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Using molecular beam epitaxy (MBE) with pure ozone, YBa2Cu3Oy/ PrBa2Cu3Oy (YBCO/ PBCO) layered structures (multilayers) have been fabricated successfully. We also adopted differential pumping making a stable rate of evaporation and the effective oxidation of films possible. The characterization of these films, such as the surface morphology, was performed using x-ray diffraction (XRD) spectra and atomic force micrograph (AFM). The superconductivity of these multilayers was measured by a SQUID magnetometer and magnetization measurements were useful to observe the dynamics of flux in very thin superconductive layers.

Digital-to-analog (D/A) converters based on rapid single flux quantum (RSFQ) technology are under development for ac voltage standard applications. We present design and test results on a prototype 10-bit version integrated on a single chip. The 10-bit chip includes over 6000 Josephson junctions and consumes a bias current exceeding 1 A. To reduce the effects of the high bias current on circuit operation, a custom design method was employed in part and large circuit blocks were divided into smaller ones. The 10-bit chips were fabricated and tested at low speed. The test results suggested that our design approach could manage large bias currents on the order of 1 A per chip.

Microscopic properties of high temperature superconductors (HTS) were investigated by a low temperature scanning tunneling microscope at 4.2K. We have obtained a topographic image of YBa2Cu3O7 at 4.2K and were able to observe twin boundaries on the cleaved surface for a large area scan (153×153 nm2). Spectroscopic studies were carried out on the same cleaved surface. A superconducting energy-gap (2Δ) of 20–35 meV at 4.2K measured in the present study.

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.

We are developing a tungsten transition edge sensor (W-TES) for x-ray imaging spectroscopy applications. The conventional readout scheme for TES array has a great difficulty in reading out multi-channel x-ray signals. Therefore we have considered a new readout scheme for a W-TES array. This scheme is based on a cooling time measurement in a saturation mode. In this scheme, we only need time information, therefore we can expect to simplify a readout system. Although the currently achieved energy resolution of our W-TES (0.5 mm×0.5 mm) using the cooling time measurement is 55 eV (FWHM) at 5.9 keV, much higher energy resolution is expected by making smaller devices.

We are developing an Ir-TES x-ray microcalorimeter with high energy-resolution. First we fabricated an Ir film of 0.5 mm×0.5 mm. The measured energy resolution was 194 eV (FWHM) for 5.9 keV x-rays and this value is extremely worse than the theoretical resolution. We considered the limitation in the energy resolution arises from non-uniformity in the heat diffusion process. To improve the heat diffusion property, we have developed a new small pixel geometry that has a shorter length between electrodes and some thin slits to separate the heat diffusion area of the Ir-TES. This device has shown an improved energy resolution of 47 eV (FWHM) at 5.9 keV. Furthermore the risetime and falltime map has shown the possibility of identifying the pixel position which would be useful for an imaging TES array.