Showing papers by "Alexandre Avraamovitch Golubov published in 2013"

Theoretical model of superconducting spintronic SIsFS devices

Abstract: Motivated by recent progress in the development of cryogenic memory compatible with single flux quantum (SFQ) circuits, we have performed a theoretical study of magnetic SIsFS Josephson junctions, where “S” is a bulk superconductor, “s” is a thin superconducting film, “F” is a metallic ferromagnet, and “I” is an insulator. We calculate the Josephson current as a function of s and F layers thickness, temperature, and exchange energy of F film. We outline several modes of operation of these junctions and demonstrate their unique ability to have large product of a critical current IC and a normal-state resistance RN in the π state, comparable to that in superconductor–insulator–superconductor tunnel junctions commonly used in SFQ circuits. We develop a model describing switching of the Josephson critical current in these devices by external magnetic field. The results are in good agreement with the experimental data for Nb-Al/AlO x -Nb-Pd0.99Fe0.01-Nb junctions

Andreev bound states and current-phase relations in three-dimensional topological insulators

Abstract: To guide the search for the Majorana fermion, we theoretically study superconductor/topological-insulator/superconductor (S/TI/S) junctions in an experimentally relevant regime. We find that the striking features present in these systems, including the doubled periodicity of the Andreev bound states (ABSs) due to tunneling via Majorana states, can still be present at high electron densities. We show that via the inclusion of magnetic layers, this 4π periodic ABS can still be observed in three-dimensional (3D) topological insulators, where finite angle incidence usually results in the opening of a gap at zero energy and hence results in a 2π periodic ABS. Furthermore, we study the Josephson-junction characteristics and find that the gap size can be controlled and decreased by tuning the magnetization direction and amplitude. These findings pave the way for designing experiments on S/3DTI/S junctions.

Theoretical Model of Superconducting Spintronic SIsFS Devices

Abstract: Motivated by recent progress in development of cryogenic memory compatible with single flux quantum (SFQ) circuits we have performed a theoretical study of magnetic SIsFS Josephson junctions, where 'S' is a bulk superconductor, 's' is a thin superconducting film, 'F' is a metallic ferromagnet and 'I' is an insulator. We calculate the Josephson current as a function of s and F layers thickness, temperature and exchange energy of F film. We outline several modes of operation of these junctions and demonstrate their unique ability to have large product of a critical current $I_{C}$ and a normal-state resistance $R_{N}$ in the $\pi$ state, comparable to that in SIS tunnel junctions commonly used in SFQ circuits. We develop a model describing switching of the Josephson critical current in these devices by external magnetic field. The results are in good agreement with the experimental data for Nb-Al/AlO${_x}$-Nb-Pd$_{0.99}$Fe$_{0.01}$-Nb junctions.

Manifestations of impurity induced s +- = s++ transition: multiband model for dynamical response functions

Abstract: We investigate the effects of disorder on the density of states, the single-particle response function and optical conductivity in multiband superconductors with s± symmetry of the order parameter, where s± → s++ transition may take place. In the vicinity of the transition, the superconductive gapless regime is realized. It manifests itself in anomalies in the above-mentioned properties. As a result, intrinsically phase-insensitive experimental methods such as angle-resolved photoemission spectroscopy, tunneling and terahertz spectroscopy may be used to reveal information about the underlying order parameter symmetry.

Theory of supercurrent transport in SIsFS Josephson junctions

Abstract: We present the results of a theoretical study of current-phase relations (CPRs) Js(φ) in Josephson junctions of SIsFS type, where S is a bulk superconductor and IsF is a complex weak link consisting of a superconducting film s, a metallic ferromagnet F, and an insulating barrier I. At temperatures close to critical, T≲Tc, calculations are performed analytically in the frame of the Ginsburg-Landau equations. At low temperatures a numerical method is developed to solve self-consistently the Usadel equations in the structure. We demonstrate that SIsFS junctions have several distinct regimes of supercurrent transport and we examine spatial distributions of the pair potential across the structure in different regimes. We study the crossover between these regimes, which is caused by shifting the location of a weak link from the tunnel barrier I to the F layer. We show that strong deviations of the CPR from sinusoidal shape occur even in the vicinity of Tc , and these deviations are strongest in the crossover regime. We demonstrate the existence of temperature-induced crossover between 0 and π states in the contact and show that the smoothness of this transition strongly depends on the CPR shape.

