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N. L. Bobrov

Publications and source records attributed to N. L. Bobrov.

At least 19 recordsLinked to original sources
arXiv2025

Nonequilibrium effects in ballistic point contacts Ta-Cu and 2H-NbSe2-Cu. Two-gap superconductivity in 2H-NbSe2

Ta-Cu and NbSe2-Cu heterocontacts have been studied. For Ta-Cu contacts, a theoretical estimation of the delta-functional barrier at the boundary, arising due to the mismatch of the Fermi parameters of the contacting metals, has been carried out, and good agreement between the calculation and experiment has been obtained. An expression for estimating the diameter of the heterocontact on both sides of the boundary is obtained. The value of the jump-like reduction of the excess current (and superconducting gap) associated with the phase transition of the superconducting region near the point contact to a spatially inhomogeneous state when the critical concentration of nonequilibrium quasiparticles is reached is determined. The dependence of the additive differential resistance on the displacement at the contact arising after the phase transition due to the excess charge of quasiparticles and the associated reverse current (or additive voltage) is obtained. In 2H-NbSe2, a two-zone superconductivity character with an 8-fold difference in the energy gap values takes place. Under the action of current injection of nonequilibrium quasiparticles, there is a sequential phase transition of layers adjacent to the point contact into a spatially inhomogeneous state with a suppressed gap, which is accompanied by a stepwise change in the slope of the I-V curve with a discrete increase in the differential resistance.

cond-mat.supr-con↗
arXiv2021

Reconstruction of the Electron-Phonon Interaction Function for Superconductors Using Inhomogeneous Point-Contacts and Background Correction in Yanson Spectra

The application of inhomogeneous niobium point-contacts in the superconducting stata for reconstructing the electron-phonon interaction function is considered. The method is based on the use of the nonlinearity of current-voltage curve, which is due to the inelastic processes of suppressing excess point contact current when nonequilibrium phonons are scattered from the electrons undergoing Andreev reflection. A new model of background occurrence in point-contact Yanson spectra and some ways to correct this background are proposed.

cond-mat.supr-con↗
arXiv2021

Spectroscopy of electron-phonon interaction of superconducting point contacts: experimental aspects

The recovering procedure of the electron-phonon interaction (EPI) functions from the additional nonlinearities of the current-voltage curve ($I-V$ curve) of point contacts associated with an excess current is considered. The approach proposed takes into account both inelastic scattering, which causes suppression of the excess current in the reabsorption of nonequilibrium phonons by electrons undergoing Andreev reflection (Andreev electrons), and elastic processes associated with the electron-phonon renormalization of the energy spectrum in a superconductor. The results obtained are systematically expounded for both the ballistic contacts, wherein the second derivatives of the $I-V$ curve in the normal state are proportional to the EPI functions, and inhomogeneous contacts (with dirty constrictions and clean banks), whose second derivatives in the normal state are either free of phonon singularities or weakly pronounced.

cond-mat.supr-con↗
arXiv2019

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

The experimentally observed nonlinearities of the current-voltage characteristics (CVCs) of tantalum-based point homo- and hetero- contacts in both normal and superconducting states related to electron-phonon interaction (EPI) were analyzed. It was taken into account that additional nonlinearity of CVCs arising upon contact transition to the superconducting state (superconducting spectral component) is formed not only near the constriction in the region roughly equal to the contact diameter (as is the case for the normal state, and as predicted theoretically for the superconducting state), but also in a markedly larger region that is about the size of the coherence length. In this case, a considerable role in the formation of this superconducting component is played by nonequilibrium phonons with low group velocity, which account for the experimentally observed sharpening of the phonon peaks in the EPI spectra (the second derivatives of the CVCs) during the superconducting transition of the contacts, instead of the theoretically expected peak broadening (spreading), and for the increase in the superconducting contribution to the point contact spectrum in the low and medium energy regions. The high-energy part of the EPI spectrum changes much less significantly during the superconducting transition, which is attributable to suppression of the excess contact current by nonequilibrium quasi-particles. A detailed procedure was proposed for the recovery of the EPI spectral function from the point contact spectrum contribution (the second derivative of the CVC) that arises during the superconducting transition of one or both contacting metals.

cond-mat.supr-con↗
arXiv2017

Influence of nonequilibrium phonons on the amplitude of magnetoquantum oscillations in the point-contact resistance

For metallic point contacts with Be and Al the magnetoquantum oscillations in the contact resistance have been investigated as a function of the applied voltage over the contact. For one set of point contacts the oscillation amplitude is found to vary nonmonotonously with the applied voltage with similarities to the point-contact spectrum of the electron-phonon interaction. The other part of the investigated point contacts shows a decrease of the oscillation amplitude with increasing bias voltage. For the understanding of the voltage dependence of the amplitude of the point-contact magnetoresistance oscillations the influence of nonequilibrium phonons generated by the ballistically injected electrons will be discussed.

