Fuminori Hirayama

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.

Strong, easy-to-manufacture, transition edge x-ray sensor

We developed a membrane structure with a silicon-on-insulator (SOI) wafer by using a micromachining technique to create a transition edge x-ray sensor. In this membrane structure, the part of the SOI layer between the silicon nitride (SiNx) film and the buried oxide layer was etched from the front side to form the SiNx membrane. Advantages of this membrane are that (a) it is stronger than conventional membranes and is therefore suitable for large format arrays, (b) the Si etching time is reduced from 12 h (for conventional etching) to 4 h, and (c) all the fabrication processes are done from the front of the wafer, thus simplifying the manufacturing process.

An interface circuit for a Josephson-CMOS hybrid digital system

For broadband data communication between Josephson and CMOS digital systems, amplification of small Josephson-output signals and synchronization between the systems are important issues. We present an interface circuit for a Josephson-CMOS hybrid digital system. The interface circuit consists of a parallel-in-parallel-out (PIPO) circuit and built-in Josephson-MOS amplifiers. The PIPO circuit, implemented based on 4JL latching logic technology, performs synchronized data transfer between the Josephson and CMOS circuits. The Josephson-MOS amplifiers consists of stacked Josephson junctions (Suzuki stacks) and MOS inverters which are monolithically integrated on a chip. The circuits have been designed, fabricated and tested. We have successfully confirmed correct operation of the circuits.

Design of series SQUID array suppressing Josephson oscillation interference between element-SQUIDs

We have designed and fabricated single-stage series SQUID arrays usable for current amplifiers with filtering resistors in its input coil. From experimental results, it was revealed that filtering in each SQUID in the array successfully suppressed the interference between element-SQUIDs. An array with 1000 SQUIDs showed relatively smooth flux-voltage curves, large voltage swing (/spl sim/50 mV) and current-voltage conversion coefficient (/spl sim/1000 V/A).

Growth behavior and surface morphology of homoepitaxial YBa2Cu3O7−δ thin films on flux‐grown single crystals

Atomically flat terraces and steps with a height of one c‐axis lattice parameter have been observed with atomic force microscopy (AFM) on the surface of a flux‐grown YBa2Cu3O7−δ (YBCO) single crystal. Homoepitaxial growth of YBCO on the flux‐grown single crystal has been investigated using reflection high‐energy electron diffraction (RHEED). Initial stage of the RHEED intensity oscillation without anomalous peaks usually observed in heteroepitaxial growth suggests epitaxial growth of YBCO on the substrate surface from the beginning of deposition. A surface image of AFM for the homoepitaxial film reveals coalescent step‐like structures and two‐dimensional islands of one c‐axis height on the growing surface of YBCO. The observed RHEED oscillation and AFM image are consistent with the two‐dimensional nucleation growth of the film.

Development of an X-ray imaging microcalorimeter with a pixel-type Ir transition edge sensor

Abstract We are developing an X-ray microcalorimeter with high energy resolution and position sensitivity by using an Iridium transition edge sensor. The Ir-TES with a new pixel-type geometry has ten small pixel TES array. Each pixel size is 80 μm ×200 μm . This device has shown a good energy resolution of 26 eV (FWHM) at 5.9 keV . Furthermore, the Ir-TES film was scanned along each pixel by a collimated X-ray microbeam and signal pulses were measured at each incident position. The pulse height and rise time map has shown ten different regions where pulses are concentrated which correspond to each pixel. This might be useful for an imaging TES array.

White noise of Nb-based microwave superconducting quantum interference device multiplexers with NbN coplanar resonators for readout of transition edge sensors

White noise of dissipationless microwave radio frequency superconducting quantum interference device (RF-SQUID) multiplexers has been experimentally studied to evaluate their readout performance for transition edge sensor (TES) photon counters ranging from near infrared to gamma ray. The characterization has been carried out at 4 K, first to avoid the low-frequency fluctuations present at around 0.1 K, and second, for a feasibility study of readout operation at 4 K for extended applications. To increase the resonant Q at 4 K and maintain low noise SQUID operation, multiplexer chips consisting of niobium nitride (NbN)-based coplanar-waveguide resonators and niobium (Nb)-based RF-SQUIDs have been developed. This hybrid multiplexer exhibited 1 × 104 ≤ Q ≤ 2 × 104 and the square root of spectral density of current noise referred to the SQUID input √SI = 31 pA/√Hz. The former and the latter are factor-of-five and seven improvements from our previous results on Nb-based resonators, respectively. Two-directional...

TES x-ray microcalorimeters with single element superconductors

To develop high energy resolution x-ray microcalorimeters, we have fabricated single element superconductor TES detectors with W and Ir. In the W-TES, the detector is operated in the saturation mode because of high sensitivity of the W film. Energy resolution of 34.2eV (FWHM) at 5.9 keV is obtained by integrating the reduced current signals. In the Ir-TES detector, a position dependent response of pulse height distribution was observed, however, 15.4 eV (FWHM) energy resolution and 60μsec response time are obtained for collimated 3 keV x-rays.

Study of YBCO/Au Surface Using Low-Temperature Scanning Tunneling Microscopy/Scanning Tunneling Spectroscopy

A low-temperature scanning tunneling microscope (LTSTM) has been used to study the surface electronic state of a 30-nm-thick Au film deposited on (103)-oriented YBCO thin film. Conductance spectra with the BCS-like dip induced by the superconductivity proximity effect were observed in a small area. Also, spectra with zero-bias conductance peaks were observed in another limited area. Most parts of the areas studied showed constant conductance spectra. Since this fact suggests the existence of a degraded layer on most surfaces of the YBCO film, the Josephson coupling in YBCO/Au/Nb is assumed to occur primarily in this degraded layer.

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.