Josephson Tunnel Junctions Research Articles

Lévy noise-induced effects in a long Josephson junction in the presence of two different spatial noise distributions

We analyze the impact of Lévy-distributed stochastic fluctuations on the average switching time and voltage drop across a current-biased long Josephson tunnel junction. We compare the system’s response for two spatial configurations of time-dependent noise, i.e., homogeneous and distributed along the junction length. The response of the Josephson junction is explored by varying the characteristic parameter of the Lévy source, i.e., the α stability index and the noise intensity. These findings offer an effective tool to characterize a Lévy component possibly embedded in an unknown noise signal.

Anodization-free fabrication process for high-quality cross-type Josephson tunnel junctions based on a Nb/Al-AlO x /Nb trilayer

Josephson tunnel junctions form the basis for various superconductor electronic devices. For this reason, enormous efforts are routinely taken to establish and later on maintain a scalable and reproducible wafer-scale manufacturing process for high-quality Josephson junctions. Here, we present an anodization-free fabrication process for Nb/Al-AlO x /Nb cross-type Josephson junctions that requires only a small number of process steps and that is in general intrinsically compatible with wafer-scale fabrication. We show that the fabricated junctions are of very high quality and, compared to other junction types, exhibit not only a significantly reduced capacitance but also an almost rectangular critical current density profile. Our process hence enables the usage of low capacitance Josephson junctions for superconductor electronic devices such as ultra-low noise dc-superconducting quantum interference devices (SQUIDs), microwave SQUID multiplexers based on non-hysteretic rf-SQUIDs and RFSQ circuits.

Superconductivity in Freestanding Infinite-Layer Nickelate Membranes.

The observation of superconductivity in infinite-layer nickelates has attracted significant attention due to its potential as a new platform for exploring high-Tc superconductivity. However, thus far, superconductivity has only been observed in epitaxial thin films, which limits the manipulation capabilities and modulation methods compared to two-dimensional exfoliated materials. Given the exceptionally giant strain tunability and stacking capability of freestanding membranes, separating superconducting nickelates from the as-grown substrate is a novel way to engineer the superconductivity and uncover the underlying physics. Herein, this work reports the synthesis of the superconducting freestanding La0.8Sr0.2NiO2 membranes ( ), emphasizing the crucial roles of the interface engineering in the precursor phase film growth and the quick transfer process in achieving superconductivity. This work offers a new versatile platform for investigating superconductivity in nickelates, such as the pairing symmetry via constructing Josephson tunneling junctions and higher Tc values via high-pressure experiments.

The effect of under and over develop time in Josephson junction fabrication process using electron beam lithography

In the 1960s, the physicist Brian Josephson introduced the concept of Josephson tunnel junctions, which involve the tunneling of Cooper pairs across a barrier between superconducting electrodes. The typical methods of fabricating Josephson junction such as Dolan bridge and cross-type technique require specialized equipment for angle deposition. In this work, we present Josephson junction fabrication process using electron beam lithography that eliminates the need for angle deposition. The dose test was performed to optimize the right dose and we also explored various develop time. We observed that for PMMA A9 film with thickness 1.3 μm, the electron dose 280 μC/cm2 and develop time between 1.30 – 2.00 minutes produces clear pattern with sharp features. The effect of under and over develop time is shown such that there are resist residue left on the substrate after lift-off process and the longer develop time created an undercut.

Tuning the inductance of Josephson junction arrays without SQUIDs

It is customary to use arrays of superconducting quantum interference devices (SQUIDs) for implementing magnetic field-tunable inductors. Here, we demonstrate an equivalent tunability in a (SQUID-free) array of single Al/AlOx/Al Josephson tunnel junctions. With the proper choice of junction geometry, a perpendicularly applied magnetic field bends along the plane of the superconductor and focuses into the tunnel barrier region due to a demagnetization effect. Consequently, the Josephson inductance can be efficiently modulated by the Fraunhofer-type supercurrent interference. The elimination of SQUIDs not only simplifies the device design and fabrication but also facilitates a denser packing of junctions, and, hence, a higher inductance per unit length. As an example, we demonstrate a transmission line, the characteristic impedance of which is field-tuned in the range of 4–8 kΩ, centered around the important value of the resistance quantum h/(2e)2≈6.5 kΩ.