cond-mat.mes-hall↗
arXiv2017

Superconducting energy gap in $\rm Ba_{1-x}K_xBiO_3$: Temperature dependence

The superconducting energy gap of $\rm Ba_{1-x}K_xBiO_3$ has been measured by tunneling. Despite the fact that the sample was macroscopically single phase with very sharp superconducting transition $T_c$ at 32~$K$, some of the measured tunnel junctions made by point contacts between silver tip and single crystal of $\rm Ba_{1-x}K_xBiO_3$ had lower transition at 20~$K$. Local variation of the potassium concentration as well as oxygen deficiency in $\rm Ba_{1-x}K_xBiO_3$ at the place where the point contact is made can account for the change of $T_c$. The conductance curves of the tunnel junctions reveal the BCS behavior with a small broadening of the superconducting-gap structure. A value of the energy gap scales with $T_c$. The reduced gap amounts to $2Δ/kT_c = 4÷4.3$ indicating a medium coupling strength. Temperature dependence of the energy gap follows the BCS prediction.

cond-mat.supr-con↗
arXiv2017

Tunneling measurements of the electron-phonon interaction in $\rm Ba_{1-x}K_xBiO_3$

The conductance curves of point-contact tunnel junctions between Ag and $\rm Ba_{1-x}K_xBiO_3$ ($x\simeq 0.4$) reveal a BCS behavior with low leakage current at zero voltage and some broadening of the superconducting-gap structure. In the energy range above the superconducting energy gap, the structure in the voltage dependence of the second derivative $d^2V/dI^2$ of the voltage with respect to the current of the tunnel junction has been investigated in detail in magnetic fields up to $10\ T$. While part of this structure is rapidly changing in a magnetic field, three reproducible peaks in $d^2V/dI^2(V)$ remain stable up to the transition temperature from the superconducting to the normal state with only additional broadening in the applied magnetic field. An analysis of this structure in terms of strong-coupling effects yields the spectral function $α^2F$ for the electron-phonon interaction. The obtained spectral weight in the energy region 20-70~$meV$ points to the importance of the oxygen optical modes in the electron-phonon coupling for the superconductivity of $\rm Ba_{1-x}K_xBiO_3$.

cond-mat.supr-con↗
arXiv2017

Critical current density in the superconducting ceramic $\rm La_{1.8}Sr_{0.2}CuO_4$ in S-c-N point contacts

Measurement of the current-voltage characteristics of point contacts makes it possible to study the properties of individual crystallites in a superconducting ceramic. The critical current density in the superconducting regions of the ceramic $\rm La_{1.8}Sr_{0.2}CuO_4$, with a size of the order of several tens of angstroms, is found to attain values of $10^8\ A/cm^2$, which are of the same order of magnitude as the pair-breaking current density, as evaluated from the formulas of the standard theory of superconductivity.

cond-mat.supr-con↗
arXiv2017

Structure and physical properties of superconducting compounds Y(La)-Ba(Sr)-Cu-O

The structure and physical properties of superconducting compounds Y(La)-Ba(Sr)-Cu-O are studied, the compounds being prepared by the method of cryogenic dispersion of a charge consisting of premix oxides and carbonates. Electrical conductivity and critical current density of the superconductors are measured over a wide temperature range of 10~$mK$ to 300~$K$. Degradation of the superconductor critical parameters in time and structural characteristics, magnetic susceptibility, heat capacity and acoustic properties are studied, and current-voltage characteristics are determined.

cond-mat.supr-con↗
arXiv2017

Reversible recovery of excess current in $Y(Ho)-Ba-Cu-O$-normal metal point contacts under high voltages

A polarity-dependent reversible change in the current-voltage characteristics between states corresponding to different values of the excess current ${{I}_{exc}}$ is observed for bias voltages of several hundred millivolts in $Ag-ReB{{a}_{2}}C{{u}_{3}}{{O}_{7-x}}$ $\left( Re\text{ }=\text{ }Y,\text{ }Ho \right)$ point contacts in the current-carrying state. The observed effect has a high intensity (right up to a complete suppression or recovery of ${{I}_{exc}}$) and is not accompanied by a noticeable variation of the energy gap or the critical temperature in the point contact region. It is also established that the states with intermediate values of $I_{exc}$ are stable for zero bias voltages across the point contacts, at least up to $110 \ K$. It is assumed that the variation of ${{I}_{exc}}$ and the corresponding modification of IVC are due to the oxygen migration processes induced by the electric field and current, resulting in a local variation of the superconducting parameters in the point-contact region.

cond-mat.supr-con↗
arXiv2017

Electron-quasiparticle interaction in $\rm DyNi_2B_2C$ measured by point-contact spectroscopy

The electron-quasiparticle interaction (EQI) spectral function has been measured for $\rm DyNi_2B_2C$ in the normal state at low temperatures by means of point-contact spectroscopy (PCS). A low-frequency peak is found around $eV\sim 5\ meV$. It becomes measurable at $T_m^*\simeq 15\ K$ and grows in intensity with constant width as the temperature is lowered. We argue that this peak arises from the strong interaction of conduction electron with coupled crystal-electric-field-phonon excitations whose branches cross at low energy. The comparison with PC spectra for $\rm HoNi_2B_2C$ suggests that a similar peak also exists for this compound. The magnitude of the point-contact EQI parameter $λ_{PC}$ for $\rm DyNi_2B_2C$ is estimated. $©$2000 Published by Elsevier Science B.V. All rights reserved.