Josephson phi-junctions based on structures with complex normal/ferromagnet bilayer

Abstract: We demonstrate that Josephson devices with a nontrivial phase difference 0 < phivg < π in the ground state can be realized in structures composed of longitudinally oriented normal metal (N) and ferromagnet (F) films in the weak link region. Oscillatory coupling across the F-layer makes the first harmonic in the current–phase relation relatively small, while coupling across the N-layer provides a negative sign of the second harmonic. To derive quantitative criteria for a phiv-junction, we have solved the two-dimensional boundary-value problem in the frame of Usadel equations for overlap and ramp geometries of S–NF–S structures. Our numerical estimates show that phiv-junctions can be fabricated using up-to-date technology.

Theory of Tunneling Spectroscopy of Multi-Band Superconductors

Abstract: We present the derivation of boundary conditions on a wave function at the normal metal/superconductor (N/S) interface by extending the tight-binding approach developed for semiconducting heterostructures [Phys. Rev. 27 (1983) 3519]. Based on these boundary conditions, we formulate a quantitative theory for tunneling spectroscopy in N/S junctions, where a superconductor is characterized by complex non-parabolic energy spectrum beyond effective mass approximation. As an application to single-band unconventional superconductors, we re-derive the known conductance formula [Phys. Rev. Lett. 74 (1995) 3451] with generalized definition of a normal-state conductance. We further apply the model to junctions between normal metals (N) and multi-band iron-based superconductors (FeBS). Our calculations show that tunneling studies of (100) oriented N/FeBS junctions allow to distinguish between the s ± and the s ++ order parameter symmetry in FeBS. In low transparent N/FeBS junctions with the s +− symmetry in FeBS, finite energy subgap Andreev bound states are formed due to sign change of pair potential between different Fermi surface pockets. Another fingerprint of the s +− symmetry in FeBS is suppressed Andreev conductance in high transparent (100) N/FeBS junctions compared to the case of the s ++ symmetry. Our results may serve as a basis for quantitative tunneling spectroscopy of FeBS

Multiband Description of Optical Conductivity in Ferropnictide Superconductors

Abstract: We study optical properties of the multiband superconductors with an s± order parameter symmetry By comparing results of our theory with experimental data on optical conductivity for Ba068K032Fe2As2 single crystals, we show that satisfactory description of the novel superconductors can be obtained only considering a strong electron-boson coupling We reexamine the effect of disorder and demonstrate that multiband superconductors are more robust with respect to it than naively expected by simple analogy with paramagnetic impurities in single-band superconductors Moreover, disorder may give rise to new effects, in particular to a phase transition s±→s++ We discuss how the systematic study of disorder impact on the density of states and the optical conductivity may provide information on the underlying order parameter structure

Effect of Born and unitary impurity scattering on the Kramer-Pesch shrinkage of a vortex core in a chiral p-wave superconductor

Abstract: We theoretically investigate a non-magnetic impurity effect on the temperature dependence of the vortex core shrinkage (Kramer–Pesch effect) in a chiral p-wave superconductor. The Born limit and the unitary limit scattering are compared within the framework of the quasiclassical theory of superconductivity. We find that the robustness of the Kramer–Pesch effect against the impurity scattering in the Born limit is lost in the unitary limit.

Erratum : Josephson supercurrent through a topological insulator surface state (Nature Materials (2012) 11 (417-421))

Abstract: In the version of this Letter originally published, in Fig. 2b, the label on the y axis should have been 'mΩ'. On page 1, the end of the second paragraph should have read “Now the existence of the topological surface states has been established it is time to study the interaction with other materials. Efforts have been made to contact a topological insulator to a superconductor in the search for the Majorana fermion and supercurrents have been reported22, 23. In this Letter, we report the observation of the two hallmarks of the Josephson effect: a Fraunhofer-like magnetic field modulation of the critical current and the appearance of Shapiro steps on microwave irradiation, resulting from the d.c. and a.c. Josephson effects, respectively. We demonstrate the existence of a Josephson supercurrent in the surface state of the topological insulator Bi2Te3.” These errors have been corrected in the HTML and PDF versions.

Comment on "Unified Formalism of Andreev Reflection at a Ferromagnet/Superconductor Interface"

Abstract: A Comment on the Letter by T. Y. Chen et al., Phys. Rev. Lett. 109, 146602 (2012). The authors of the Letter offer a Reply.