We experimentally demonstrate that a thin dirty superconducting (S) strip covered by low resistive normal metal (N) approaches closer to the depairing current than a single S strip, which makes its non-linear properties stronger. The obtained result comes from proximity-induced superconductivity in the N layer, its large contribution to the superconducting properties of the SN bilayer and larger sensitivity to the current than that of the host S layer. We argue that such an SN bilayer could be a promising system for different applications based on the current-dependent kinetic inductance. In addition, we also find that in the presence of the N layer the maximal vortex velocity in the resistive state considerably increases.

We performed spectral measurements of two low-temperature discrete Josephson junction (JJ) arrays on a Fourier-transform spectrometer. The arrays contain up to 9000 niobium based junctions located in the area 5 × 5 mm and composed of different designs. The spectrum of each array demonstrates a narrow peak in the frequency range near 140 GHz with the line width 160-170 MHz corresponding to the resolution limit of the spectrometer. It implies the coherent subterahertz radiation from niobium JJ arrays that is necessary for most applications where the JJ array based on similar technology would be implemented. The radiation coherence follows from numerical simulation of Josephson antennas with the self-made algorithm combining the Finite-Difference Time Domain Method with the Resistively and Capacitively Shunted Junction (RCSJ) model. Exploring the spectrum of Beverage-like Josephson antenna simulated by this algorithm we revealed the pure monochromatic emission of all junctions.

Phase-locking of oscillators leads to superradiant amplification of the emission power. This is particularly important for development of THz sources, which suffer from low emission efficacy. In this work we study large Josephson junction arrays containing several thousands of Nb-based junctions. Using low-temperature scanning laser microscopy we observe that at certain bias conditions two-dimensional standing-wave patterns are formed, manifesting global synchronization of the arrays. Analysis of standing waves indicates that they are formed by surface plasmon type electromagnetic waves propagating at the electrode/substrate interface. Thus we demonstrate that surface waves provide an effective mechanism for long-range coupling and phase-locking of large junction arrays.

We study the perspective of thin bilayer strips made of highly resistive (in the normal state) superconductor (S) and low resistive normal metal (N) as a detector of electromagnetic radiation. A small minigap in the quasiparticle spectra, induced in the N layer due to proximity induced superconductivity from the S layer and the ability of low resistive N layer to carry a larger superconducting current than the host dirty superconductor leads to steep temperature dependence of critical current of SN strip in wide temperature ranges. It results in higher sensitivity of current-biased SN strip to the electromagnetic radiation in comparison with S strip. We present experimental results on several microns wide MoN/Cu and MoN strips, which confirm this statement in the bolometric regime. We also discuss possible advantages of single photon counting by narrow SN strips.

We performed experimental investigations of the interaction between separate discrete niobium Josephson junction (JJ) arrays. It was found out that after tuning the arrays to the equal Josephson frequency the amplification of power is registered. The arrays are straight strip lines with 332 JJs oriented in parallel and located on common silicon substrate as well as on different substrates configured in a stack. The effect of amplification is observed at frequencies of up to 204 GHz while the gain relatively to the sum of power from alone radiated arrays can be as large as 3. These results indicate the mutual synchronization of niobium JJ arrays that would forces the submillimeter generation technique based on large JJ arrays with superwavelength dimensions.

This paper presents a brief overview of the current state-of-the-art of Josephson junctions for Quantum-based Voltage Standards fabricated with High-Temperature Superconductors (HTS). A short introduction on the history and technical evolution of Low Temperature Superconductors (LTS) technology is provided for non-specialists. Then HTS technology is summarized and discussed in the context of quantum voltage standard applications. Finally, the two most promising technologies: bicrystal and Focused Helium-Ion Beam junctions are discussed with more detail, analyzing strength, limitations and perspectives in both cases.

Highly disordered thin films of niobium nitride and the specific features of the proximity effect in NbN (S)–aluminum (N) bilayers with a large resistivity ratio, ρNbN/ρAl $$ \gg $$ 1, are studied. It is shown that magnetic screening and the critical current Ic of such SN structures significantly increase compared to the S layer. The observed effect is associated with the induced superconductivity in the N layer due to the proximity effect. The Josephson effect is demonstrated in NbN/Al–NbN–NbN/Al variable thickness bridges made of such SN bilayers.

