Mutsuo Hidaka

High-temperature superconducting edge-type Josephson junctions with modified interface barriers

This paper describes recent results on the fabrication, electrical characteristics, and microstructure of high-temperature superconducting edge-type Josephson junctions with modified interface barriers. The barriers are formed by surface modification of the YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// base layer. This process involves structural and chemical modification by ion irradiation and crystallization by annealing. The junctions showed resistively and capacitively shunted junction-like current-voltage characteristics and excellent uniformity. The spread in the critical current for one hundred junctions was smaller than 1/spl sigma/=10% at 4.2 K. The uniformity is now approaching 1/spl sigma/=5%. The junction characteristics have remained the same after two-year room-temperature storage. They also showed no change after high-temperature processing at about 700/spl deg/C. High-resolution transmission electron microscopy revealed that both the crystal structure and chemical composition in relatively thick barriers are different from those of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//.

ATOMIC STRUCTURE AND COMPOSITION OF THE BARRIER IN THE MODIFIED INTERFACE HIGH-TC JOSEPHSON JUNCTION STUDIED BY TRANSMISSION ELECTRON MICROSCOPY

The atomic structure and composition of modified interface junctions which showed reproducible critical current Ic (Ic1σ<8% for 100 junctions) are investigated by transmission electron microscopy. Transmission electron microscopic observations show the existence of a thin barrier (1–2 nm) homogeneously covering the ion milled edge of the base YBa2Cu3Oy film although there is no barrier deposition and annealing process. High-resolution electron microscopy images and energy dispersive x-ray analysis with a spot size of 1 nm indicates that the barrier is a Ba-based perovskite-like structure, (Y1−xCux)BaOy with x<0.5. A thin amorphous layer whose composition deviates from YBa2Cu3Oy is formed due to the preferential sputtering of Cu. The amorphous layer recrystallizes into the nonequilibrium phase (Y1−xCux)BaOy after heating up to the deposition temperature.

Resonant tunneling transport in YBaCuO/PrBaCuO/YBaCuO edge-type Josephson junctions

YBaCuO/PrBaCuO/YBaCuO edge-type Josephson junctions were fabricated and their transport properties were studied. Josephson effect in these structure was confirmed by the observation of microwave induced Shapiro steps and periodic magnetic field dependence of the critical current. Normal electron transport in the junctions was well explained by Glazman and Matveevs theory for hopping transport via localized states. In addition, the supercurrent transport in PrBaCuO is possibly supported by the resonant tunneling via the localized states. We estimated the density of the localized states to be 10/sup 16/ cm/sup -3/.<<ETX>>

DIRECT-INJECTION HIGH-TC DC-SQUID WITH AN UPPER YBA2CU3O7-X GROUND PLANE

We have fabricated direct-injection high-Tc superconducting quantum interference devices (SQUIDs) with integrated YBa2Cu3O7−x (YBCO) ground planes in the upper part of structure. The YBCO/PrBa2Cu3O7−x(PBCO)/YBCO edge junctions with 10-nm-thick PBCO barriers showed RSJ-like current-voltage (I–V) characteristics before and after the ground plane formation. We measured the voltage modulations of these SQUIDs with two different hole sizes to evaluate the line inductance. At a temperature of 30 K, the estimated line inductance under the ground plane was about 1.0 pH/□, while the estimated line inductance without the ground plane was 2.8 pH/□.

Circuit design for a high-T/sub c/ Josephson sampler

We propose a high-speed and high-sensitivity sampler circuit using high T/sub c/ superconducting (HTS) Josephson junctions. This circuit consists of five non-latching junctions, superconducting lines and a superconducting ground-plane. A main feature of this circuit is a non-latching comparator junction whose switching generates a circulating current in a superconducting loop. Computer simulation and prototype layout were performed to study its ability and feasibility.<<ETX>>

High-T/sub c/ superconductor sampler system for digital signal waveform measurement

We are developing a system using a high-T/sub c/ superconductor (HTS) sampler for measuring a 40-Gbps digital signal waveform, which will be used in next generation ultra-high-speed communications. To measure a 40-Gbps digital signal, the sampler bandwidth needs to be more than 120 GHz. The HTS sampler has the potential to achieve this bandwidth. As a first step in developing this system, we have developed a prototype whose target bandwidth is 40 GHz. In the prototype system, the sampler chip is cooled down to 35 K by a single-stage Stirling cryocooler, and the digital signal is transmitted to the sampler chip via a 40-GHz bandwidth assembly line. This system is the first to integrate an HTS device, a cryocooler, and a high-frequency assemble line. A 5.9-Gbps digital waveform was reconstituted by the prototype system with the correct period.

High-resolution current measurement system using a high-T/sub c/ superconductor sampler

We are developing a prototype system for measuring high-frequency current flowing through a room-temperature sample without contact. It is based on a high-T/sub c/ superconductor sampler that can measure current waveforms at picosecond and microampere resolution. The sampler chip is housed in a vacuum chamber and is cooled down to its operating temperature by a GM-pulse tube cryocooler. Current flowing through the sample, which is placed just below and outside the vacuum chamber, generates a magnetic field. The sampler can measure this current because a superconducting loop, which includes the comparator junction of the sampler, picks up the magnetic field. Sinusoidal current flowing through a 50-ohm microstrip line was successfully observed up to 7 GHz by the system. Frequency dependence of measurement sensitivity is a problem to overcome for applying it to practical uses.

High-resolution measurement by a high-T/sub c/ superconductor sampler

We measured a signal current waveform by using a high-T/sub c/ superconductor (HTS) sampler with a 1-ps delay between every sampling point. The maximum time differential obtained in the measured waveform was 12 /spl mu/A/ps with a 2.5-/spl mu/A current sensitivity at 25 K. This result guarantees that the sampler is able to measure current waveforms correctly when their maximum time differential is less than 12 /spl mu/A/ps. The superior temporal response was achieved by using high-speed single-flux-quantum pulses generated in the HTS circuit. A unique feature of the sampler is that it directly measures the current with picosecond and microampere resolutions. Measurement of current flowing through wiring in a semiconductor large-scale integrated circuits is a promising application for the HTS sampler.

High-temperature superconducting edge junctions with modified interface barriers

This paper summarizes our recent investigations on the fabrication process, microstructure, and characteristics of high-temperature superconducting edge-type Josephson junctions with modified interface barriers. The junctions were fabricated by an in situ edge fabrication process using an epitaxial insulator layer as an etching mask. The barriers were formed not by intentional deposition but by interface modification. The modified interface barrier material was identified as a Ba-based perovskite: Ba(Y1-x Cux )Oy . The junctions showed resistively and capacitively shunted junction-like current-voltage characteristics and excellent uniformity. 100 junctions showed a spread in the critical current as low as 1 = 8% at 4.2 K and the junction characteristics remained the same after a high-temperature process at about 700 °C.

Development of two prototype systems based on a high-Tc superconductor sampler

Abstract We are developing two prototype systems based on high- T c superconductor (HTS) samplers for measuring high-frequency electrical waveforms. One is a digital waveform measurement system in which high-frequency digital signals are directly inserted into the sampler circuit via a 40-GHz frequency assembly. The system can reconstitute a 5.9-Gbps digital waveform at 37 K. The other system is a current-waveform measurement system in which the signal current is picked up by a superconducting closed loop, and the waveform is measured by the sampler included in the loop. A sinusoidal waveform at 7 GHz was successfully measured using this system at 38 K.