Searcharxiv⌕ Search

arXiv subjects

N. L. Bobrov

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

At least 37 records · Page 2Linked to original sources
arXiv2017

Point-contact studies of high-temperature superconductor $\rm YBa_2Cu_3O_{7-δ}$

Point-contacts formed by the high-temperature superconductor $\rm YBa_2Cu_3O_{7-δ}$ prepared by cryochemical technique, and a noble metal Ag or Cu) are investigated. The maximum value of the energy gap $Δ\simeq 40~meV$ and the ratio $2Δ/kT_c\simeq 12$ are obtained. It is found that along certain crystallographic axes of the superconductor under investigation, the electrical resistivity $\lesssim 10^{-5} Ω\cdot cm$.

cond-mat.supr-con↗
arXiv2017

Investigation of energy gap and observation of electron-phonon interaction in high-temperature superconductor $La_{1.8}Sr_{0.2}CuO_{4}$ - normal metal point contacts

Electrical characteristics $(I(eV), dV/dI(eV), d^{2}V/dI^{2}(eV))$ are studied comprehensively for pressed submicron point contacts between fresh fractures of LaSrCuO ceramic pellets and a normal metal (Cu). Point contacts (PC) with the direct-type conductivity are investigated, which are characterized by a considerable excess current due to Andreev reflection of quasiparticles at an NS boundary of high transparency. Basic requirements for point contacts suitable for PC measurements are formulated. The models of investigated point contacts permitting the interpretation of the observed characteristics are considered. The results of measurement of the energy gap reveal that LaSrCuO is a superconductor with a strong coupling since the maximum value of the ratio $2Δ/kT\simeq 11$. It is found that superconducting properties may vary appreciably over distances of several tens of angstroms, resulting in some cases in the simultaneous emergence of two gaps corresponding to two critical temperatures. The $dV/dI(eV)$ and $d^{2}V/dI^{2}(eV)$ curves display some high-intensity singularities whose position on the eV-axis is practically independent of temperature and correlates with the phonon spectrum obtained in neutron scattering experiments. This points to a strong electron-phonon interaction in LaSrCuO.

cond-mat.supr-con↗
arXiv2017

Excess conduction of YBaCuO point contacts between 100 and 200 K

$YBaCuO-Ag$ pressure point contacts with direct conduction are investigated. The excess (relative to the normal state) conductivity mainly caused by fluctuational pairing of electrons above $T_c$ is measured in the temperature interval 100-200~$K$. The superconductivity above 120~$K$ is found to be of the two-dimensional type. The obtained preliminary results indicate the presence of small amount of an unknown phase with $T'_c\gtrsim 200~K$ in $YBaCuO$.

cond-mat.supr-con↗
arXiv2017

Reversible transitions in high-$T_c$ cuprates based point contacts

The influence of electric felds and currents has been investigated in the high-$T_c$ superconductors $YBaCuO$ and $BiSrCaCuO$ using a point-contact geometry with $Ag$ as the counterelectrode, which reveal switching transitions between states of a different resistance. The origin of this effect in point contacts is associated with electromigration of the oxygen, driven by the electric feld as well as by the currentinduced "electron wind". The switching effect preserves its basic features at elevated temperatures up to room temperature and in high magnetic felds up to 10 T.

cond-mat.supr-con↗
arXiv2017

Point-contact measurements of the energy gap of lead-chalcogenide-based superconducting superlattices

The energy gap of superconducting $PbTe-PbS$ semiconductor superlattices has been analyzed with the help of point contacts for the first time below the critical temperature, $T_{c}=3.9~K$, and in the fluctuation region $T>T_{c}$. The size of the gap and its dependence on the temperature and field are determined by the position of the point contact inside the superlattice. \pacs{73.40.Jn, 74.25.Kc, 74.45.+c, 74.50.+r., 73.40.-c, 74.78.-w, 74.78.Fk, 74.20.Mn, 74.40.+k.-n, 74.45.+c, 74.62.Dn, 74.70.Ad

cond-mat.supr-con↗
arXiv2017

Point-contact studies of semiconductor $PbTe-PbS$ superconducting superlattice as a model of HTS