Nonequilibrium Fractional Josephson Effect.

Josephson tunnel junctions exhibit a supercurrent typically proportional to the sine of the superconducting phase difference ϕ. In general, a term proportional to cos(ϕ) is also present, alongside microscopic electronic retardation effects. We show that voltage pulses sharply varying in time prompt a significant impact of the cos(ϕ) term. Its interplay with the sin(ϕ) term results in a nonequilibrium fractional Josephson effect (NFJE) ∼sin(ϕ/2) in the presence of bound states close to zero frequency. Our microscopic analysis reveals that the interference of nonequilibrium virtual quasiparticle excitations is responsible for this phenomenon. We also analyze this phenomenon for topological Josephson junctions with Majorana bound states. Remarkably, the NFJE is independent of the ground state fermion parity unlike its equilibrium counterpart.

In-plane tunnel spectroscopy of cuprate superconductors with nano-scale Josephson junctions

Superconducting tunnel spectroscopy has been used for decades to gather valuable information about the electronic density of states and the energy gap in superconductors. For thin film materials, this has, for the most part, only been possible in directions perpendicular to the substrate. For anisotropic materials like the cuprate superconductors, this has hindered knowledge about the density of states in the a–b plane of the film where superconductivity is strongest. The advent of helium ion beam fabricated planar Josephson junctions has fundamentally changed how we can perform tunnel spectroscopy. In this work, we utilize nanoscale Josephson tunnel junctions orientated along different crystalline directions and report the anisotropy of the superconducting energy gap of cuprates at both the micro- and nanoscale. The smaller nanoelectrodes exhibit more variation that roughly correlates with the granularity of the film. We also observe that the gap energy is inversely proportional to the material's conductivity in the voltage state.

Tunnel Josephson Junction with Spin-Orbit/Ferromagnetic Valve.

We have theoretically studied the transport properties of the SIsNSOF structure consisting of thick (S) and thin (s) films of superconductor, an insulator layer (I), a thin film of normal metal with spin-orbit interaction (SOI) (NSO), and a monodomain ferromagnetic layer (F). The interplay between superconductivity, ferromagnetism, and spin-orbit interaction allows the critical current of this Josephson junction to be smoothly varied over a wide range by rotating the magnetization direction in the single F-layer. We have studied the amplitude of the spin valve effect and found the optimal ranges of parameters.

Phase Control of Bipolar Thermoelectricity in Josephson Tunnel Junctions

Not so long ago, thermoelectricity in superconductors was believed to be possible only by breaking explicitly the particle-hole (PH) symmetry. Recently, it has been theoretically predicted that a superconducting tunnel junction can develop bipolar thermoelectric phenomena in the presence of a large thermal gradient owing to nonequilibrium spontaneous PH symmetry breaking. The experimental realization of the thermoelectric Josephson engine then followed. Here, we give a more extended discussion and focus on the impact of the Josephson contribution on thermoelectricity modulating the Cooper pairs' transport in a double-loop superconducting quantum interference device. When the Cooper pairs' current prevails on the quasiparticle one, the Josephson contribution short circuits the junction thereby screening the thermoelectric effect. We demonstrate that the thermoelectric generation due to the pure quasiparticle transport is phase independent, once Josephson contribution is appropriately removed from the net current measured. At the same time, we investigate an additional metastable state at $V\ensuremath{\approx}0$ determined by the presence of the Josephson coupling, which peculiarly modifies the hysteretic behavior of our thermoelectric engine realized. At the end, we also discuss how the current-voltage characteristics are affected by the presence of multiple thermoelectric elements, which improve the generated output power.

