Alexandre Avraamovitch Golubov

Coupling of two superconductors through a ferromagnet: evidence for a pi junction

We report 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, with x around 0.5. With decreasing temperature I(c) generally increases, but for specific thicknesses of the ferromagnetic interlayer, a maximum is found followed by a strong decrease down to zero, after which I(c) 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 pi phase state at low temperatures.

Multiband model for tunneling in MgB2 junctions

A theoretical model for quasiparticle and Josephson tunneling in multiband superconductors is developed and applied to MgB2-based junctions. The gap functions in different bands in MgB2 are obtained from an extended Eliashberg formalism, using the results of band structure calculations. The temperature and angle dependencies of MgB2 tunneling spectra and the Josephson critical current are calculated. The conditions for observing one or two gaps are given. We argue that the model may help to settle the current debate concerning two-band superconductivity in MgB2.

Specific heat of MgB2 in a one- and a two-band model from first-principles calculations

The heat capacity anomaly at the transition to superconductivity of the layered superconductor MgB2 is compared to first-principles calculations with the Coulomb repulsion, µ*, as the only parameter which is fixed to give the measured Tc. We solve the Eliashberg equations for both an isotropic one-band model and a two-band model with different superconducting gaps on the π-band and σ-band Fermi surfaces. The agreement with experiments is considerably better for the two-band model than for the one-band model.

Probing spin polarization with Andreev reflection: A theoretical basis

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.

Superconductivity in MgB2: Clean or Dirty?

A large number of experimental facts and theoretical arguments favor a two-gap model for superconductivity in MgB2. However, this model predicts strong suppression of the critical temperature by interband impurity scattering and, presumably, a strong correlation between the critical temperature and the residual resistivity. No such correlation has been observed. We argue that this fact can be understood if the band disparity of the electronic structure is taken into account, not only in the superconducting state, but also in normal transport.

Effect of magnetic and nonmagnetic impurities on highly anisotropic superconductivity

We generalize Abrikosov-Gor{close_quote}kov solution of the problem of weakly coupled superconductors with impurities to the case of a multiband superconductor with arbitrary interband order parameter anisotropy, including interband sign reversal of the order parameter. The solution is given in terms of the effective (renormalized) coupling matrix and describes not only T{sub c} suppression but also renormalization of the superconducting gap basically at all temperatures. In many limiting cases we find analytical solutions for the critical temperature suppression. We illustrate our results by numerical calculations for two-band model systems. {copyright} {ital 1997} {ital The American Physical Society}

Josephson supercurrent through a topological insulator surface state

The long-sought yet elusive Majorana fermion is predicted to arise from a combination of a superconductor and a topological insulator. An essential step in the hunt for this emergent particle is the unequivocal observation of supercurrent in a topological phase. Here, direct evidence for Josephson supercurrents in superconductor (Nb)-topological insulator (Bi(2)Te(3))-superconductor electron-beam fabricated junctions is provided by the observation of clear Shapiro steps under microwave irradiation, and a Fraunhofer-type dependence of the critical current on magnetic field. Shubnikov-de Haas oscillations in magnetic fields up to 30 T reveal a topologically non-trivial two-dimensional surface state. This surface state is attributed to mediate the ballistic Josephson current despite the fact that the normal state transport is dominated by diffusive bulk conductivity. The lateral Nb-Bi(2)Te(3)-Nb junctions hence provide prospects for the realization of devices supporting Majorana fermions.

Band Filling and Interband Scattering Effects in MgB2: Carbon versus Aluminum Doping

We argue, based on band structure calculations and the Eliashberg theory, that the observed decrease of T(c) of Al and C doped MgB2 samples can be understood mainly in terms of a band filling effect due to the electron doping by Al and C. A simple scaling of the electron-phonon coupling constant lambda by the variation of the density of states as a function of electron doping is sufficient to capture the experimentally observed behavior. Further, we also explain the long standing open question of the experimental observation of a nearly constant pi gap as a function of doping by a compensation of the effect of band filling and interband scattering. Both effects together generate a nearly constant pi gap and shift the merging point of both gaps to higher doping concentrations, resolving the discrepancy between experiment and theoretical predictions based on interband scattering only.

Multiband model for penetration depth in MgB2

The results of first-principles calculations of the electronic structure and the electron-phonon interaction in MgB2 are used to study theoretically the temperature dependence and anisotropy of the magnetic-field penetration depth. The effects of impurity scattering are essential for a proper description of the experimental results. We compare our results with experimental data and we argue that the two-band model describes the data rather well.

Nonmonotonic critical temperature in superconductor ferromagnet bilayers

The critical temperature Tc of a superconductor/ferromagnet (SF) bilayer can exhibit nonmonotonic dependence on the thickness df of the F layer. SF systems have been studied for a long time; according to the experimental situation, a ?dirty? limit is often considered which implies that the mean free path in the layers is the second smallest spatial scale after the Fermi wavelength. However, all calculations reported for the dirty limit were done with some additional assumptions, which can be violated in actual experiments. Therefore, we develop a general method (to be exact, two independent methods) for investigating Tc as a function of the bilayer parameters in the dirty case. Comparing our theory with experiment, we obtain good agreement. In the general case, we observe three characteristic types of Tc(df) behavior: (1) a nonmonotonic decay of Tc to a finite value exhibiting a minimum at particular df; (2) a reentrant behavior, characterized by a vanishing of Tc in a certain interval of df and finite values otherwise; and (3) a monotonic decay of Tc and a vanishing at finite df. Qualitatively, the nonmonotonic behavior of Tc(df) is explained by the interference of quasiparticles in the F layer, which can be either constructive or destructive depending on the value of df.