Critical Current-normal Resistance Product Research Articles

Unconventional long range triplet proximity effect in planar YBa2Cu3O7/La0.7Sr0.3MnO3/YBa2Cu3O7 Josephson junctions

The proximity effect between superconductors and ferromagnets may become long range due to the generation of triplet pairs. The recent finding of a long, one micron-range unconventional Josephson effect between YBa2Cu3O7 high Tc cuprates separated by a half metallic La0.7Sr0.3MnO3 manganite ferromagnet has uncovered a novel unconventional triplet proximity effect. In this paper, we examine the temperature dependence of the critical current in planar Josephson junctions. We find that the critical current—normal resistance product follows the predictions of traditional superconductor-normal metal-superconductor junctions, which implies that triplet pairs in a ferromagnet are transported in the diffusive limit similarly to singlet pairs in a normal metal. This result calls for theoretical studies of the new triplet Josephson effect and underlines its potential in future superconducting spintronics.

Nb/yttria-stabilized-zirconia/Nb nonhysteretic Josephson junctions

Yttria-stabilized zirconia (YSZ) is used as an insulating barrier for the fabrication of niobium edge junctions. Using the ion beam assisted deposition technique, Nb/YSZ/Nb edge junctions with YSZ thin film of a few nanometers in thickness have been fabricated. The junctions are nonhysteretic with critical current densities up to 4 mA/μm2 and critical current normal resistance products of ∼1 mV.

Interface-engineered Josephson junctions with high IcRN products

Interface-engineered Josephson junctions were fabricated using high-temperature superconducting films, and large characteristic voltages were obtained in sub-micrometer junctions. The original 2 μm -wide junctions had large critical current densities and showed wide-junction effects as the temperature was reduced below 70 K. When the junctions were narrowed to sub-micrometer width they showed characteristics with greatly reduced wide-junction effects. Critical current–normal resistance products were over 5 mV at 4.0 K, and we observed Shapiro steps under irradiation at 93.6 GHz that were consistent with the resistively-shunted junction model including thermal noise.

Investigation for the basic characteristics of HTS SFQ logic gate

The characteristics of a simple logic gate which consists of an rf-Superconducting QUantum Interference Device (SQUID) and dc-SQUID has been investigated in connection with the Single-Flux-Quantum (SFQ) logic operation and Josephson junction characteristics. The Josephson junctions were made of Neodymium Barium Copper Oxide thin film employing Focused Ion Beam technology. We confirmed the generating SFQ in the rf-SQUID and simultaneous read-out by the dc-SQUID up to 50 K. It has been found that the temperature dependence of output voltage level decreased more rapid than that of critical current normal resistance product. The origin of the discrepancy was studied using a phenomenological model of the Josephson junction and numerical simulation.

Critical current–gate voltage characteristics in short- and long-gated Josephson junctions

We studied the gate controllability of the critical current and the normal resistance in superconductor–semiconductor–superconductor junctions. The junctions used a two-dimensional electron gas (2DEG) in the InAs-inserted InAlAs/InGaAs heterostructure. It is shown that the interface barrier between the superconductor and the 2DEG affects the controllability in a short-gated junction. In a split-gated junction, the critical current–normal resistance product is almost constant against gate voltage. This is due to quantization of both the critical current and the conductance in a narrow and short semiconductor channel. The long-gated junction in the quasi-ballistic transport regime shows rapid suppression of the critical current by gate voltage.

High T/sub c/ superconducting step-edge junctions and their properties in millimeter-wave band

Superconducting step-edge junctions were fabricated on step-edges in (100) LaAlO/sub 3/ substrates by sputter-deposited epitaxial YBaCuO and TlBaCaCuO thin films. The substrate steps were patterned by ion beam etching using metal film as a mask. The transport properties of the junctions were characterized by four-probe electrical measurements. The critical current-normal resistance product (I/sub c/R/sub N/) was larger than 0.2 mV (77 K) for most of the junctions. Clear Shapiro steps were observed in the current-voltage characteristics of the junctions with 36 GHz or 95 GHz millimeter-wave irradiation. The high frequency properties of these high T/sub c/ Josephson junctions showed that they were quite suitable for the Josephson harmonic mixer used in the digital phase locked millimeter and sub-millimeter wave sources.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Step-edge and stacked-heterostructure high-T/sub c/ Josephson junctions for voltage-standard arrays

We have explored two high-transition-temperature Josephson junction technologies for application in voltage standard arrays: step-edge junctions made with YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// and Au normal-metal bridges, and stacked series arrays of Josephson junctions in selectively doped, epitaxially grown Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8/ heterostructures. For both kinds of junctions, Shapiro steps induced by a microwave bias were characterized as a function of power. We compare the technologies with respect to critical current and normal resistance uniformity, maximum achievable critical current, critical-current normal-resistance product, and operating temperature.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

YBa2Cu3O7−δ Josephson junctions on directionally ion beam etched MgO substrates

Shallow, 1–40 nm deep steps were ion beam milled at 60° angle of incidence to the normal of the MgO substrate in order to form Josephson junctions. Microbridges were patterned across the step boundary in films of YBa2Cu3O7−δ. They displayed critical current densities and critical current-normal resistance products that, on the whole, decreased with increasing step height, leveling off at about 5 nm. Transmission electron microscopy showed the presence of 45° [001]-tilt regions in the films close to the steps. Interferometers (dc-SQUIDs) showed magnetic field response at temperatures above 77 K and a noise level of about 1.5×10−8 Φ02/Hz at 10 Hz and 4.2 K.

Hysteretic YBa/sub 2/Cu/sub 3/O/sub 7- delta /-Au- YBa/sub 2/Cu/sub 3/O/sub 7-/ delta proximity effect Josephson junctions

The superconducting proximity effect can be used to produce Josephson junctions in high-temperature superconducting structures with a normal conducting carrier. The authors describe work on edge-microbridge junctions made by defining 10-30- mu m-wide Au bridges to link the upper and lower layers of a YBa/sub 2/Cu/sub 3/O/sub 7- delta /Y/sub 2/O/sub 3/-YBa/sub 2/Cu/sub 3/O/sub 7- delta / trilayer. The structures are patterned by photolithography and ion milling from multilayers grown in situ by electron-beam evaporation. The best junctions operate up to around 20 K, exhibit hysteresis in their current-voltage characteristics, and have an I/sub c/R/sub N/ (critical-current normal-resistance product) greater than 200 mu V at 4.2 K. With a 400-nm-long microbridge, the maximum junction T/sub c/ observed was 25 K, and the largest I/sub c/R/sub N/ product at 4.2 K was 400 mu V.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High-T C superconductor-normal metal-superconductor Josephson microbridges with high-resistance normal metal links

We have developed an in situ process for fabricating high transition temperature superconductor-normal metal-superconductor microbridges using a step edge to define the normal metal length. Critical current-normal resistance products over 1 mV have been measured at low temperature in devices with high-resistivity Ag-Au alloy bridges. Results on samples with Ag bridges are compared with the alloy data as an initial test of recent theories of SNS Josephson junctions. Josephson effects have been demonstrated in these devices at temperatures higher than 80 K. Clearly defined rf steps have been observed, with power dependence qualitatively similar to theoretical predictions.