Showing papers by "Alexandre Avraamovitch Golubov published in 2001"
TL;DR: Measurements of the temperature dependence of the critical current, I(c), in Josephson junctions consisting of conventional superconducting banks of Nb and a weakly ferromagnetic interlayer of a CuxNi1-x alloy show a sharp cusp with decreasing temperature.
Abstract: We report measurements of the temperature dependence of the critical current, Ic, in Josephson junctions consisting of conventional superconducting banks of Nb and a weakly ferromagnetic interlayer of a CuxNi1-x alloy, with x around 0.5. With decreasing temperature Ic generally increases, but for specific thicknesses of the ferromagnetic interlayer, a maximum is found followed by a strong decrease down to zero, after which Ic rises again. Such a sharp cusp can be explained only by assuming that the junction changes from a 0-phase state at high temperatures to a I� phase state at low temperatures.
929 citations
TL;DR: In this article, the Eliashberg equations for both an isotropic one-band and a two-band model with different superconducting gaps on the Fermi surfaces were solved.
Abstract: The heat capacity anomaly at the transition to superconductivity of the layered superconductor MgB$_2$ is compared to first-principles calculations with the Coulomb repulsion, $\mu^\ast$, as the only parameter which is fixed to give the measured $T_c$. We solve the Eliashberg equations for both an isotropic one-band and a two-band model with different superconducting gaps on the $\pi$ and $\sigma$ Fermi surfaces. The agreement with experiments is considerably better for the two-band model than for the one-band model.
205 citations
TL;DR: In this paper, a general theory of Andreev reflection for spin-polarized systems and arbitrary Fermi surface in two limiting cases of ballistic and diffusive transport is presented.
Abstract: Andreev reflection at the interface between a ferromagnet and a superconductor has become a foundation of a versatile technique of measuring the spin polarization of magnetic materials. In this article we will briefly outline a general theory of Andreev reflection for spin-polarized systems and arbitrary Fermi surface in two limiting cases of ballistic and diffusive transport.
177 citations
TL;DR: The current-phase relationship I(varphi) of symmetric 45 degrees YBa2Cu3O7-x grain boundary Josephson junctions is measured, in good agreement with a microscopic model of Josephsonjunctions between d-wave superconductors.
Abstract: We have measured the current-phase relationship $I(\ensuremath{\phi})$ of symmetric 45\ifmmode^\circ\else\textdegree\fi{} ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ grain boundary Josephson junctions. Substantial deviations of the Josephson current from conventional tunnel-junction behavior have been observed: (i) The critical current exhibits, as a function of temperature $T$, a local minimum at a temperature ${T}^{*}$. (ii) At $T\ensuremath{\approx}{T}^{*}$, the first harmonic of $I(\ensuremath{\phi})$ changes sign. (iii) For $Tl{T}^{*}$, the second harmonic of $I(\ensuremath{\phi})$ is comparable to the first harmonic, and (iv) the ground state of the junction becomes degenerate. The results are in good agreement with a microscopic model of Josephson junctions between $d$-wave superconductors.
156 citations
TL;DR: In this paper, the authors studied the influences of deposition conditions such as pressure and temperature, the substrate material, and annealing-procedures on superconducting thin MgB2 films.
Abstract: Superconducting thin MgB2 films have been prepared using pulsed-laser deposition. We have studied the influences of deposition conditions such as pressure and temperature, the substrate-material, and annealing-procedures. Various approaches have been pursued to obtain the right Mg content in the film during ablation and annealing. Special care has been taken to avoid oxidation of Mg in the laser plasma and deposited film, by optimizing the background pressure of Ar gas in the deposition chamber. The annealing procedure was found to be the most critical to obtain superconducting films.
58 citations
TL;DR: In this article, superconducting thin films were prepared on Si-substrates, using pulsed laser deposition from a target composed of a mixture of Mg and MgB2 powders.
Abstract: Superconducting thin films have been prepared on Si-substrates, using pulsed laser deposition from a target composed of a mixture of Mg and MgB2 powders. The films were deposited at room temperature and post-annealed at 600 degrees C. The zero resistance transition temperatures were 12 K, with an onset transition temperature of 27 K. Special care has been taken to avoid oxidation of Mg in the laser plasma and deposited film, by optimizing the background pressure of Ar gas in the deposition chamber. For this the optical emission in the visible range from the plasma has been used as indicator. Preventing Mg from oxidation was found to be essential to obtain superconducting films.
57 citations
TL;DR: In this article, the depairing current behavior of ferromagnet/superconductor/ferromagnetic trilayers was investigated as a function of the thickness of the superconducting layers.