cond-mat.supr-con↗
arXiv2017

Point-contact spectroscopy of superconducting energy gap in $\rm DyNi_2B_2C$

The superconducting energy gap in $\rm DyNi_2B_2C$ has been investigated using a point-contact technique based on the Andreev reflection from a normal (N)-superconductor (S) boundary, where N is Ag. The observed differential resistance $dV/dI$ is well described by the Blonder-Tinkham-Klapwijk (BTK) theory based on the BSC density of states with zero broadening parameter. Typically, the intensity of the gap structure amounts to several percentage of the normal state resistance, which is an order of magnitude less than predicted by the theory. For $\rm DyNi_2B_2C$ with $T_c<T_N$ (the Neel temperature), we found gap values satisfying the ratio of $2Δ_0/k_BT_c=3.63\pm 0.05$ similar to other superconducting nickel-borocarbides, both nonmagnetic and magnetic with $T_c\geq T_N$. The superconducting gap nonlinearity is superimposed on the antiferromagnetic structure in $dV/dI(V)$ which is suppressed at the magnetic field of the order of 3T applied nominally in the $ab$-plane and temperature $\geq 11~K$. The observed superconducting properties depend on the exact composition and structure at the surface of the crystal.

cond-mat.supr-con↗
arXiv2017

Size-effect of Kondo scattering in point contacts (revisited)

The size-effect of Kondo-scattering in nanometer-sized metallic point contacts is measured with the simplified, mechanically-controlled break-junction technique for CuMn alloy of different Mn concentrations: 0.017; 0.035; and 0.18 ($\pm$0.017) at.\%. The results are compared with our previous publication on nominally 0.1 at.\% CuMn alloy \cite{1,2}. The increase of width of the Kondo resonance and enhanced ratio of Kondo-peak intensity to electron-phonon scattering intensity is observed for contacts with sizes smaller than 10 $nm$. From the comparison of electron-phonon scattering intensity for the pressure-type contacts, which correspond to the clean orifice model, we conclude that the size effect is observed in \emph{clean} contacts with the shape of a \emph{channel} (nanowire).

cond-mat.mes-hall↗
arXiv2017

Point-contact spectroscopy of tantalum, molybdenum, and tungsten

Electron-phonon interaction in superconducting transition metals (Ta, Mo, W) is investigated by the method of point-contact spectroscopy. The spectra obtained are compared with model densities of the phonon states $F(ω)$ computed from experiments on neutron scattering, and with the tunnel electron-phonon interaction function $g(ω)$ for Ta. Close agreement between the appropriate spectra is obtained for Ta and Mo, while a significant discrepancy is observed for W, which is the appearance of a blurred maximum instead of the two expected separated peaks on the point-contact spectra. The absolute intensities of a number of point-contact spectra $G(eV)$ and electron-phonon coupling constants $λ$ are calculated. For all three metals the values obtained turned out to be considerably below those expected. The possible reasons for this disagreement are discussed.

cond-mat.mes-hall↗
arXiv2017

"Internal" coherent effects in HTS-normal metal point contacts

Electrical characteristics are measured in submicron YBaCuO-normal metal point contacts with a direct conductivity. In a microwave field, such contacts reveal a nonstationary Josephson effect which can occur in the current concentration region not only in the initial weak bonds in YBaCuO but also the weak bonds of a phase-slip center type, formed as a result of a significant current injection (as well as a microwave field). The existence of internal weak bonds in the HTS-electrode contact area produces a narrow minimum in the differential resistance near zero biases. Moreover, the effect of magnetic flux quantization is observed for separate granules in magnetic fields stronger than the first critical field.

cond-mat.supr-con↗
arXiv2017

Point-contact spectroscopy of superconducting YBaCuO single crystals of tetragonal modification

The point-contact spectra of tetragonal $\rm YBa_2Cu_3O_{7-δ}$ with $T_c=50-60~K$ are studied at various temperatures. The single-electron energy ($2Δ_0/kT_c\simeq 17$) and its temperature dependence are determined. The characteristic energies of quasiparticle excitations interacting with conduction electrons in this high-$T_c$. superconductor are found to correlate with the characteristic phonon energies. The possibility of a pair gap, which increases with temperature, is revealed near the critical temperature.

cond-mat.supr-con↗
arXiv2017

Point-contact spectroscopy of the high-temperature superconductor BiSrCaCuO

The maximum value of the energy gap $Δ\simeq 8\ meV$ and the ratio $2δ/kT_c\simeq~2.5$ are determined for a high-temperature superconductor $\rm Bi_2Sr_2CaCu_2O_{8+y}$ by using point contacts. It was found that the high-temperature superconductor is transformed under the effect of current injection of quasiparticles to a new modified state with a reduced gap, which is stable in a wide range of injection intensity.

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