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L. F. Rybal'chenko

Publications and source records attributed to L. F. Rybal'chenko.

22 records · Page 2Linked to original sources
arXiv2015

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

The current-voltage characteristics (IVC) of $S-c-N$ point contacts of superconductors with a small coherence length ${ξ_{0}}$ reveal steps with discrete values of the differential resistance. This peculiarity is associated with a transition of the contact region of the superconductor to a spatially inhomogeneous state under the influence of the current injection of nonequilibrium quasiparticles penetrating the superconductor to a depth ${{l}_{E}}$ . The role of the relaxation of the disbalance between the occupancies of electron- and hole-like branches of the quasiparticle spectrum is manifested in the displacement of the position of the singularities on the IVC towards higher energies upon an increase in the magnetic field and/or temperature. This effect was observed in superconductors with different ratios of the contact diameter $d$ and ${ξ_{0}}$ or ${{l}_{E}}$ in the series $Ta\ (d\ll {ξ_{0}},\ {{l}_{E}})\to NbS{{e}_{2}}$, $N{{b}_{3}}Sn\ (d\gtrsim {ξ_{0}},\ {{l}_{E}})\to YB{{a}_{1.25}}S{{r}_{0.75}}C{{u}_{3}}{{O}_{7-δ}}(d\gg {ξ_{0}},\ {{l}_{E}})$. Apparently, a jumplike displacement of the boundary between regions with suppressed and equilibrium values of the energy gap near the contact is responsible for the oscillations observed on the IVC of point contacts between single crystals of high- temperature superconductors (HTS) and a normal metal. The resistance-periodic step-like structure of the IVC allows us to estimate the penetration depth ${{l}_{E}}$ of the electric field in $NbS{{e}_{2}}$ and $YBaSrCuO$.

cond-mat.supr-con↗
arXiv2015

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 $Δ$ 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 $\partial ω/\partial q\simeq 0$ in $Ta$. 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 $Δ$ 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.

cond-mat.supr-con↗
arXiv2015

Point-contact electron-phonon interaction function in tantalum

Tantalum is studied by the method of point-contact (PC) spectroscopy: the parameter ${{λ}_{pc}}$ and the absolute intensity of the PC EPI function are determined, and the form of the EPI function is determined more accurately. Both homocontacts $Ta-Ta$ and heterocontacts $Ta-Cu$ and $Ta-Au$ were studied. It was found that the contributions of copper and gold to the spectrum of the heterocontact are not noticeable, though the forms of the PC spectra of $Ta$ in homo- and heterocontacts are different. The intensities of the spectra of $Ta-Ta$ homocontacts and $Ta-Cu$ heterocontacts turned out to be close to one another. Simple expressions, enabling numerical calculations of the PC characteristics for geometrically symmetric heterocontacts in the approximation of a spherical Fermi surface, are derived, based on the theory. It is shown that the use of the free-electron approximation in the numerical estimates leads to an error. It is proposed that the effective mass approximation be used for such estimates.

cond-mat.mes-hall↗