Microwave-induced Shapiro Steps Research Articles

Existence of Shapiro Steps in the Dissipative Regime in Superconducting Weak Links

We present measurements of microwave-induced Shapiro steps in a superconducting nanobridge weak link in the dissipative branch of a hysteretic current-voltage characteristic. We demonstrate that Shapiro steps can be used to infer a reduced critical current and associated effective local temperature. Our observation of Shapiro steps in the dissipative branch hows that a finite Josephson coupling exists in the dissipative state and thus can be used to put an upper limit on the effective temperature and on the size of the region that can be heated above the critical temperature. This work provides evidence that Josephson behaviour can still exist in thermally-hysteretic weak link devices and will allow extension of the temperature ranges that nanobridge based single flux quantum circuits, nanoSQUIDs and Josephson voltage standards can be used.

Nanoscale detection of metastable states in porous and granular media

Microseismicity in subsurface geologic environments, such as sandstone gas reservoirs, is expected in the presence of liquid or gas injection. Although difficult to predict, the potential for microseismic events is important to field-scale projects, such as geologic storage of CO2, whereby the gas is injected into natural sandstone formations. We conjecture that a primary factor causing microseismicity is the existence of metastable states in a granular porous medium and provide experimental evidence for its validity. External perturbation triggers abrupt relaxation events which, with a certain probability, can grow into macroscopic microseismic events. Here, the triggering perturbation is produced by cooling to a cryogenic temperature. As the “sensor” for the abrupt relaxation events, we use thin Al films deposited on the sandstone surface. We show that as the temperature is varied, the films' resistance exhibits sharp jumps, which we attribute to mechanical restructuring or microfractures in the fabric of the sandstone. We checked the superconducting characteristics of the Al thin films on the sandstone and found microwave-induced Shapiro steps on the voltage–current diagrams. Such quantized steps provide indications that the film is made of a network of nanobridges, which makes it even more sensitive to abrupt relaxation events occurring in the substrate, i.e., in the underlying sandstone.

High-quality in situ fabricated Nb Josephson junctions with black phosphorus barriers

Owing to appealing physical properties such as broad tunability in bandgaps and structural anisotropy, black phosphorus (BP) holds great potential in exploring novel electronic devices. However, it is extremely challenging to use BP to fabricate electronic devices, since it is prone to deteriorate in air. To address this challenge, we demonstrate an in situ fabrication technique which enables us to minimize interfacial degradation and to fabricate vertical Josephson junctions by employing few-layer BP as a barrier between two closely spaced Nb electrodes. The current–voltage characteristics of the junctions are hysteretic at low temperatures and become nonhysteretic when approaching the junction critical temperature. In the resistive state the differential conductance increases with decreasing voltage. Microwave-induced Shapiro steps were observed, confirming the presence of the ac Josephson effect. We present different models to analyze the current–voltage characteristics and conclude that resistive state of the current–voltage characteristics points to a zero bias anomaly, which is presumably caused by Andreev reflections. Our in situ fabrication technique represents a viable way to incorporate air-unstable materials for electronics and offer a chance to explore their unique functionalities.

Investigation of Dayem Bridge NanoSQUIDs Made by Xe Focused Ion Beam

Superconducting QUantum Interference Devices (SQUIDs) based on nanobridge junctions have shown increasing promise for single particle detection. This paper describes the development of the fabrication of improved and reproducible nanobridge junctions fabricated by focused ion beam (FIB) milling from niobium thin films. Although the very low noise properties of nanobridge SQUIDs are well known, the nature of the milling process is little understood at the level of local superconducting properties. In this paper, we report the results for nanobridge Josephson devices and SQUIDs, which we believe are the first to be made by Xenon (Xe) FIB milling. Temperature-dependent current–voltage behavior, microwave-induced Shapiro steps, and SQUID response to magnetic fields have been measured. We make preliminary comparisons with nominally identical devices milled from Nb thin films using either Xe or Ga ions.

Weak link nanobridges as single flux quantum elements

This paper investigates the feasibility of using weak link nanobridges as Josephson junction elements for the purpose of creating Josephson circuits. We demonstrate the development of a single-step electron beam lithography procedure to fabricate niobium nanobridges with dimensions down to . The single-step process facilitates fabrication that is scalable to complex circuits that require many junctions. We measure the IV-characteristics (IVC) of the nanobridges between temperatures of and and find agreement with numerical simulations and the analytical resistively shunted junction (RSJ) model. Furthermore, we investigate the behaviour of the nanobridges under rf irradiation and observe the characteristic microwave-induced Shapiro steps. Our simulated IVC under rf irradiation using both the RSJ model and circuit simulator JSIM are in agreement with the experimental data. As a potential use of nanobridges in circuits requiring many junctions, we investigate the theoretical performance of a nanobridge-based Josephson comparator circuit using JSIM.

NB/AL2O3/AL/MGB2 LARGE AREA THIN FILMS HETEROSTRUCTURES: POSSIBLE OBSERVATION OF TUNNELING FROM BOTH DIRTY AND CLEAN LIMIT MGB2

We report the demonstration of dc and ac Josephson effects as well as conductance spectra measurements on Nb/Al2O3/Al/MgB2 thin films heterostructure. The heterostructure exhibits moderately hysteretic current-voltage characteristic with a dc Josephson current branch and regular microwave-induced Shapiro steps. Conductance spectra measurements point toward a two-gap scenario and are explained with the probable presence of tunneling both from dirty limit regions, reflecting a proximized order parameter Δdirty ≈ 2 meV , and from clean limit regions, reflecting a proximized order parameter Δ3D ≈ 0.9 meV .

