Joonhee Kang

EFFECT OF CU-O LAYER SPACING ON THE MAGNETIC FIELD INDUCED RESISTIVE BROADENING OF HIGH-TEMPERATURE SUPERCONDUCTORS

Abstract For H‖c-axis, the magnetic field induced broadening of the resistive transitions of high-Tc superconductors (HTS) is shown to depend strongly on the Cu-O layer spacing. For the highly anisotropic HTS, we show experimental evidence that flux motion results from a thermally activated crossover from three dimensional (3D) vortex lines to 2D independent pancake-like vortices in the Cu-O layers, which is intrinsic to the material and occurs when kBT exceeds the Josephson coupling energy of these layers. At low temperatures, however, thermally activated conventional depinning (which can be sample dependent) or melting in the uncoupled 2D Cu-O layers is also required for flux motion. For YBa2Cu3O7, this dimensional crossover does not occur belowHc2, presumably because the conducting Cu-O chains short-circuit the Josephson interlayer coupling, leading to better superconducting properties in a magnetic field. These results show that strong interlayer coupling is a key to finding good alternatives.

Josephson sigma-delta modulator for high dynamic range A/D conversion

A Josephson sigma-delta modulator suitable for use in high dynamic range conversion of MHz bandwidth signals was designed and simulated. Input signal voltages were integrated as current in a superconducting inductor. A single junction quantizer provided analog-to-digital (AID) conversion at 40 GHz and fed back single-flux-quantum (SFQ) voltage pulses to balance the input. A JSIM calculation for a 13,110,000-ps interval indicated ideal first-order noise shaping of a 2.44-MHz voltage-source signal. Spur free dynamic range was 102 dB and signal-to-noise-and-distortion was 98 dB on the DC 10-MHz signal band, equivalent to a 16-b, 20-MSample/s, Nyquist-rate A/D. The modulator transfer function was derived for a current-source signal applied to an input resistor. First-order quantization noise suppression was still observed for signal bands wider than the 2 pi R/L modulator cutoff frequency. The superconductive sigma-delta modulator combines the speed of Josephson junctions for GHz sampling and digital filtering with the quantum mechanical accuracy of SFQ feedback to obtain superior A/D performance.<<ETX>>

Superconductivity in thin films of the Bi-Ca-Sr-Cu-O system

Abstract Films of Bi-Sr-Ca-Cu-O have been made by multi-target magnetron sputtering. They show a superconductivity of ≈90–110 K and a resistive T c 0 of 72 K. Preliminary X-ray diffraction analysis suggests the material to be predominantly tetragonal with a cell size of 5.41 A × 5.41 A × 30.8 A.

Meandering grain boundaries in YBa2Cu3Oy bi‐crystal thin films

Artificially induced [001] tilt grain boundaries in epitaxial YBa2Cu3Oy (YBCO) thin films were prepared by deposition onto SrTiO3 bi‐crystal substrates and subsequently examined by transmission electron microscopy and atomic force microscopy (AFM). It was found that the YBCO grain boundary deviated from the path defined by the underlying substrate boundary, with the ‘‘meandering’’ YBCO boundary only generally following the path defined by the boundary in the underlying substrate. The AFM studies suggest this ‘‘meandering’’ behavior is related to the nucleation and growth mechanisms of the film, and based on this, we were able to vary the magnitude of the meandering by changing the growth conditions. The implications of this meandering behavior are significant, suggesting potential variations in electrical behavior from point to point along these boundaries. This effect is likely to be exacerbated by reduced junction linewidths and may lead to inconsistent behavior in devices which utilize this type of bou...

