which are frequently observed in experiments [10].…”

“…Moreover, there is no equilibrium in a superconductor between quasiparticle excitations and the condensate, which is manifested in the disbalance between the population densities of the electron-and hole-like branches in the spectrum of quasiparticle excitations. It has been shown by Yanson et al [6,7] that the jumplike decrease in the excess current with increasing bias voltage V for S − c − N point contacts T a − T a, T a − Cu, and T a − Au in which the spectral regime of current passage is observed in the entire interval of applied bias voltages, i.e., the inequality d < l ε , (l i l ε )…”

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

“…It turns out that even the complete correspondence of the point contact parameters to the requirements of the theory does not ensure that the experimental curves follow the theoretical model. For example, as it was ascertained for ballistic point contacts based on tantalum [10][11][12], the superconducting transition is accompanied by a radical change in the shape, intensity, and positions of phonon effects. Furthermore, differences from theory predictions are observed for both hetero-and homocontacts.…”

“…This phenomenon is associated with the attained critical density of nonequilibrium quasi-particles in the near-contact region and is considered in detail in Refs. [10][11][12]. Since the contacts investigated here are ballistic, most of electrons lose excess energy within the point contact banks, when scattered on nonequilibrium phonons.…”