Title: Very Large Scale Integration of Nano-Patterned YBa2Cu3O7-delta Josephson Junctions in a Two- Dimensional Array

Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. In the last two decades there has been considerable effort aimed at developing a high-transition temperature (T c ) superconductor Josephson junction technology capable of producing large numbers of junctions with uniform electrical properties, namely junction critical current I 0 and normal state resistance R n . 1 This is especially challenging in high-T c materials compared with conventional metallic low-T c superconductors because the superconducting coherence length ξ is much shorter and highly anisotropic, typically 2 nm in the ab plane and 0.2 nm along the c-axis direction. REPORT DATE MAR 2010 2 However, they may not be suitable for devices operating at liquid nitrogen temperatures, because the operating temperature is typically below 50 K. 14 Furthermore, the inter-junction spacing can only be scaled down to a few µm, 15 which does not allow them to be used in high frequency devices that may require junction spacings of the order of 100 nm. To create this array, a 200-nm thick YBCO thin film 35 was thermally coevaporated on a sapphire wafer followed by a gold contact layer deposited in situ. We patterned the films using photolithography and Ar + ion milling to fabricate the microstrip with 15,255 SQUID loops etched into it ( hard-baked photoresist that served as the main ion stopping layer. A 25-nm layer of germanium was electron-beam evaporated on top of the resist and served as an etch stop. We next spun 100 nm of polymethylmethacrylate (PMMA) resist on to the Ge for electron-beam lithographic patterning ( etches two orders of magnitude faster then Ge in an oxygen plasma. Completed devices were attached to a printed circuit board equipped with π-filters and a silicon diode thermometer. The board was mounted inside a vacuum probe and cooled in a liquid nitrogen bath. We measured the resistance of the device as a function of decreasing temperature using a lock-in amplifier and a 22-Hz, 8-µA peak current. As shown in T characteristic, or equivalently to the peaks in dR/dT vs. T . Thus, the ion damage reduced T c by about 7.6 K. The full-width at half maximum (FWHM) for the dR/dT peak of the irradiated material, 1.4 ± 0.2 K, was similar to that of the electrodes, 1.3 ± 0.2 K. Thus, the spread in the T c of the junctions in the 565 segments in series is close to the spread in the T c of the unirradiated YBCO. This implies that the spread in T c along each parallel segment is likely to be comparable. These measurements suggest that the ion damage is quite uniform across the array, implying that the resistances of the weak links are similarly uniform. We note that we previously observed a similar spread in the T c of a 1D array of 280 SQUIDS. shown in To study the response of the array to a magnetic field, the array was biased with a static (neg- shown in ative We also observe a linear tilt in V vs. B, visible in where − → J is the current density and − → B is the magnetic field. This force causes vortices to drift with a steady velocity across the array, creating an electric field in the direction of the bias current. On our measurements this appeared as magnetoresistance. For bias currents in the opposite direction, the asymmetry had the opposite sign, consistent with our interpretation. We analyzed our data using the Bardeen-Stephen equation 42 ρ = Φ 0 B/ηB, which relates the flow resistivity ρ to the magnetic field B, to determine η, the vortex viscosity coefficient. Using the bias current and junction geometrical parameters we converted V to ρ, and determined Φ 0 B/η from a linear fit of the data. We estimate η = 3 × 10 −7 kg m −1 s −1 at a current bias of 100 µA. We are not aware of any other measurements of Josephson vortex motion in 2D arrays of this type for comparison. However this value is typical for the viscosity coefficient of Abrikosov vortices in high-transition temperature superconductors. In may allow for substantial increases in I 0 R n , opening up this process to more applications such as rapid single flux quantum logic or precision digital-to-analog conversion

Series arrays of planar long Josephson junctions for high dynamic range magnetic flux detection

