V. A. Oboznov

Implementation of superconductor-ferromagnet-superconductor pi-shifters in superconducting digital and quantum circuits

Interrupting a superconducting loop with a thin ferromagnetic film creates a so-called π-Josephson junction that shifts the phase of a current flowing in the loop by 180°. A demonstration of the use of π-junctions in a variety of device structures suggests they could enable the development of a new class of superconducting logic circuits.

Superconductor-ferromagnet-superconductor π-junctions

Superconductor—ferromagnet—superconductor (SFS) π- junctions1 were investigated experimentally. The transitions to π-state and back to the conventional 0-state were found to result from temperature decrease or ferromagnetic interlayer thickness increase. Thickness dependence of the critical current was studied and nonmonotonic behavior was observed. The SF-proximity effect was also examined on SF-bilayers. It is shown that, due to the spatial oscillations of the induced superconducting order parameter in the ferromagnet, the critical temperature of the SF-bilayer has a minimum when the thickness of the ferromagnetic layer is close to a quarter of the oscillation period. PACS numbers: 74.50.+r, 85.25.Cp.

Proximity effect and spontaneous vortex phase in planar SF structures

The proximity effect in SF structures is examined. It is shown that, due to the oscillations of the induced superconducting order parameter in a ferromagnet, the critical temperature of an SF bilayer becomes minimal when the thickness of the ferromagnetic layer is close to a quarter of the period of spatial oscillations. It is found that the spontaneous vortex state arising in the superconductor due to the proximity of the magnetic domain structure of a ferromagnet brings about noticeable magnetoresistive effects.

Phase‐locked flux‐flow Josephson oscillator

We report on the observation of large rf induced steps due to phase‐locking of unidirectional flux‐flow motion in long quasi‐one‐dimensional Josephson junctions. The external microwave irradiation in the frequency range 62–77 GHz was applied from the edge of the junction at which the fluxons enter. The dependence of the amplitude of the phase‐locked step on external magnetic field and microwave power has been measured. The observed zero‐crossing steps have potential application in Josephson voltage standards. A simple model for the flux‐flow as determined by the microwave driven boundary gate at the edge of the junction is presented.

Supercurrents through the superconductor–ferromagnet–superconductor (SFS) junctions

Abstract We have observed and studied critical supercurrents, I c , in cross-type Nb–Cu/Ni–Nb junctions with Josephson layers prepared from diluted ferromagnetic alloys. For suitable values of the exchange energy in the ferromagnet and the F-layer thickness we have observed I c ( T ) oscillation with vanishing I c for definite values of T and d F .

A single flux quantum circuit with a ferromagnet-based Josephson π-junction

We report on the functionality of a Nb-based superconducting single flux quantum (SFQ) toggle flip-flop (TFF) circuit, comprising a complementary superconductor–ferromagnet–superconductor (SFS) Josephson π-junction. The SFS junction was used as a phase shifting element inserted in the storage loop of the TFF. The fabricated circuits demonstrated correct functionality with the operation parameter ranges of ± 20%. The application of SFS π-junctions makes the SFQ circuits very compact, may substantially improve their stability, and may also be suitable for integration with Josephson quantum circuits (qubits).

Experiments with solitons in annular Josephson junctions

Abstract It is known that the long annular Josephson junction allows to avoid the influence of boundaries and soliton(fluxon)- soliton collisions on the dynamics of soliton chains. With new experimental methods and junction design a fully controlable way of introducing individual solitons into such a system is realized. For the homogeneous junction crossover at high velocities of the fluxon chain is seen and explained in terms of the fluxon bunching. In the periodically modulated junction a new resonant mode called a supersoliton is observed.

Imaging spontaneous currents in superconducting arrays of π-junctions

A charge current can flow between two superconductors separated by a thin barrier. This phenomenon is the Josephson effect, which enables a current to tunnel at zero voltage1, typically with no phase shift between the superconductors in the lowest-energy state. Recently, Josephson junctions with ground-state phase shifts of π, proposed by theory three decades ago2, have been demonstrated3,4,5. In superconducting loops, π-junctions cause spontaneous circulation of persistent currents in zero magnetic field2, in analogy to spin-1/2 systems6. Here we use a scanning superconducting quantum interference device microscope7 to image the spontaneous zero-field currents in superconducting networks of temperature-controlled π-junctions with weakly ferromagnetic barriers3. We find an onset of spontaneous supercurrents at the 0–π transition temperature of the junctions, Tπ≈3 K. We image the currents in non-uniformly frustrated arrays consisting of cells with even and odd numbers of π-junctions. Such arrays are attractive model systems for studying the exotic phases of the two-dimensional XY-model8,9 and achieving scalable adiabatic quantum computers10.

Absence of the transition into Abrikosov vortex state of two-dimensional type-II superconductor with weak pinning.

The resistive properties of thin amorphous

Anomalous temperature dependence of H⊥c1 in superconducting Nb/ Cu multilayer

{\mathrm{NbO}}_{x}