Josephson effect in Nb/Al2O3/Al/MgB2 large-area thin-film heterostructures

We report the demonstration of dc and ac Josephson effects in Nb/Al2O3/Al/MgB2 thin-film heterostructure. The heterostructure exhibits moderately hysteretic current–voltage characteristic with a dc Josephson current branch and regular microwave-induced Shapiro steps. From conductance spectrum, a gap of about 2 meV at 7.7 K is estimated for the proximized surface of MgB2 electrode.

Focused electron beam damaged YBCO Josephson junctions for THz device applications

We investigate the high-frequency properties of focused electron beam (FEB) damaged Josephson junctions for THz device applications. The FEB damaged YBCO junctions exhibit the resistively shunted junction (RSJ)-like current-voltage (I-V) curve and the microwave-induced Shapiro steps for all operation temperatures. At 4.2 K, the junctions exhibited the microwave-induced Shapiro steps up to 3 mV in dV/dI-V curves suggesting that the junctions can respond up to about 1.5 THz. To estimate the high frequency performance of the junctions, direct irradiation by a far infrared (FIR) laser at 0.76 THz is carried out and the clear first Shapiro step is observed for the junctions with I/sub C/R/sub N/ product of about 2 mV and at the operation temperature of 6 K.

Intrinsic Josephson junctions in oxygen-deficient YBa2Cu3O7−δ film deposited on a substrate step

Using c-axis-oriented oxygen-deficient YBa2Cu3O7−δ film deposited across a low-angle step on a SrTiO3 substrate, we successfully demonstrated intrinsic Josephson effects. In addition to several voltage jumps of large amplitudes (a few millivolts) and remarkable hysteresis on the dc current–voltage curves, we observed upturns on the current–voltage curves under microwave irradiation which appeared at increasingly high voltages with increasing microwave power. We proposed to explain this observation in terms of high-order microwave-induced Shapiro steps.

Josephson junction arrays on Si membrane using focused electron beam irradiation

Series arrays of two Josephson junctions were fabricated by focused electron beam (FEB) irradiation on YBCO films. The arrays show single junction like behavior with the coherent oscillations of the Shapiro steps in a array up to 2.8 mV. Microwave-induced Shapiro steps correspond to the double voltages Vn′=2NVj where VJ=f°h/2e in two-junctions arrays. The microwave power dependence of I–V curves shows the coherently oscillating steps corresponding to the resistively shunted junction (RSJ) model.

Superconducting YBa 2Cu 3O 7- x based edge junctions with Y 0.1Ca 0.3Ba 2Cu 3O 7- x barriers

High-temperature superconductor YBa 2cU 3O 7- x (YBCO) Josephson junctions were fabricated in an edge junction geometry using Y 0.7Ca 0.3Ba 2O 7- x as a barrier material. The films were deposited by pulsed-laser deposition and patterned by argon ion milling. The edge morphology of the bottom electrode after ion milling was investigated by atomic force microscopy. The yield of devices in terms of supercurrent continuity is nearly 100%. Critical current densities exceeding 10 6 A/cm 2 at 75 K were obtained through crossovers of YBCO/YBCO under our processing conditions. Resistively shunted junction current-voltage characteristics and microwave-induced Shapiro steps were observed in a wide temperature range from 75 K to 84 K for the junctions.

High quality YBa2Cu3O7 Josephson junctions made by direct electron beam writing

High-Tc Josephson junctions have been fabricated by direct electron beam writing over YBa2Cu3O7 thin-film microbridges, using scanning transmission electron microscope (STEM) with an accelerating voltage of 80–120 kV. Annealing at 330–380 K increases Tc and Ic of the junctions and makes them more stable. In the operating range of a few degrees below Tc, the junctions show 100% magnetic field modulation of the critical current, microwave-induced Shapiro steps oscillating according to the resistively shunted junction (RSJ) model, and RSJ current-voltage characteristics with IcRn product up to 0.5–0.6 mV at 75 K and 0.3 mV at 77 K.

Josephson effects and dc SQUID behaviour in break junction of Y-Ba-Cu-O at 77 K

Break junctions of Y-Ba-Cu-O superconductor were realized by creating fresh crack in the bar shape bulk samples mounted in a specially designed probe kept in the liquid nitrogen bath. While the bulk sample was kept static, a sharp tip of a shaft was moved downward from the outside of the dewar in order to break the sample. It was possible to change the critical current of the break junctions by adjusting pressure on the tip of the shaft. The effects of magnetic field and microwave power on the I-V characteristic of the junction were studied. The junction was found to show periodic V-B behaviour and the microwave-induced Shapiro steps were clearly observed at 77 K. The flux noise spectrum of the junction was also studied.

Grain boundary Josephson effect in constricted Bi-Sr-Ca-Cu-O

The current-voltage characteristics for Bi-Sr-Ca-Cu-O bulk specimen incorporating manually carved constriction show both zero voltage pair current and quasiparticle tunnelling current. The pair current amplitude modulates with applied d.c. magnetic fields. Microwave-induced Shapiro steps are clearly seen in the I-V curves. The Josephson effects arise due to the presence of arrays of grain boundary Josephson junctions.

Josephson effects in a constricted Bi-Sr-Ca-Cu-O bulk superconductor

The current-voltage characteristics for a Bi-Sr-Ca-Cu-O bulk specimen incorporating a manually carved constriction show both zero-voltage and finite-voltage pair currents, flux flow and quasi-particle tunnelling regimes. The finite voltage pair current regime may be understood to arise due to non-equilibrium enhancement of Josephson supercurrents. The pair current amplitude modulates with applied DC magnetic fields. Microwave-induced Shapiro steps are clearly seen in the I-V curves. The Josephson effects arise due to the presence of three-dimensional arrays of inter-grain junctions.