Abstract: We have investigated the behavior of the depairing current ${J}_{\mathrm{dp}}$ in ferromagnet/superconductor/ferromagnet $(F/S/F)$ trilayers as function of the thickness ${d}_{s}$ of the superconducting layers. Theoretically, ${J}_{\mathrm{dp}}$ depends on the superconducting order parameter or the pair-density function, which is not homogeneous across the film due to the proximity effect. We use a proximity-effect model with two parameters (proximity strength and interface transparency), which can also describe the dependence of the superconducting transition temperature ${T}_{c}$ on ${d}_{s}.$ We compare the computations with the experimentally determined zero-field critical current ${J}_{c0}$ of small strips (typically $5\ensuremath{-}\ensuremath{\mu}\mathrm{m}$ wide) of Fe/Nb/Fe trilayers with varying thickness ${d}_{\mathrm{Nb}}$ of the Nb layer. Near ${T}_{c}$ the temperature dependence ${J}_{c0}(T)$ is in good agreement with the expected behavior, which allows extrapolation to $T=0.$ Both the absolute values of ${J}_{c0}(0)$ and the dependence on ${d}_{\mathrm{Nb}}$ agree with the expectations for the depairing current. We conclude that ${J}_{\mathrm{dp}}$ is correctly determined, notwithstanding the fact that the strip width is larger than both the superconducting penetration depth and the superconducting coherence length, and that ${J}_{\mathrm{dp}}{(d}_{s})$ is correctly described by the model.
48 citations
TL;DR: In this article, a general model for calculating the density of states and the Cooper pair potential in proximity-coupled superconducting bi-and trilayer films is presented.
Abstract: This article presents a general model for calculating the density of states and the Cooper pair potential in proximity-coupled superconducting bi- and trilayer films. It is valid for any kind of bilayer S1-S2, whatever the quality of the materials S1 and S2, the quality of the S1-S2 interface, and the layer thicknesses. The trilayer model is valid for a thin S3 layer, whereas the other two layers have arbitrary thicknesses. Although the equations of the dirty limit are used, it is argued that the model stays valid in clean bi-and trilayer films. The typical example of superconducting tunnel junctions is used to show that existing models, which apply to very thin or very thick layers or to perfectly transparent S1-S2 interfaces, are too restrictive to apply to an arbitrary bilayer. The new model is applied to practical junctions, with layer thicknesses intermediate between the “thick” and the “thin” approximation.
43 citations
TL;DR: In this paper, the influence of the barrier transparency on critical current, IC, and normal resistance, RN, by preparing SIS and SINIS junctions under identical technological conditions and comparing their transport properties was performed.
Abstract: The transparency of the tunnel barriers in double-barrier junctions influences the critical current density and the form of the current–voltage characteristics (IVC). Moreover, the barrier asymmetry is an important parameter, which has to be controlled in the technological process. We have performed a systematic study of the influence of the barrier transparency on critical current, IC, and normal resistance, RN, by preparing SIS and SINIS junctions under identical technological conditions and comparing their transport properties. We have fabricated Nb/Al2O3/Nb and Nb/Al2O3/Al/Al2O3/Nb devices with different current densities using a conventional fabrication process, varying pressure and oxidation time. The thickness of the Al middle electrode in all Nb/Al2O3/Al/Al2O3/Nb junctions was 6 nm. Patterning of the multilayers was done using conventional photolithography and the selective niobium etching process. The current density of SIS junctions was changed in the range from 0.5 to 10 kA/cm2. At the same conditions the current density of SINIS devices revealed 1–100 A/cm2 with non-hysteretic IVC and characteristic voltages, ICRN, of up to 200 μV. By comparing the experimental and theoretical temperature dependence of the ICRN product we estimated the barrier transparency and its asymmetry. The comparison shows a good agreement of experimental data with the theoretical model of tunneling through double-barrier structures in the dirty limit and provides the effective barrier transparency parameter γeff≈300. A theoretical framework is developed to study the influence of the barrier asymmetry on the current–phase relationship and it is proposed to determine the asymmetry parameter by measuring the critical current suppression as function of applied microwave power. The theoretical approach to determine the non-stationary properties of double-barrier junctions in the adiabatic regime is formulated and the results of calculations of the I–V characteristics are given in relevant limits. The existence and the magnitude of a current deficit are predicted as function of the barrier asymmetry.
12 citations
TL;DR: In this paper, the authors present an application of the generalised proximity effect theory to determine the energy gap (Δg) in proximised transition metal/aluminium bilayer structures.
Abstract: We present an application of the generalised proximity effect theory. The theory has been used to determine the energy gap (Δg) in proximised transition metal/aluminium bilayer structures such as Nb/Al, Ta/Al, V/Al and Mo/Al. These bilayers have different film thicknesses ranging from 5 to 260 nm. For the cases of Nb/Al, Ta/Al and V/Al bilayers, the interface parameters γ and γBN (here we define γ as the ratio of the products of normal state resistivity and coherence length in each film of the bilayer while γBN is the ratio of the boundary resistance between films 1 and 2 to the product of the resistivity and coherence length in the second film), which were used as input parameters to the model, were inferred experimentally from an existing bilayer of each kind and then suitably modified for different film thicknesses. This experimental assessment is therefore based on a comparison of measurements of the critical temperature and the energy gap at 300 mK with the predictions from the model for various values of γ, γBN. The energy gap of the bilayer was experimentally determined by using symmetrical superconducting tunnel junctions (STJs) of the form S–Al–AlOx–Al–S, where each electrode corresponds to a proximised bilayer. However for the case of Mo/Al bilayers the interface parameters were determined theoretically since currently no STJ data for this configuration are available. The results for the Nb/Al, Ta/Al and V/Al bilayers have also then been compared to experimentally determined energy gaps found for a series of STJs with different film thicknesses. The correspondence between experiment and theory is very good.
