S.V. Bakurskiy

Theoretical model of superconducting spintronic SIsFS devices

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

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

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 supercurrent transport in SIsFS Josephson junctions

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 magnetic rotary valve

We propose a control element for a Josephson spin valve. It is a complex Josephson device containing ferromagnetic (F) layer in the weak-link area consisting of two regions, representing 0 and π Josephson junctions, respectively. The valves state is defined by mutual orientations of the F-layer magnetization vector and boundary line between 0 and π sections of the device. We consider possible implementation of the control element by introduction of a thin normal metal layer in a part of the device area. By means of theoretical simulations, we study properties of the valves structure as well as its operation, revealing such advantages as simplicity of control, high characteristic frequency, and good legibility of the basic states.

Josephson junctions with nonsinusoidal current-phase relations based on heterostructures with a ferromagnetic spacer and their applications

A detailed review of theories describing the current-phase relations in Josephson junctions based on heterostructures with ferromagnetic layers is presented. The particular attention has been focused on the possibilities of making the so-called ϕ-junctions in which the ground state in the absence of the current is realized when the Josephson phase ϕ is nonzero. The recently popular speculations concerning the possibility of applying the Josephson π- and ϕ-junctions, which are made on the basis of heterostructures with ferromagnetic layers, to the formation of quantum bits (qubits) have been illustrated. An attempt to formulate the requirements for the characteristics of the Josephson heterostructures based on the first principles of the quantum-mechanical description of superconducting interferometers with a low inductance has been made.

Current-phase relations in SIsFS junctions in the vicinity of 0- π transition

We consider the current-phase relation (CPR) in Josephson junctions with complex insulator-superconductor-ferromagnetic interlayers in the vicinity of the 0-π transition. We find a strong impact of the second harmonic on the CPR of the junctions. It is shown that the critical current can be kept constant in the region of 0-π transition, while the CPR transforms through multivalued hysteretic states depending on the relative values of tunnel transparency and magnetic thickness. Moreover, the CPR in the transition region has multiple branches with distinct ground states.

Beyond Moore’s technologies: operation principles of a superconductor alternative

The predictions of Moore’s law are considered by experts to be valid until 2020 giving rise to “post-Moore’s” technologies afterwards. Energy efficiency is one of the major challenges in high-performance computing that should be answered. Superconductor digital technology is a promising post-Moore’s alternative for the development of supercomputers. In this paper, we consider operation principles of an energy-efficient superconductor logic and memory circuits with a short retrospective review of their evolution. We analyze their shortcomings in respect to computer circuits design. Possible ways of further research are outlined.

Superconducting phase domains for memory applications

In this work, we study theoretically the properties of S-F/N-sIS type Josephson junctions in the frame of the quasiclassical Usadel formalism. The structure consists of two superconducting electrodes (S), a tunnel barrier (I), a combined normal metal/ferromagnet (N/F) interlayer, and a thin superconducting film (s). We demonstrate the breakdown of a spatial uniformity of the superconducting order in the s-film and its decomposition into domains with a phase shift π. The effect is sensitive to the thickness of the s layer and the widths of the F and N films in the direction along the sIS interface. We predict the existence of a regime where the structure has two energy minima and can be switched between them by an electric current injected laterally into the structure. The state of the system can be non-destructively read by an electric current flowing across the junction.

Anomalous surface states at interfaces in p-wave superconductors

We present the results of theoretical study of surface state properties in a two-dimensional model for triplet p -wave superconductors. We derive boundary conditions for Eilenberger equations at rough interfaces and develop the approach for self-consistent solution for the spatial dependence of

Symmetrical Josephson vortex interferometer as an advanced ballistic single-shot detector

p_x