Superconducting (SC) superlattices (SL) of the epitaxial semiconductor $PbTe-PbS$/(001) $KCl$ with two-dimensional ordered misfit dislocation (MD) networks across heteroboundaries (HB) are studied, and the obtained results are compared with the corresponding data for $YBaCuO$. An analysis of the fluctuational region showed that the pairing of electrons in SL takes place at the sites of dislocation networks. In the region of SC fluctuations, the IVC derivatives of point-contacts (PC) based on SL and HTS display a metal-insulator transition associated with delocalization of carriers from the vicinity of the structural elements responsible for the emergence of SC. The critical current density in a superlattice is determined, and a mechanism is proposed for formation of excess current on the IVC of PC based on SL and HTS. The values of the energy gap $Δ_0$ at low temperatures in units of $kT_c$ are found to be anomalously large for SL and HTS. For anisotropic three-dimensional SL with a strong bond between two-dimensional SC planes, two gaps emerge from the MD. For quasi-two-dimensional SL with a weak bond between two-dimensional SC planes, the $Δ(T)$ dependence is nonmonotonic, and the fluctuation region for these SL and for HTS reveals a gap that is practically independent of temperature. The IVC derivatives of a $Cu$-SL PC reveal an oscillating structure which is restricted to an energy of $90\ meV$ and is apparently associated with the electron-phonon interaction. It is assumed that there is a similarity between the SC mechanisms in SL and HTS. pacs 73.40.Jn, 74.25.Kc, 74.45.+c, 74.50.+r., 73.40.-c, 74.78.-w, 74.78.Fk, 74.20.Mn, 74.40.+k.-n, 74.45.+c, 74.62.Dn, 74.70.Ad

cond-mat.supr-con↗
arXiv2017

Point-contact function of the electron-phonon interaction in zirconium

Experimental study of the electron-phonon interaction (EPI) spectra of zirconium was carried out using Yanson's point-contact (PC) spectroscopy. The EPI spectral function and constant were determined. Both homocontacts ($Zr-Zr$) and heterocontacts ($Zr-Cu$, $Zr-Ag$, and $Zr-Au$) were studied. No contribution by copper, silver or gold to the heterocontact spectra was detected. The positions of five phonon features of the EPI function were observed at energies 5-6, 9-11, 13-15, 18-20, and 27-29 $meV$. The observed diversity of the PC spectra was attributed to the anisotropy of $Zr$. pacs{71.38.-k, 73.40.Jn, 74.25.Kc, 74.45.+c}

cond-mat.mes-hall↗
arXiv2017

Point contact spectroscopy of the high-temperature superconductor $La_{1.8}Sr_{0.2}CuO_{4}$

The energy gap has been measured and the characteristic phonon energies determined for the superconducting compound $La_{1.8}Sr_{02}CuO_4$ from the current-voltage characteristics of point contacts with a copper counterelectrode. The temperature dependence of the energy gap was determined. pacs 73.40.Jn, 74.25.Kc, 74.45.+c, 73.40.-c, 74.20.Mn, 74.70.Ad, 74.72.-h, Dn

cond-mat.supr-con↗
arXiv2016

Phonon singularities on volt-ampere curves of niobium point contacts

The volt-ampere curves and their second derivatives were studied for niobium point contacts at low temperatures in the voltage range corresponding to the characteristic phonon energies. It was found that while for the dirty contacts in the normal state no PC spectra of phonons could be detected, in the superconducting state there were singularities in the I-V curves corresponding to maxima either in the first or in the second derivatives. The singularities observed were due to the energy dependence of the excess current. We suppose that the origin of these singularities is due to the inelastic transitions of electrons between chemical potentials of Cooper pairs at both sides of the contact, which differ in energy by $eV$. These transitions are possible if $ξ\left(0 \right)\gtrsim d \sim{Λ_{\varepsilon}}$ ($ξ(0)$ being the coherence length, $d$, the contact diameter, ${Λ_{\varepsilon}}={{({{l}_{i}}\cdot {{l}_{\varepsilon}})}^{1/2}}$, where ${{l}_{i}}$ and ${{l}_{\varepsilon}}$ being the elastic and inelastic electron mean free paths, respectively).

cond-mat.supr-con↗
arXiv2016

Spectroscopy of phonons in dirty superconducting contacts

The nonlinearities of current-voltage characteristics (IVC) of superconducting niobium point contacts for voltages of the order of the characteristic phonon frequencies are investigated. It is shown that in limiting by dirty contacts ${{l}_{1}}\ll d$ , (${{l}_{1}}$ is the momentum mean-free path of electrons), features are observed on the IVC corresponding to maxima in the first or second derivatives and are situated at energies close to the characteristic phonon energies. These features are due to the energy dependence of the excess current, since the nonlinearity of the IVC in the normal state is several times smaller in absolute magnitude, and its second derivative is monotonic. For a contact diameter $d$ of the order of several tens of angstroms (the contact resistance is of the order of 100 $Ω$), the thermal effects can be neglected. It is assumed that in the region of the constriction, the characteristic lengths of the system satisfy the relation $ξ\gtrsim d>{{Λ}_{\varepsilon }}$ (${{Λ}_{\varepsilon }}$ is the diffusion length for energy relaxation of the electrons, $ξ$ is the coherence length) which makes possible inelastic transitions of electrons between pair chemical potentials differing in energy by an account $eV$ ($V$ is the voltage drop across the contact)

cond-mat.supr-con↗
arXiv2016

Point-contact spectra of $2H-NbS{{e}_{2}}$ in the superconducting state

A qualitative study of the electron-phonon interaction (EPI) spectra in $2H-NbS{{e}_{2}}$ is carried out by the method of point-contact spectroscopy in the superconducting state. Anisotropy of the EPI spectra for contacts oriented in the principal crystallographic directions (along the $c$ axis and parallel to the basal surface) was observed. Approximate values of the positions of the main features in the phonon density of states were established for both directions. The additional low-frequency peaks observed in the spectra for separate crystallographic orientations are associated with the collective oscillations of the amplitude and phase of charge density waves.