The temperature dependence of the linear electrodynamic response of thin-film superconductor (MoN)–normal metal (Al) hybrid structures with a high conductivity ratio in the normal state has been theoretically and experimentally investigated. Low-frequency measurements of the coefficient of mutual induction of two coils with a sample placed between them indicate an increase in the magnetic screening of the superconductor–normal metal (SN) structures with an increase in the Al layer thickness dAl near liquid-helium temperatures. Measurements of the frequency shift δf of a microwave dielectric resonator, brought into contact with the sample, as a function of temperature and dAl showed that (i) the character of the dependence δf(T) depends strongly on dAl and (ii) the resonance frequency shift of SN structures at temperatures close to the critical temperature Tc is not described by dependence const/(1 – T/Tc), which is typical of thin superconducting films. Numerical calculations performed within the Usadel model well describe the observed effects. Thus, these anomalies of the electrodynamic properties of SN structures can be explained by the presence of a minigap in the spectrum of quasiparticles due to the proximity effect in a normal-metal layer, which depends on dAl, and by the high conductivity of the Al layer.

Temperature dependence of linear electrodynamic response of thin-film hybrid structures superconductor (MoN) — normal metal (Al) with large ratio of normal-state conductivities was studied theoretically and experimentally. Low-frequency measurements of the mutual inductance of two coils with a sample placed between them indicated an increase in magnetic screening ability of the superconductor – normal metal (SN) hybrid structures at liquid helium temperatures as $d_Al$ increases, where $d_Al$ is the thickness of the Al layer. Measurements of the frequency shift $\delta f$ of the microwave dielectric resonator, which was in contact with the SN samples as a function of temperature and $d_Al$ demonstrated that (i) type of the $\delta f(T)$ dependence depends significantly on $d_Al$ and (ii) the shift of resonant frequency of the SN structures at temperatures close to the critical temperature Tc cannot be approximated by a functional dependence $const/(1–T/T_c)$, which is typical for thin superconducting films. Numerical calculations performed within the Usadel model describe the observed effects quite well. Thus, the mentioned anomalies of the electrodynamic properties of the SN hybrid structures can be explained by an appearance of a mini-gap in the spectrum of quasi-particle excitation caused by the proximity effect in the normal metal layer, which depends on $d_Al$ as well as by the high conductivity of the Al layer.

The highly disordered thin films of niobium nitride and features of the manifestation of the proximity effect in the NbN (S) –Aluminium (N) bilayers with a high ratio of their specific resistances NbN / Al >> 1were investigated. It was shown that the magnetic screening and critical current Ic of such SN structures significantly increase compared with the S layer. The observed effect is associated with the appearance of induced superconductivity in the N layer due to the proximity effect. The Josephson effect was demonstrated in the variable thickness bridges NbN / Al - NbN - NbN / Al made from such SN bilayers.

A commercially available compact Josephson voltage standard N4-21 based on high-temperature superconductor Josephson junctions with dry nitrogen temperature cooling is presented. The cryocooler unit of the voltage standard including the cryogenic system and the high-temperature superconductivity Josephson array chip is described. The measured relative standard uncertainties between output voltages of N4-21 and the primary state voltage standard of Russia are only a few parts in 10 8 . Such low uncertainties are sufficient for the application of voltage standard N4-21 in the highest-level metrology. The implementation of the link between Josephson junctions and Zener diodes largely eliminates the uncertainty contributions associated with Zeners nonlinear drift, environmental effects, and the impact due to shipping.

Mutual synchronization of many Josephson junctions is required for superradiant enhancement of the emission power. However, the larger the junction array is, the more difficult is the synchronization, especially when the array size becomes much larger than the emitted wavelength. Here, we study experimentally Josephson emission from such larger-than-the-wavelength Nb/NbSi/Nb junction arrays. For one of the arrays we observe a clear superradiant enhancement of emission above a threshold number of active junctions. The arrays exhibit strong geometrical resonances, seen as steps in current-voltage characteristics. However, radiation patterns of the arrays have forward-backward asymmetry, which is inconsistent with the solely geometrical resonance (standing-wave) mechanism of synchronization. We argue that the asymmetry provides evidence for an alternative mechanism of synchronization mediated by unidirectional traveling-wave propagation along the array (such as a surface plasmon). In this case, emission occurs predominantly in the direction of propagation of the traveling wave. Our conclusions are supported by numerical modeling of Josephson traveling-wave antenna. We argue that such a nonresonant mechanism of synchronization opens a possibility for phase locking of very large arrays of oscillators.

