Applications In Superconducting Electronics Research Articles

A gate- and flux-controlled supercurrent diode effect

Non-reciprocal charge transport in supercurrent diodes (SDs) has polarized growing interest in the last few years for their potential applications in superconducting electronics (SCE). So far, SD effects have been reported in complex hybrid superconductor/semiconductor structures or metallic systems subject to moderate magnetic fields, thus showing limited potentiality for practical applications in SCE. Here, we report the design and realization of a monolithic device that shows a valuable SD effect by exploiting a Dayem bridge-based superconducting quantum interference device. Our structure allows reaching rectification efficiencies (η) up to ∼6%. Moreover, the absolute value and the polarity of η can be selected on demand by the modulation of an external magnetic flux or by a gate voltage, thereby guaranteeing high versatility and improved switching speed. Furthermore, our SD operates in a wide range of temperatures up to about 70% of the superconducting critical temperature of the titanium film composing the interferometer. Our SD effect can find extended applications in SCE by operating in synergy with widespread superconducting technologies such as nanocryotrons, rapid single flux quanta, and memories.

Effect of the substrate on the electrical transport and fluctuation processes in NbRe and NbReN ultrathin films for superconducting electronics applications

NbRe-based superconducting thin films recently received relevant interest in the field of low-temperature electronics. However, for these materials the electrical conduction mechanisms, in particular in the normal state, still need to be investigated in more detail. Here, NbRe and NbReN films of different thicknesses have been deposited on two different substrates, namely monocrystalline Si and text {SiO}_2 buffered Si. The films were characterized by DC electrical transport measurements. Moreover, a connection with the charge carriers fluctuation processes has been made by analyzing the electrical noise generated in the normal state region. Despite the films morphology seems not to be affected by the substrate used, a lower noise level has been found for the ones grown on text {SiO}_2, in particular for NbReN. From this study it emerges that both NbRe and NbReN ultrathin films are of very good quality, as far as the low-temperature electrical noise and conduction are concerned, with noise levels competitive with NbN. These results may further support the proposal of using these materials in a nanowire form in the field of superconducting electronics.

Critical currents in conventional Josephson junctions with grain boundaries

It has been hypothesized that the variation of the critical currents in Nb/Al–AlOx/Nb junctions is due to, among other effects, the presence of grain boundaries in the system. Motivated by this, we examine the effect of grain boundaries on the critical current of a Josephson junction. We assume that the hopping amplitudes are dependent on the interatomic distance and derive a physically realistic model of distance-dependent hopping amplitudes. We find that the presence of a grain boundary and associated disorder is responsible for a very large drop in the critical current relative to a clean system. We also find that when a tunnel barrier is present, grain boundaries cause substantial variations in the critical currents due to the disordered hoppings near the tunnel barrier. We discuss the applicability of these results to Josephson junctions presently intended for use in superconducting electronics applications.

Phase-dependent noise in Josephson junctions

In addition to the usual superconducting current, Josephson junctions (JJs) support a phase-dependent conductance related to the retardation effect of tunneling quasi-particles. This introduces a dissipative current with a memory-resistive (memristive) character that should also affect the current noise. By means of the microscopic theory of tunnel junctions we compute the complete current autocorrelation function of a Josephson tunnel junction and show that this memristive component gives rise to both a previously noted phase-dependent thermal noise, and an undescribed non-stationary, phase-dependent dynamic noise. As experiments are approaching ranges in which these effects may be observed, we examine the form and magnitude of these processes. Their phase dependence can be realized experimentally as a hysteresis effect and may be used to probe defects present in JJ based qubits and in other superconducting electronics applications.

Point-contact spectroscopy in MgB2: from fundamental physics to thin-film characterization

In this paper we highlight the advantages of using point-contact spectroscopy (PCS) in multigap superconductorslike MgB2, both as a fundamental research tool and as a non-destructive diagnostictechnique for the optimization of thin-film characteristics. We first presentsome results of crucial fundamental interest obtained by directional PCS inMgB2 single crystals, for example the temperature dependence of the gaps and of thecritical fields, and the effect of a magnetic field on the gap amplitudes. Then, weshow how PCS can provide useful information about the surface properties ofMgB2 thin films(e.g., Tc, gap amplitude(s), clean- or dirty-limit conditions) in view of their optimization for thefabrication of tunnel and Josephson junctions for applications in superconductingelectronics.

MgB2 junctions and SQUIDs fabricated by focused ion beam

We have recently developed a technique for fabricating SNS junctions in MgB2 thin films using a focused ion beam. These junctions show a strong modulation of the critical current by applied magnetic field and microwaves. They also show large IcRN products in excess of 1 mV at 4.2 K making them attractive candidates for a range of superconducting electronics applications. We have made SQUIDs with directly coupled pick-up loops using this technique which show large voltage modulations (175 μV at 10 K) and noise comparable to HTS SQUIDs. Here we will present an overview of our devices along with our latest results.

Direct writing of low Tc superconductor-normal metal-superconductor junctions using a focused ion beam

Using a focused ion beam, we have produced superconductor-normal metal-superconductor junctions by controllably removing a portion of the top layer of a patterned superconductor-normal metal thin film. The high-quality junctions showed Josephson coupling which scaled qualitatively with barrier properties and temperature as expected. The largest product of a junction’s critical current and the normal state resistance measured is 98 μV at 4.2 K. The method has good reproducibility and could be exploited in a number of superconducting electronics applications.

Hot electron tunable supercurrent

A Josephson junction has been realized in which the supercurrent flow is regulated by a “normal” control current traversing the normal metal in between the superconducting electrodes. The principle of operation of the devices is based on the existing relation between the magnitude of the supercurrent and the electronic distribution function in the junction. This method for controlling the supercurrent has clear advantages over other known methods and is relevant for superconducting electronics applications.

Upper critical field measurement and penetration depth determination for superconducting NbCN films

High T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> , low resistivity NbCN films were prepared by rf sputtering in a reactive gas mixture of Ar, N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> and cyanogen. Upper critical field for parallel and perpendicular orientation of the films to the applied field were measured systematically from T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> down to 1.5 K. The critical field anisotropy is anomalous i.e., <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H\min{c2}\max{\perp} &gt; H\min{c2}\max{\parallel}</tex> for films sputtered at lower substrate temperature, whereas it shows normal behavior for films deposited at T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</inf> ∼ 1100°C. This behavior is similar to that in sputtered NbN. The measured critical current in the presence of an applied magnetic field is lower than that in NbN of comparable properties. The penetration depth, λ(0), was determined by measuring the self inductance of these NbCN films using it as an inductor in an LC oscillator circuit driven by a tunnel diode. It appears that NbCN films have lower λ(0) than sputtered NbN films. The fact that NbCN films have higher T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> , lower ρ and lower λ(0) makes this a somewhat better superconductor for superconducting electronics applications.