Shogo Kiryu

Superconducting properties and normal‐state resistivity of single‐crystal NbN films prepared by a reactive rf‐magnetron sputtering method

In this letter, we report some superconducting properties and normal‐state resistivities of single‐crystal NbN films prepared by a reactive rf‐magnetron sputtering method. It was found that the prepared single‐crystal NbN films have Tc’s above 16 K, normal‐state resistivities in the range from 12.2 to 14.3 μΩ cm (at 20 K), and magnetic penetration depths in the range from 93 to 100 nm (at 4.2 K). From a temperature dependence of the upper critical magnetic field for a single‐crystal NbN film, the zero‐temperature Ginzburg–Landau coherence length ξGL(0) for the film was calculated to be 6.9 nm.

RSFQ-based D/A converter for AC voltage standard

Digital to analog converters based on the Josephson effect are promising for voltage standards, because they produce voltage steps with ultimate precision and stability. In this paper, we describe a project to develop a Josephson D/A converter designed for synthesizing a sinusoidal waveform with metrological accuracy. The D/A converter is based on RSFQ (Rapid Single Flux Quantum) logic circuits, and consists of a frequency multiplier (FM), a pulse distributor (PD), and a number of voltage multipliers (VMs). Each VM circuit, corresponding to the n-th bit digital code, multiplies the number of SFQ pulses by a factor of 2/sup n/. By gating the input SFQ pulses from the FM to the VMs using the PD circuits, a programmable output voltage is obtained. Possible sources of uncertainties in the measurement of the rms value of the synthesized sine wave are discussed.

Surface resistance of epitaxial and polycrystalline NbCN films in submillimeter wave region

Surface resistances, R/sub s/s, of epitaxial and polycrystalline NbCN films in submillimeter-wave region were evaluated from measurements of Fiske-resonant modes in Josephson tunnel junctions. It is found that epitaxial NbCN films have about one order of magnitude smaller R/sub s/ values than those for polycrystalline NbCN films. It is also found that the frequency dependence of R/sub s/ of epitaxial and polycrystalline NbCN films can be explained by the two-fluid model.<<ETX>>

Operation of a Josephson arbitrary waveform synthesizer with optical data input

In order to establish a quantum ac voltage standard at NMIJ/AIST, we have developed a Josephson arbitrary waveform synthesizer based on overdamped Josephson junction arrays (JJAs) using optoelectronics and a liquid helium free compact cryocooler. We have succeeded in the generation of sine waves at 60 Hz and 152.6 kHz. To verify the performance of the system, two different types of JJAs with different characteristic frequencies have been developed. An application to a quantized voltage noise source for Johnson noise thermometry is also discussed, with experimental results.

New Design of Quantized Hall Resistance Array Device

National Metrology Institute of Japan have been proposed and fabricated the quantized Hall resistance (QHR) array devices, and confirmed the performance with uncertainty of as low as 10-8. The new combinations of Hall bars have been designed ranging from 100 Ω to 1 MΩ. This new combinations realize the quantized resistance values more closely to decade values with fewer numbers of Hall bars. In the new design, 10-kΩ QHR array device consists of only 16 Hall bars, and this number is about 16 times lower than that in the previous design however the nominal value is same to previous one. This fact might allow us to evaluate each Hall bar in the array device.

Observation of Quantized Voltage Steps Using a Josephson Junction Array Driven by Optoelectronically Generated Pulses

In this paper, quantized voltage steps were observed by driving an overdamped Josephson junction array (JJA) with current pulse trains. The current pulse trains were generated by triggering a metal-semiconductor-metal photodetector installed close to JJA at 4.2 K with a mode-locked fiber laser. The intervals of the constant voltage steps were 10 muV, which agrees with the repetition frequency of the light source (49.7 MHz) and the number of Josephson junctions (100) in the JJA. This is the first step toward establishing an ac voltage standard at the National Institute of Advanced Industrial Science and Technology

Effects of parasitic capacitance in a magnetically-coupled voltage multiplier

Effects of parasitic capacitance in a magnetically-coupled voltage multiplier (VM) are described. We found that parasitic capacitance between SQUIDs and JTLs in a VM decreases its operating margin. We also found that separation of electric grounds for the output terminal of a VM from those for the JTLs is effective to improve the operating margin. Using this method, a 64-stage VM was fabricated and well-defined output voltage was obtained.

Stress in NbCxN1-x Films Prepared by Reactive Rf Magnetron Sputtering

Stress in reactively sputter-deposited NbCxN1-x films was measured. It was found that the stress in NbCxN1-x films strongly depended on the total sputtering gas pressure Pt. Films deposited at Pt≥2.0 Pa had very small stress values (<2.0×108 Pa). On the other hand, films deposited at Pt≤1.1 Pa had large compressive stress values. The results of X-ray diffraction measurements showed that the stress in NbCxN1-x films was strongly related to the ratio of (111) and (200) grains in the films. We also observed the surface morphology of deposited films by scanning electron microscopy and found that the stress in deposited films is affected by the microstructure of the films.

Magnetic Penetration Depths and Normal-State Resistivities of Epitaxial and Polycrystalline NbCxN1-x Films

We have studied the magnetic penetration depth λ and the normal-state resistivity ρ of epitaxial and polycrystalline NbCxN1-x films as a function of film thickness t and sputtering gas pressure P during the deposition of the films. It was found that λ(~200 nm) and ρ(~50 µΩcm) of epitaxial films are almost independent of P and t, while λ(>300 nm) and ρ(>110 µΩcm) of polycrystalline films increase considerably with increasing P(>2 Pa) or decreasing t(<80 nm). The pressure and thickness dependence of λ and ρ of polycrystalline films can be explained by a model of Josephson coupling between grains.

Operation of Josephson Junctions With Current Pulses Generated by Triggering a Cold Photo Detector With an Optical Comb

We report the experimental results on a Josephson junction array (JJA) driven by current pulse trains generated by triggering a photo detector located in a cryostat with an optical comb. The key components in the experimental system are a mode locked fiber laser, a metal-semiconductor-metal (MSM) photoconductive switch, and an overdamped JJA. Optical pulse trains generated at room temperature are transmitted via an optical fiber to the MSM-photo detector installed at 4.2 K, and then converted into electric current pulse trains to drive the overdamped JJA. Quantized voltage steps have been verified at multiples of 10 muV, which agrees with the repetition rate of the pulse laser and the number of Josephson junctions in the JJA.