This work is licensed under a Creative Commons Attribution 4.0 International License.We investigated series arrays of closely spaced, planar long Josephson junctions for magnetic field transduction in Earth’s field, with a linear response and high dynamic range. The devices were fabricated from thin film high-temperature superconductor YBa2Cu3O7−δ (YBCO) thin films, using focused helium ion beam irradiation to create the Josephson barriers. Four series arrays, each consisting of several hundreds of long junctions, were fabricated and electrically tested. From fits of the current-voltage characteristics, we estimate the standard deviation in critical current to be around 25%. Voltage-magnetic field measurements exhibit a transfer function of 42 mV/mT and a linear response over a range of 303 μT at 71 K, resulting in a dynamic range of 124 dB.FOSR FA 9550-17-C-0006FA 9550-15-1-0218ARO Grant W911NF1710504NSF Grant No. 166444

Magnetic effects in sulfur-decorated graphene

The interaction between two different materials can present novel phenomena that are quite different from the physical properties observed when each material stands alone. Strong electronic correlations, such as magnetism and superconductivity, can be produced as the result of enhanced Coulomb interactions between electrons. Two-dimensional materials are powerful candidates to search for the novel phenomena because of the easiness of arranging them and modifying their properties accordingly. In this work, we report magnetic effects of graphene, a prototypical non-magnetic two-dimensional semi-metal, in the proximity with sulfur, a diamagnetic insulator. In contrast to the well-defined metallic behaviour of clean graphene, an energy gap develops at the Fermi energy for the graphene/sulfur compound with decreasing temperature. This is accompanied by a steep increase of the resistance, a sign change of the slope in the magneto-resistance between high and low fields, and magnetic hysteresis. A possible origin of the observed electronic and magnetic responses is discussed in terms of the onset of low-temperature magnetic ordering. These results provide intriguing insights on the search for novel quantum phases in graphene-based compounds.Comment: 6 pages and 5 figure

Planar Josephson junctions and arrays by electron beam lithography and ion damage

In the years to come, the size and cost of cryo-coolers will get smaller and the demand for a VLSI high- temperature superconducting (HTS) Josephson junction technology will increase. One possible candidate to fill this need is the 'ion-damage' Josephson junction. These junctions are fabricated by using ion bombardment to create localized narrow regions of defects in the plane of a thin film superconductor. These regions have a superconducting transition temperature lower than that of the bulk film and act as non-hysteretic Josephson junctions. The advantage of these junctions over other technologies is that they have no interfaces between different materials, and can be placed over 10 times closer to each other in comparison to competing techniques. Individual junctions and multi-junction serial arrays were fabricated and characterized. Current-voltage characteristics were well described by the restively shunted junction model at temperatures close to the critical temperature of the weak link. At lower temperatures the junctions became strongly coupled and exhibit flux flow characteristics. It is suggested that this is due to the barrier changing from non- superconducting to superconducting, which allows Abrikosov vortices to enter the junction. Junction arrays have flat giant Shapiro steps at N times the voltage predicted by the ac Josephson relation, where N is the number of junctions in the array. Modulation of the critical current was measured to observe the dc Josephson effect. Unlike other planar junction types these junctions do not exhibit flux-focusing effects in magnetic field measurements and are more comparable with classical sandwich type Josephson junctions. Low temperature (100̕C) anneals substantially increased the critical current and operating temperature possibly from the recombination of vacancy-interstitial pairs formed during implantation. Junctions were also fabricated from the low temperature superconductor MgB₂ for comparisons with the HTS junctions and to show that this technology is not limited to HTS. Overall it has been shown that this technique can be used to reproduce junctions with uniform resistances and critical currents. With a few innovations in materials and/or lithography, ion damage Josephson junctions may become common place in the medical, communications, and defense industrie

Temporal stability of Y Ba Cu O nano Josephson junctions from ion irradiation

We investigate the temporal stability of YBa2Cu3O7 Josephson junctions created by ion irradiation through a nano-scale implant mask fabricated using electron beam lithography and reactive ion etching. A comparison of current-voltage characteristics measured for junctions after fabrication and eight years of storage at room temperature show a slight decrease in critical current and increase in normal state resistance consistent with broadening of the weaklink from diffusion of defects. Shapiro step measurements performed 8 years after fabrication reveal that device uniformity is maintained and is strong evidence that these devices have excellent temporal stability for applications