N. V. Klenov

Bi-SQUID: a novel linearization method for dc SQUID voltage response

We show that the voltage response of a dc SQUID can be substantially linearized by the introduction of a nonlinear inductance. The inductance tuning allows us to achieve response linearity close to 120 dB. Such a nonlinear inductance can be easily formed using Josephson junction inductance. The additional junction and main inductance form a single-junction SQUID and hence the device can be called a bi-SQUID. To obtain high dynamic range commensurate to the high response linearity, one can use a serial array of nonlinear inductance dc SQUIDs. Experimental studies of a single bi-SQUID and serial arrays of bi-SQUIDs are reported and discussed.

Active Electrically Small Antenna Based on Superconducting Quantum Array

We introduce Superconductive Quantum Arrays and propose to use these structures as active Electrically Small Antennas (ESA). Several prototypes of the active ESA were fabricated using Nb process with a critical current density of 4.5 kA/cm2 and experimentally evaluated. The magnetic field to voltage transfer function linearity up to 70 dB was measured, and transfer factor dV/dB up to 6.5 mV/μT was observed for the ESA prototype containing 560 cells.

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

Linear Bi-SQUID Arrays for Electrically Small Antennas

Recently we proposed so-called bi-SQUID based on a 3-junction SQUID circuit capable of providing highly linear voltage response. In this report, we present the experimental evaluation of series arrays of 20 and 128 bi-SQUIDs fabricated with a 4.5 kA/cm2 Nb process as well as a prototype of an active electrically small antenna based on series array of 12 bi-SQUIDs. Both the origins of imperfections of the observed response linearity and the possible ways of the linearity improvement are discussed.

Design and Experimental Evaluation of SQIF Arrays With Linear Voltage Response

Differential circuits consisting of two series arrays of 10-junction parallel SQIFs were developed, designed and fabricated with 4.5 kA/cm2 Nb HYPRES process. The differential voltage response evolution with applied magnetic field providing opposite frustration of the serial arrays was analysed in detail. Linear differential response with amplitude as high as 22 mV was observed for the serial arrays of 108 parallel SQIFs. It was shown that the response linearity is kept within some range of the applied frustrating magnetic field.

Performance Advantages and Design Issues of SQIFs for Microwave Applications

We consider applications of SQIFs as amplifiers for gigahertz frequency range. SQIF-like structures are able to provide much higher dynamic range and linearity than a dc SQUID. We also analyse design limitations imposed by finite coupling inductances and stray capacitances. Possible ways of resolving design issues are discussed.

High Linearity SQIF-Like Josephson-Junction Structures

Recently we reported design approaches for the synthesis of multi-SQUID structures capable of providing high linearity voltage response. These structures were developed to form periodic voltage responses. This paper presents possible design solutions for multi-element structures providing SQIF-like high linearity voltage response. The approach is based on the use of a differential scheme of two parallel SQIFs with arrays oppositely frustrated by applied magnetic field deltaB. The differential scheme enables a high efficiency synthesis of highly linear SQIF response by subtraction of deviations from linear law.

Development of SQIF-Based Output Broad Band Amplifier

High-performance single flux quantum (SFQ) pulse amplifier (driver) based on superconducting quantum interference filter (SQIF) or near regular array of SQUIDs has been developed, fabricated, and tested. The driver part coupling method and circuit optimizations are discussed. The first test results of the driver prototype are reported. The experimental results confirm performance advantages of this driver design approach.

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.