Applied Superconductivity最新文献

We discuss recent developments for HTS Josephson junctions: (i) isolated (S/I) and resonantly coupled (S/I/S) Andreev interface states, (ii) d-wave models with resonantly coupled midgap states for ab-plane junctions and (iii) resonant tunneling models for c-axis junctions.

We present experimental data on superconductor/semiconductor hybrid systems which are based on the two-dimensional electron gas at the surface of p-type InAs. A short distance (≈200 nm) between the Nb contacts is obtained using step geometry. The best junctions show a high product of critical current times normal resistance of about 1 mV at T=4.2 K.

We have designed, fabricated, and operated an SFQ RS flip–flop based on YBCO grain boundary junctions formed on a SrTiO₃ bi-crystal substrate. The circuit consisted of a read SQUID and an RS flip–flop, which were magnetically coupled together. The circuit operated correctly in the temperature range of 65–71 K. Especially, at 71 K, this circuit operated correctly over the 100 reset–set operations without making errors. We also measured the effect of noise on switching a Josephson junction to the voltage state in an SFQ circuit.

The recent achievements of critical currents exceeding 1,000,000 amps/cm² at 77 K in YBCO deposited over suitably textured substrate have stimulated interest in the potential applications of coated conductors at high temperatures and in high magnetic fields. Currently, ion-beam assisted deposition (IBAD), and rolling assisted bi-axially textured substrate (RABiTS), represent two available options for obtaining textured substrates. For applying suitable coatings of buffer and high temperature superconductor (HTS) material over textured substrates, several options are available which include sputtering, electron-beam evaporation, laser ablation, electrophoresis, chemical vapor deposition (including metal organics chemical vapor deposition), sol-gel, metal organics decomposition, electrodeposition and aerosol/spray pyrolysis. A commercial continuous long-length wire/tape manufacturing scheme developed out of any suitable combination of the above techniques would consist of operations involving preparation of the substrate and application of buffer, HTS and passivation/insulation materials and special treatment steps such as post-annealing. These operations can be effected by various process parameters that can be classified into chemistry, materials, engineering and environmental related parameters. To carry out an engineering evaluation: (i) the process flow schemes were developed for various candidate options identifying the major operating steps, process conditions, and process streams; (ii) to evaluate quantifiable parameters such as process severity (e.g. temperature and pressure), coating thickness and deposition rate for HTS material, achieved maximum J_c value, and cost of chemicals and material utilization efficiency, the multi-attribute method was used to determine attributes/merits for various parameters and candidate options. To determine similar attribute values for the non-quantifiable parameters, a subjective evaluation was used. Results of the two evaluations were then combined to calculate the overall merit/utility of a given option using weighting factors. To evaluate the effect of arbitrarily determined weighting factors, a sensitivity analysis was carried out by using three different sets of weighting factors. In the end, the results of the overall utility/merit values calculated using different sets of weighting factors were utilized to determine the preliminary ranking among the candidate options. The process flow schemes developed for various options, as well as the results of the engineering evaluation carried out for the candidate options using these schemes, are given in this paper.

We have employed HTS rf washer SQUIDs with coplanar resonators for magnetocardiography measurements. The white field noise of such a magnetometer (rf washer SQUID, 3.5 mm in diameter, flip-chip mounted on a flux concentrator 13 mm in diameter) inside a 4 layer μ-metal shielding was 16 fT/√Hz. MCG measurements were performed with a magnetometer in the Berlin magnetically shielded room (BMSR) and with a first order gradiometer in a standard shielded room in Jülich (JMSR). The results are compared regarding the fragmentation score, which is supposed to be a risk marker for sudden cardiac death. Despite different environments, score values determined with the magnetometer and the gradiometer were almost identical.

Fluxon modes in stacked Josephson junctions (SJJ's) are studied both theoretically and experimentally. It is shown that the critical current J_c across the stack is multiple valued due to a possibility of having different fluxon configurations (modes), which is in good agreement with experiment on Bi₂Sr₂CaCu₂O_8+x, the temperature and magnetic field dependencies of the critical current were calculated for a stack with nonidentical junctions.

A fully integrated device on a single 2″-MgO wafer for a 4 bit instantaneous frequency measurement between 9.5 and 10.5 GHz has been developed with coplanar delay lines and power dividers. Due to integration of power divider resistors and a large number of air-bridges, improved transmission characteristics are achieved in a more reproducible way than with surface mounted chip-resistors and ultrasonically bonded air-bridges. The passive integrated circuit has 4 output signals and after threshold detection yields a one step binary code with a maximum resolution of ±31.25 MHz at a bandwidth of ∼1 GHz and a center frequency of ∼10 GHz. A 4 bit test vehicle with Nb-delay lines on a 2″-MgO substrate yields a total nonlinearity of 1/5 LSB corresponding to 12 MHz. Another test vehicle with YBCO-delay lines on a 2″-MgO substrate will also be discussed.

We have fabricated Coulomb blockade devices which act as current mirrors. Each sample consists of two nominally identical one-dimensional arrays of small tunnel junctions. Each electrode of one array is capacitively coupled to two electrodes in the other array. By measuring the current voltage characteristics of the two arrays simultaneously, we observe a coupling between the currents in the two arrays. We have studied the properties both in the superconducting state and in the normal state, and we interpret the data in terms of correlated transport of charges in the two arrays. The locking between the currents in the two arrays is strongest for intermediate magnetic fields where the electrodes are still superconducting. At best we find current locking over a range of ±6.7 pA

Residue number system (RNS) arithmetic has a promising role for fault-tolerant high throughput superconducting single flux quantum (SFQ) circuits for digital signal processing (DSP) applications. We have designed one of the basic computational blocks used in DSP circuits, one-decimal-digit RNS adder. A new design for its main component, the single-modulus adder, has been developed. It combines simple and robust RSFQ elementary cells, both combinational and sequential. The central units are a circular shift register, a code converter, and the clock control circuitry. Our mod5 adder employs 195 Josephson junctions, consumes 50 μW of power, and occupies an area of less than 2 mm². Chips were fabricated at HYPRES, Inc. using 1 kA/cm² low-T_c Niobium technology. The mod5 adder was successfully tested at low speed, and gave experimental bias margins of ±26%.