Simulation framework for microwave SQUID multiplexer optimization

So far, performance prediction and optimization of microwave SQUID multiplexers have largely been based on simple approximate analytical models and experimental results. This is caused by the complexity of the underlying physics and the intricacy of operation and readout parameters. As a simplified description can never account for all potential effects occurring in a real device, we have developed a software framework to simulate the characteristics and performance of a microwave SQUID multiplexer. Our simulation framework is a powerful tool to guide understanding and optimization of microwave SQUID multiplexers and other related devices. It includes common readout schemes such as open-loop or flux ramp-modulated readout as well as the nonlinear behavior of Josephson tunnel junctions. Moreover, it accounts for the non-zero response time of superconducting microwave resonators with high loaded quality factors as well as the most significant noise contributions such as amplifier noise, resonator noise, as well as SQUID noise. This ultimately leads to a prediction of device performance that is significantly better compared to simple analytical methods. Using the simulation framework, we discuss first steps toward full microwave SQUID multiplexer optimization and highlight some other applications for which our simulation framework can be used.

Methods to Create Novel La2−xSrxCuO4 Devices with Multiple Atomically Sharp Interfaces

We present methods to create devices that utilize the high-temperature superconductor La2-xSrxCuO4 grown by atomic layer-by-layer molecular beam epitaxy (ALL-MBE). The ALL-MBE synthesis technique provides atomically precise interfaces necessary for the tunnel junctions, Josephson junctions, and dyon detection devices that will be considered. A series of microfabrication processing steps using established techniques are given for each device, and their details are discussed. These procedures are easily extended to generate more complex designs and could be suitable for a wider variety of materials.

Evidence of Long-Range Coherence in Superconducting Networks

Systematic experimental investigations are reported of charge transport in double-comb and star-shaped planar arrays obtained by coupling superconducting islands via Josephson tunnel junctions. The fabrication of the structures is based on a standard niobium trilayer technology with superconducting transition temperature close to 9 K. Noticeable enhancements of Josephson supercurrents and energy gap are recorded. Complementarity between experimental data and theoretical predictions is employed as a tool to clarify the role of different graph topologies in conditioning the singular behavior of synthetic graph-shaped networks. The predictions of the theoretical models are based on the Bose-Einstein topological condensation and a de Gennes-Alexander approach for granular superconductors. Evidence is shown of the dependence, for both gap and Josephson critical currents, on node-to-node connections in the networks, namely on array topology.

Предельные параметры СИС-переходов в теории и технологические возможности их достижения

Tunneling Josephson junctions of the superconductor-insulator-superconductor (SIS) type have a history of more than 50 years, and theoretical estimates of the ultimate parameters of devices for receiving and processing signals based on them look very promising. In practice, in many cases, the actually achieved parameters turn out to be much worse than the theoretical ones, so for niobium SQUIDs the characteristic voltage Vc=IcRn at best reaches 200 µV, and according to theory it should be up to 2 mV. For Terahertz SIS mixers and oscillators, the main problems are a large specific capacitance, hysteresis, and leakage currents. These problems may be related to the morphology and crystal structure of superconductor films. In practice, films are granular, tunnel barriers are nonuniform, the effective area is about 10% of geometric area, leakage currents, parasitic capacitances occur. The crystal structure determines fundamentally different properties of the same elements, for example, for carbon it is diamond, graphite, fullerenes, nanotubes. Important components of a promising superconducting technology are: the use of single-crystal substrates matched in lattice constant and orientation with the grown films, optimization of growth temperature conditions, controlled formation of an oxide or nitride tunnel barrier. One option is to use a Schottky barrier for the semiconductor interlayer instead of a dielectric or normal metal one. This review presents the results of studying films by X-ray diffraction diagnostics, atomic force microscopy, and electron microscopy, showing the main bottlenecks of the existing technology with the deposition of niobium, niobium nitride, and aluminum films on oxidized standard silicon substrates, as well as the results of quasi-epitaxial growth of films on single-crystal substrates at various temperature conditions. Reproducible manufacturing of high-quality tunnel junctions can be achieved by implementing atomically smooth surfaces of tunnel contacts, which will improve the signal and noise characteristics of superconducting devices for receiving and processing information.