Current recycling and SFQ signal transfer in large scale RSFQ circuits

The practical implementation of RSFQ technology in most digital electronics application areas requires much more complexity than the presently developed circuits. There are two important issues in building large-scale RSFQ circuits: 1) the recycling of the bias currents and 2) the transfer of SFQ pulses between circuits located far apart. RSFQ circuits are well known to operate with DC current bias. Even though the DC current biasing is more forgiving than the problematic AC biasing, it can still be a big concern when the circuit size becomes large. Dramatic reduction of the total bias current can be achieved by biasing several RSFQ circuits in series, where each circuit is positioned on a separate ground plane. In this work, we have used magnetically coupled Josephson transmission lines as inputs and outputs of an isolated shift register to show the feasibility of using the concept of serial biasing in current recycling. The circuit was simulated, fabricated with Nb technology, and tested at a temperature of 4.2 K. Test results show that SFQ pulses were transferred into the shift register built on a separate ground plane, clocked through it, and sent out back to the circuit on the original ground plane. We also studied on how to transfer SFQ pulses over an extended length, an important issue in building large RSFQ circuits. We have designed the circuits to test our microstrip line and multichip module approaches. We designed, optimized, fabricated and tested the circuits. Test results show that SFQ pulses can be successfully transmitted over an extensive distance in a chip and between chips.

Large anisotropy in the upper critical field of sputtered thin films of superconducting Tl‐Ba‐Ca‐Cu‐O

The upper critical field Bc2 provides the most direct measure of the intrinsic anisotropy of a superconductor. For highly oriented, sputtered thin films of the high‐temperature superconductor Tl2Ba2CaCu2Ox, we find a Bc2 anisotropy of at least 70, which exceeds similar measurements on thin films and single crystals of the high‐temperature superconductors Bi‐Sr‐Ca‐Cu‐O and YBa2Cu3Oy. We discuss why the midpoints of the resistive transitions, used in these measurements, may be appropriate for defining the intrinsic Bc2.

High critical field anisotropy of superconducting Bi‐Sr‐Ca‐Cu oxide from highly oriented thin films

Superconducting films of Bi‐Sr‐Ca‐Cu oxides have been grown on MgO substrates by multitarget magnetron sputtering. After post‐annealing in an oxygen atmosphere, zero resistance was obtained at about 80 K. X‐ray diffraction indicates a high degree of preferential growth of the c axis perpendicular to the substrate. Measurements of the upper critical field Bc2(T) of these films show critical field slopes (B’c2≡−dBc2/dT at Tc) of 8.5 and 0.56 T/K, respectively, for B parallel and perpendicular to the substrate. The anisotropy of ∼15 and the Bc2∥ are the highest values yet reported for high‐temperature superconductors.

Margins and yields of SFQ circuits in HTS materials

An analytical model has been developed to project the yield of superconductive integrated circuit chips as a function of circuit operating margins, fabrication process control, and component count. For Gaussian distributed deviations of critical component values from design specifications, chip yield was a highly nonlinear (threshold) function of the ratio of circuit margin to process standard deviation. Computer simulations of single-flux-quantum (SFQ) logic gates with model high-temperature superconductor (HTS) superconductor-normal-metal-superconductor (SNS) junctions operating at GHz clock rates showed at least 50-70% of the margins of similar Nb-Al/sub 2/O/sub 3/-Nb based circuits. Margins and maximum clock rate improved as I/sub c/R/sub n/ (critical-current-normal-resistance product) was increased from 200 to 500 mV.<<ETX>>

Interface circuits for chip-to-chip data transfer at GHz rates

Interface circuits for the transfer of data between Single Flux Quantum (SPQ) circuits have been designed, fabricated, and operated at speeds up to 3 Gigabits per second. The circuit employed an improved version of the SFQ/Latch converter, a Modified Variable Threshold Logic (MVTL) OR/AND gate, a 3/spl times/ latching amplifier, and a 3/spl times/-to-10/spl times/ latching amplifier. The amplifier circuits employed stacks of latching junctions. Resistors between the parallel stacks of junctions damped residual currents to prevent flux trapping during reset. Tolerance to critical current variations in the series stacks of junctions was provided by inductive chokes on the input junction shunting resistors. Microwave modeling programs were used to ensure proper distribution of the applied current to all of the latching elements. The circuit transferred data at 3 Gigabits per second from one SFQ circuit up to room temperature and back to another SFQ circuit through 3.4 meters of 50-ohm cable.

A single flux quantum shift register operating at 65 K

We report the fabrication and quasi-static testing of a two-stage, high-temperature superconducting, Single Flux Quantum shift register. The five-junction circuit was fabricated using a single YBCO film, with step-edge grain boundary junctions. Storage of flux, and its motion in response to LOAD and SHIFT signals, was demonstrated at 65 K.<<ETX>>