TL;DR: In this paper, double-barrier SIS'IS Josephson junctions with high critical current density were considered and an intrinsic shunting resistance was estimated from I-V curves.
Abstract: New theoretical and experimental results on double-barrier SIS'IS Josephson junctions are presented (I is a tunnel barrier, S' is a thin film with critical temperature lower than that of S). The previously developed microscopic model for the stationary case, which describes the critical currents in Nb/Al/Nb junctions, is extended to the non-equilibrium regime of finite voltage. In particular, an intrinsic shunting resistance is estimated from I-V curves. We formulate the requirements for interface barriers in order to realize non-hysteretic SIS'IS junctions with high critical current density and I/sub c/R/sub N/ products. A comparison with single-barrier SIS junctions with high critical current density is carried out.
TL;DR: In this article, the SFS π-junction was used for the quantum bit implementation based on the superconducting loop with quantum Josephson junctions, and the advantages of the π junction in obtaining naturally degenerate two-level systems were discussed.
Abstract: Reentrant superconducting behavior of the critical supercurrent temperature dependencies has been observed for the Nb–Cu/Ni–Nb Josephson SFS (superconductor–ferromagnet–superconductor) junctions. The Ic(T) oscillations detected are associated with a crossover of the SFS junctions from '0'- to 'π'-state that is related to a special feature of superconducting pair flow through a ferromagnet (spatial oscillations of induced superconducting order parameter in presence of the exchange field). A triangular array of the junctions in the π-state shows evidence for a phase shift of π on the Ic(H) dependence. We have argued advantages of the SFS π-junction use for the quantum bit implementation based on the superconducting loop with quantum Josephson junctions. While designs proposed previously are based on magnetically frustrated superconducting loops, we discuss the advantages offered by the π-junctions in obtaining naturally degenerate two-level systems.
TL;DR: In this article, the authors present the results of currentvoltage characterizations of superconducting tunnel junctions, which reveal a dependence of the measured energy gap on the size of the junction.
Abstract: We present the results of current-voltage characterizations of $\mathrm{N}\mathrm{b}/\mathrm{A}\mathrm{l}$ and $\mathrm{T}\mathrm{a}/\mathrm{A}\mathrm{l}$ superconducting tunnel junctions, which reveal a dependence of the measured energy gap on the size of the junction. This implies a geometrical dependence of the energy gap, which suggests that the effective local energy gap has a lateral spatial variation on a scale of several \ensuremath{\mu}m. An extended version of the theory of the proximity effect could explain this phenomenon when lateral coherence lengths are introduced, which are of the order of the bulk coherence length in Al. One of the consequences of this theory is that the coherence length in a thin film cannot be taken to be isotropic. Another is that the effective lateral coherence length, which features in the lateral Usadel equation, actually depends on the quasiparticle energy.
Journal Article•
TL;DR: In this article, a simple scattering approach to the charge transport across a realistic superconductor-normal injector interface of a finite transmittance that is modeled by a double-barrier mesoscopic junction is presented.
Abstract: We present a simple scattering approach to the charge transport across a realistic superconductor–normal injector interface of a finite transmittance that is modeled by a double-barrier mesoscopic junction. For a d-wave pairing symmetry, our calculations combine a fully quantum-mechanical scattering formalism with a self-consistent estimation of Andreev reflection coefficients within the quasi-classical Eilenberger equation scheme for a free specular superconducting surface. Numerical simulations confirm experimental criteria of Cucolo for the unconventional superconducting origin of conductance anomalies in high-temperature oxides. A discussion of dephasing effects caused by inelastic scattering processes in the interlayer and their impact on the conductance spectra is given.
TL;DR: In this article, the critical current in a superconductor/two-dimensional electron gas junction/superconductor junction is controlled by an injection current, and the experimental results are compared to a theoretical model based in the solution of the Gor'kov equations, which takes the finite width of the sample into account.
Abstract: The critical current in a superconductor/two-dimensional electron gas junction/superconductor junction is controlled by an injection current. In case of a carrier injection from one contact to the semiconductor through the two-dimensional electron gas region of the junction to an ohmic contact at the opposite side a complete suppression of the supercurrent is obtained. The experimental results are compared to a theoretical model based in the solution of the Gor'kov equations, which takes the finite width of the sample into account.