The principles for building the multivalued N4-21 voltage standard based on the precision properties of Zener diodes and the quantum mechanical properties of high-temperature superconductor Josephson junctions operating at 77 K are discussed. The metrological characteristics of the new voltage standard are presented. The relative uncertainty in the output voltages of the standard at 1 and 10 V is found to be less than 5·10–8.

A commercially available Josephson voltage standard based on high temperature superconductor Josephson junctions with a liquid nitrogen free cooling is presented. The results of the calibrations of an electronic DC reference Fluke 732B and a Keithley nanovoltmeter 2182 with uncertainties equal to a few parts in 108 demonstrate the suitability of the application of a N4-21 in the highest-level metrology. The implementation of a link between Josephson junctions and Zener diodes largely eliminates the uncertainty contributions associated with Zeners non-linear drift, environmental effects and the impact due to shipping.

Developing the earlier results of Aslamazov et al (1986 Soy. Phys. JETP 64 1051) we perform a detailed study of the superconducting properties of a bilayer consisting of highly disordered superconducting (S) and low-resistive normal metal (N) films We demonstrate theoretically and experimentally that the screening and transport properties of such a SN bilayer can be considerably enhanced compared with the single S film. Such modification originates from the proximity-induced superconductivity in the N film and it is not related to enhanced vortex pinning in the SN hybrid. In particular, the coverage of superconducting NbN and MoN films by a thin Ag or Al layer is responsible for enhancing the diamagnetic response and the critical current I-c of the SN microbridge by several times and the appearance of the diode effect in a parallel magnetic field. Due to peculiar temperature dependence of I-c and the magnetic field penetration depth, such SN bilayers can be of potential interest for designing different types of superconducting bolometers and detectors.

We demonstrate, both theoretically and experimentally, that thin dirty superconductor-normal metal bilayer with resistivity of normal metal $ρ_N$ much smaller than normal-state resistivity of superconductor $ρ_S$ has unique superconducting properties. First of all the normal layer provides the dominant contribution to the diamagnetic response of whole bilayer structure in wide temperature interval below the critical temperature due to proximity induced superconductivity. Secondly, the presence of the normal layer may increase the critical current $I_c$ in several times (the effect is not connected with enhanced vortex pinning), provides strong temperature dependence of both $I_c$ and effective magnetic field penetration depth even at temperatures much below the critical one and leads to the diode effect in parallel magnetic field. Besides of general interest we believe that the found results may be useful in construction of different kinds of superconducting detectors.

Исследованы ВТСП бикристаллические переходы шириной до 50 mum на сапфировых подложках. Измерены зависимости величины критического тока от температуры и внешнего магнитного поля для таких контактов. При температуре 77 K получены ступени Шапиро на ВАХ с напряжением порядка 150 muV при облучении на частоте 73 GHz джозефсоновского перехода на сапфировой подложке. Проанализирована возможность использования таких контактов в эталонах напряжения при температуре 77 K в области СВЧ и терагерцовых частот. Авторы благодарят за частичную поддержку гранты РФФИ N 15-02-05793, N 15-42-02469 (р-поволжье), N 16-02-00727, а также грант РНФ N 15-12-10020. В работе использовано оборудование ЦКП "Физика и технология микро- и наноструктур". DOI: 10.21883/FTT.2017.11.45047.06k

HTS bicrystal junctions up to 50 μm wide on sapphire substrates have been studied. The dependences of the critical current on the temperature and external magnetic field of these contacts have been measured. The irradiation of the Josephson junction on a sapphire substrate at the frequency of 73 GHz at the temperature of 77 K resulted in the appearance of Shapiro steps in the current–voltage characteristic (IVC) at the voltage of 150 μV. The possibility of using such contacts in voltage standards at the temperature of 77 K in the ranges of microwave and terahertz frequencies has been analyzed.