cond-mat.supr-con↗
arXiv2016

Investigation of the electron-phonon interaction in $N{{b}_{3}}Sn$ with the aid of microcontacts

The method of microcontact spectroscopy in the superconducting state was used to investigate weak nonlinearities of the current-voltage characteristics of point contacts made of $N{{b}_{3}}Sn$ single crystals. The nature of the spectrum of the electron-phonon interaction was found to vary considerably from contact to contact, indicating considerable deviations of the composition of the surface of $N{{b}_{3}}Sn$ from stoichiometry. A correlation was established between the nature of the spectrum and the magnitude of the gap singularities of the current-voltage characteristics. In the case of "dirty" high-resistance contacts with strong gap singularities the microcontact spectra were reasonably reproducible, which made it possible to relate them sufficiently closely to the microcontact function of the electron-phonon interaction in the bulk material. It was found that microcontact spectroscopy of this interaction was possible in the superconducting state not only in dirty $S-c-S$ contacts, but also in dirty $S-c-N$ contacts.

cond-mat.supr-con↗
arXiv2016

Reabsorption of nonequilibrium phonons at superconducting point contacts

When there is a deviation from the inequality $d\ll {{l}_{\varepsilon }}$ ($d$ is the contact diameter, and ${{l}_{\varepsilon }}$ is the energy relaxation length of the electrons), structural features are produced on the current-voltage characteristics of $S-c-N$ contacts at characteristic phonon energies because of the decrease in $Δ$ due to the accumulation of nonequilibrium phonons with low group velocities near the contact.

cond-mat.supr-con↗
arXiv2016

Magnetoquantum oscillations in the resistance of metallic point contacts: Influence of nonequilibrium phonons

The amplitude of magnetoresistance quantum oscillations of Al and Be point contacts in a magnetic field parallel to the contact axis has been studied as a function of voltage applied over the contact. It was found that for one group of contacts the oscillation amplitude nonmonotonously increases with the bias voltage increase, while for the other part of the contacts a decrease of amplitude was observed. The scattering of electrons with nonequilibrium phonons and also phonon-phonon collisions will be discussed as the possible reasons of the observed effects.

cond-mat.mes-hall↗
arXiv2016

Dependence of the amplitude of magnetoquantum oscillations of the metallic point-contact resistance on the bias voltage

Magnetoquantum oscillations of resistance as functions of the potential difference applied across the contact are studied for metallic point contacts made of $Al$ and $Be$. The amplitude of resistance oscillations in a magnetic field increases with voltage and is identical to the EPI spectrum of the point contact for one group of contacts, and with the bias voltage across the contact for another. The increase in the oscillation amplitude as well as its decrease has a nonmonotonic dependence on energy. The scattering of electrons by nonequilibrium phonons and the Fermi-liquid effects in the nonequilibrium electron system are considered as the possible reasons behind the observed effects.

cond-mat.mes-hall↗
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

Inelastic electron-phonon scattering and excess current in superconducting point contacts with a short coherence length

Nonlinear electrical effects in superconducting S-c-S contacts, including the spectroscopy of electron-phonon interactions (EPI) in these systems, and the recovery of the EPI function from experimental data are discussed. The effect of a magnetic field on the current-voltage characteristics(I-V curves) and their derivatives for ErNi_2B_2C point contacts (PC) with $d\ge ξ$ (where $d$ is the diameter of the PC and $ξ$ is the coherence length) is studied. It is found that in zero magnetic fields and in near-critical fields, when the size of the superconducting gap can be neglected, the position of the peaks in $dV/dI$ coincides with the peaks in the Yanson EPI spectra. In low fields the peaks are shifted toward lower energies and in intermediate fields, the peaks split. For PC with diameters greater than or on the order of the coherence length, the relative size of the negative phonon contribution to the excess current is considerably greater than in ballistic contacts. This leads to substantial suppression of the high-frequency peaks in the spectra for the superconducting state. In order to recover the EPI function from these spectra it is necessary to correct their intensities at high energies. For "dirty" NbSe_2 and Nb point-contacts with $d\ge ξ$, which have no phonon features in the second derivative of the I-V curve in the normal state, the EPI can be reconstructed from the superconducting state.

cond-mat.supr-con↗