Superconducting devices based on coherent operation of Josephson junction arrays above 77K

Arrays of Josephson junctions operating coherently at 77K seem to be the ideal candidates for re-shaping the future of the electronics industry. Their advantages over their semiconducting counterparts (higher operating speed, lower power consumption/electronic noise) can be exploited in practice because of their practicality: cooling down to 77K is both cheap and easy to handle. These developments naturally fit into the increasing interest of some semiconductor technologies in lower operation temperatures. Therefore, it looks attractive to link semiconductor and 77K superconductor technologies to improve system level performance. Currently, however, in the vast majority of applications 4.2K superconducting technology is used as it provides significantly superior performances relative to devices build in the 77K technology. Recent significant improvements in the performance of superconducting devices operating as 77K, as well as, in their fabrication technologies may change all that. Several such examples will be considered here. Firstly, when coherency is achieved in large series SQUID-arrays magnetic flux sensors or voltage amplifiers can be build having record values for their output voltage and flux noise sensitivities outperforming even single SQUID-based devices operating at 4.2 K. Secondly, when coherency is achieved in parallel SQUID-arrays placed in a uniform magnetic field B, B-field tuned microwave generators can be build. Large parallel SQUID arrays were also implemented to achieve record values for current amplification at 77K, highly efficient ratchets with unidirectional magnetic vortices motion and integrated nano-magnetic sensors

2D SQIF arrays using 20,000 YBCO high Rn Josephson junctions, a viewpoint

This is a viewpoint on the letter by E E Mitchell et al (2016 Supercond. Sci. Technol. 29 06LT01)

Dual flux-to-voltage response of YBa2Cu3O7−δ asymmetric parallel arrays of Josephson junctions

We fabricated a parallel array of 440 YBa2Cu3O7−δ bicrystal grain boundary Josephson junctions having an inductive asymmetric loop configuration within the array. Families of current–voltage characteristics (IVCs) have been measured in the temperature range (4.7–92) K for various values of a magnetic flux applied via a control current Ictrl. For both positive and negative current biases, I current-driven chains of magnetic vortices are propagating along the array producing flux-flow current resonances on the IVCs. However, at 77 K and above, due to the system's inductive asymmetry the flux flow is suppressed (enhanced) for negative (positive) I. Consequently, the system shows a dual flux-to-voltage response. For negative I it operates like a flux-interferometer having a rather sinusoidal V (Ictrl) response. In contrast, for positive I the device's response V (Ictrl) remains periodic but highly non-sinusoidal due to the interplay between multiple flux-flow modes. Below 60 K such a dual behaviour is far less pronounced as a result of flux-flow modes being suppressed due to a decrease of the dissipation coefficient with temperature

Flux-coherent series SQUID array magnetometers operating above 77 K with superior white flux noise than single-SQUIDs at 4.2 K

A very promising direction to improve the sensitivity of magnetometers based on superconducting quantum interference devices (SQUIDs) is to build a series-array of N non-interacting SQUIDs operating flux-coherently, because in this case their voltage modulation depth, ΔV, linearly scales with N whereas the white flux noise SΦ 1/2 decreases as 1/N1/2. Here, we report the realization of both these improvements in an advanced layout of very large SQUID arrays made of YBa2Cu3O7. Specially designed with large area narrow flux focusers for increased field sensitivity and improved flux-coherency, our arrays have extremely low values for SΦ 1/2 between (0.25 and 0.44) μΦ0/Hz1/2 for temperatures in the range (77-83) K. In this respect, they outperform niobium/aluminium trilayer technology-based single-SQUIDs operating at 4.2 K. Moreover, with values for ΔV and transimpedance in the range of (10-17) mV and (0.3-2.5) kΩ, respectively, a direct connection to a low-noise room temperature amplifier is allowed, while matching for such readout is simplified and the available bandwidth is greatly increased. These landmark performances suggest such series SQUID arrays are ideal candidates to replace single-SQUIDs operating at 4.2 K in many applications. © 2015 AIP Publishing LLC