Charge–Phase Duality and Cotunneling of Fluxons in SQUID-like Nanorings

Employing charge–flux duality for Josephson junctions and superconducting nanowires, we predict a novel effect of fluxon cotunneling in SQUID-like nanorings. This process is strictly dual to that of Cooper pair cotunneling in superconducting transistors formed by a pairs of Josephson tunnel junctions connected in series. Cooper pair cotunneling is known to lift Coulomb blockade in these structures at low temperatures. Likewise, fluxon cotunneling may eliminate the magnetic blockade of superconducting phase fluctuations in SQUID-like nanorings, driving them into an insulating state.

Temperature-Biased Double-Loop Josephson Flux Transducer

We theoretically study the behavior of the critical current of a thermally-biased tunnel Josephson junction with a particular design, in which the electrodes of the junction are enclosed in two different superconducting loops pierced by independent magnetic fluxes. In this setup, the superconducting gaps can be modified independently through the magnetic fluxes threading the loops. We investigate the response of the device as a function of the magnetic fluxes, by changing the asymmetry parameter, i.e., the ratio between the zero-temperature superconducting gaps $\delta=\Delta_{10}/\Delta_{20}$, and the temperatures of the two rings. We demonstrate a magnetically controllable step-like response of the critical current, which emerges even in a symmetric junction, $\delta=1$. Finally, we discuss the optimal working conditions and the high response of the critical current to small changes in the magnetic flux, reporting good performances of the transducer, with a high transfer function that depends on the operating point and the quality of the junction.

Aluminum-ferromagnetic Josephson tunnel junctions for high quality magnetic switching devices

The competition between superconducting and ferromagnetic orderings in Josephson devices has promoted fundamental and applicative studies of high impact for superconducting digital technology, cryogenic memories, and spintronics, where the possibility of switching between different magnetic states is a crucial advantage. Here, we report on fabrication and characterization of very high quality tunnel ferromagnetic Josephson junctions (JJs) with aluminum electrodes, demonstrating hysteretic behavior of the magnetic field pattern preserving typical transport properties of Al junctions, underdamped behavior, and very low quasiparticle dissipation. The proposed technology paves the way for the possible implementation of Al tunnel-ferromagnetic JJs in superconducting quantum circuits, toward alternative approaches based on digital control of the Josephson device.

Josephson Tunnel Junctions with an Integral Superconductor–Insulator–Normal Metal Shunt

Josephson Tunnel Junctions with an Integral Superconductor–Insulator–Normal Metal Shunt

Microscopic Theory of the Current-Voltage Characteristics of Josephson Tunnel Junctions.

Deep theoretical understanding of the electrical response of Josephson junctions is indispensable regarding both recent discoveries of new kinds of superconductivity and technological advances such as superconducting quantum computers. Here, we study the microscopic theory of the dc current-biased I-V characteristics of Josephson tunnel junctions. We derive an analytical formula of the I-V characteristics of generic junctions. We identify subharmonics of the I-V characteristics and their underlying mechanism as the feedback effect of intrinsic ac currents generated by voltage pulses in the past. We apply our theory to analytically solve the Werthamer equation and describe various dc current-biased I-V characteristics as a function of softening of the superconducting gap. Strikingly, we identify voltage staircases of the I-V characteristics in a genuine Josephson junction without ac current bias or qubit dynamics. Our general analytical formalism opens new avenues for a microscopic understanding of I-V characteristics of Josephson junctions that have been limited to phenomenological models so far.

Josephson junctions based on amorphous MoGe: prospects for use in superconducting electronics

We have fabricated and characterized all-MoGe Josephson junctions with a very thin Al/AlO x /(Al) barrier, where the amorphous MoGe films exhibit superconducting transition temperatures up to 7 K. Due to the uniformity of the surface morphology of the MoGe films, the junctions demonstrate high uniformity of their tunneling properties. The experimental data on the temperature dependence of the subgap current agree well with theoretical calculations. The results obtained imply that Josephson tunnel junctions based on amorphous superconductors are promising candidates for use in superconducting electronics, especially in applications requiring multiple stacked junctions or the creation of a nonequilibrium quasiparticle distribution.

Josephson tunnel junctions with integral SIN shunting

Josephson tunnel junctions with integral SIN shunting