arXiv subjects
Publications and source records attributed to M. A. Obolenskii.
A polarity-dependent reversible change in the current-voltage characteristics between states corresponding to different values of the excess current Iexc is observed for bias voltages of several hundred millivolts in YBaCuO-Ag point contacts in the current-carrying state.
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.
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.
$YBaCuO-Ag$ 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 $YBaCuO$.
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
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.
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 $YBaSrCuO$.
We report on magnetic measurements carried out in a textured YBa$_2$Cu$_3$O$_{7-δ}$ and YBa$_2$(Cu$_{1-x}$Ag$_x$)$_3$O$_{7-δ}$ (at $x \approx$ 0.02) crystals. The so-called fishtail-effect (FE) or second magnetization peak has been observed in a wide temperature range 0.4~$<T/T_c<$~0.8 for $\textbf{H}\parallel c$. The origin of the FE arises for the competition between surface barrier and bulk pinning. This is confirmed in a non-monotonically behavior of the relaxation rate $R$. The value $H_{max}$ for Ag-doped crystals is larger than for the pure one due to the presence of additional pinning centers, above all on silver atoms.
$M(B)$-curves were experimentally investigated. Fishtail-effect (FE) was observed in MG YBa$_2$Cu$_3$O$_{7-δ}$ and YBa$_2$(Cu$_{1-x}$Ag$_x$)$_3$O$_{7-δ}$ (at $x \approx$ 0.02) crystals in a wide temperature range 0.4 $<T/T_c<$ 0.8 at the orientation of magnetic field $\textbf{H}\parallel c$. It was obtained that the influence of bulk pinning on FE is more effective at low temperatures while surface barriers dominates at high temperatures. The value $H_{max}$ for Ag-doped crystals is larger than for a pure one that due to the presence of additional pinning centers, above all on silver atoms.
$M(B)$-curves were experimentally investigated. Fishtail-effect (FE) was observed in MG YBa$_2$Cu$_3$O$_{7-δ}$ and YBa$_2$Cu$_{3-x}$Ag$_x$O$_{7-δ}$ (at $x \approx$ 0.02) crystals in a wide temperature range 40 K $<T<$ 75 K at the orientation of magnetic field $\textbf{H}\parallel c$. It was obtained that the influence of bulk pinning on FE is more effective at low temperatures while the influence of surface barriers is more effective at high temperatures. The value $H_{max}$ for Ag-doped crystals is larger than for a pure one that due to the presence of additional pinning centers, above all on silver atoms.
In present work the conductivity in the basis plane of YBaCuO single crystals slightly doped by Al and Pr with a pre-specified topology of twin boundaries has been investigated. The excess conductivity for the analyzed samples shows dependence like $Δσ\sim(1-T/T^*)\exp(Δ_{ab}^*/T)$ in wide temperature range $T_f<T<T^*$, where $T^*$ can be represents as mean field temperature of superconducting transition. The temperature dependence of pseudogap can be satisfactory described in terms of the BCS-BEC crossover theoretical model.
We present the results of an experimental study of vortex dynamics in non-twinned $YBa_2Cu_3O_{6,87}$ crystal. It is found that critical currents $J_c$ and $J_{c,dyn}$, which correspond to the pinning force in the thermal creep and flux flow mode, respectively, non-monotonically vary with the magnetic field. However, the minimum in the $J_{c,dyn}(H)$ dependence is observed in higher fields, compared with the minimum position $H_{OD}$ in the $J_c(H)$ dependence. Considering that the field $H_{OD}$ corresponds to the static order-disorder transition, this difference is explained by partial dynamic ordering of the vortex solid. It is concluded that finite transverse barriers guarantee finite density of transverse displacements of vortex lines $u_t\simeq c_La_0$ suitable for preservation of the disordered state of the moving vortex solid.
We show that rotation of the magnetic field off the plane of twin boundaries (TB's) induces transition of an ordered vortex solid phase to a disordered one. This transition arises due to appearance of transverse deformations of vortex lines near the TB's, $u_{t,TB}$, the amplitude of which satisfies the Lindemann criteria, $u_{t,rp}=c_La_0$. This order-disorder transition is accompanied by an increase in the depinning current, by crossover from an elastic creep to plastic one, and by appearance of the $S$-shaped voltage-current characteristics.
We present the transport studies of field variation of the pinning force for the flux motion across the twin boundaries in YBaCuO single crystals. It is found that the depinning current J_c decreases with an increase in the magnetic field due to reduction of the portion of vortex lines trapped by the TB's. However, at transport currents J << J_c, the vortex velocity weakly decreases with the increased field, i.e. it is determined mainly by the release of the vortex lines from the TB's.
The physical nature of the macroturbulence in the vortex matter in YBCO superconductors is verified by means of magnetooptic study of the instability in a single crystal prepared specially for this purpose. The instability develops near those sample edges where the oppositely directed flow of vortices and antivortices, guided by twin boundaries, is characterized by the discontinuity of the tangential component of the hydrodynamic velocity. This fact directly indicates that the macroturbulence is analogous to the instability of fluid flow at a surface of a tangential velocity discontinuity in classical hydrodynamics, and is related to the anisotropic flux motion in the superconductor.