Amplification of electromagnetic waves excited by a chain of propagating magnetic vortices in YBaCuO Josephson-junction arrays

Theory shows that when a soliton propagates in a discrete lattice it excites small-amplitude linear waves in its wake. In a dc current-biased Josephson-junction (JJ) array these manifest as electromagnetic (EM) waves excited by a (magnetic field induced) chain of propagating magnetic vortices. When the vortex velocity and the phase velocity of one of the excited EM waves match, phase-locking occurs. This results in the amplification of the EM radiation. We report the first observation of phase-locking-induced amplification of EM radiation at 77K and above in JJ arrays made of high temperature superconductors

Parallel array of YBa2Cu3O7−δ superconducting Josephson vortex-flow transistors with high current gains

Parallel array of YBa2Cu3O7−δ superconducting Josephson vortex-flow transistors with high current gain

Controlling Josephson dynamics by strong microwave fields

We observe several sharp changes in the slope of the current-voltage characteristics CVCs of thin-film ramp-edge Josephson junctions between YBa2Cu3O7− and Nb when applying strong microwave fields. Such behavior indicates an intriguing Josephson dynamics associated with the switching from a parametric excitation regime induced by the magnetic field of the microwave via oscillations of the Josephson critical current to an ac-current-excitation regime triggered by the electric field of the microwave. We propose a model, which describes the observed features on the CVC in terms of microwave-induced multiple switching between running and locked solutions of sine-Gordon equation

dc magnetometry of niobium thin film superconductors deposited using high power impulse magnetron sputtering

We performed a systematic investigation of the dc magnetic properties of superconducting niobium thin films deposited by high power impulse magnetron sputtering (HiPIMS) as a function of the main deposition parameters: the temperature, T, of the heated substrate and the applied dc bias voltage, V, during the sputtering process. The measured dc magnetization curves between 0 and 1000 mT were used to calculate the relative volume of each sample into which the applied magnetic field had penetrated, ðΔV=VÞM. The sample deposited at 700°C with −80 V biased substrate exhibited the least penetration by the magnetic field. ðΔV=VÞM appeared to be highly dependent on the bias voltage at both room temperature and 500°C; however, a broad range of bias voltages showed comparatively similar results at increased temperatures of 700°C. Samples deposited at 700°C exhibit smaller upper critical fields, HC2, than samples deposited at room temperature and 500°C, with the lower temperatures exhibiting a greater dependency on the applied bias. The films deposited at 700°C also display a more stable magnetization curve suggesting that an enhanced flux pinning was achieved when compared to lower temperatures. Consequently, films with stable pinning were found to have the most repeatable dc magnetic behavior. Our results are particularly relevant to the superconducting radio-frequency accelerator scientific community where thin films have been suggested as a technology which may ultimately surpass the performance of bulk niobium. They are also relevant to the fundamental area of superconducting thin films and any applied area where thin films produced by HiPIMS are used, such as superconducting electronics

DC magnetism of Niobium thin films

Niobium thin films were deposited onto a-plane sapphire with varying kinetic energy and varying substrate temperature. There were no consistent trends which related the particle energy or substrate temperature to RRR. The sample which displayed the largest RRR of 229 was then compared to both a thin film deposited with similar conditions onto copper substrate and to bulk niobium. DC magnetometry measurements suggest that the mechanism of flux entry into thin film niobium and bulk niobium may vary due to differences in the volumes of both defects and impurities located within the grains. Results also suggest that magnetic flux may penetrate thin films at small fields due to the sample geometry

Physical vapour deposition of thin films for use in superconducting RF cavities

The production of superconducting coatings for radio frequency cavities is a rapidly developing field that should ultimately lead to acceleration gradients greater than those obtained by bulk Nb RF cavities. Optimizing superconducting properties of Nb thin-films is therefore essential. Nb films were deposited by magnetron sputtering in pulsed DC mode onto Si (100) and MgO (100) substrates and also by high impulse magnetron sputtering (HiPIMS) onto Si (100), MgO (100) and polycrystalline Cu. The films were characterised using scanning electron microscopy, x-ray diffraction and DC SQUID magnetometry