Masao Koyanagi

High-Accuracy Josephson Potentiometer

A potentiometer with the extremely small relative uncertainty of a few parts in 109 has been developed by utilizing the property that a Josephson junction generates a precise voltage that is variable with frequency and step number. The evaluated total uncertainty is 2.5 × 10-9V for measurements at the 1-V level. This small uncertainty is attributed to the generation of a higher than normal Josephson voltage, that is, on the order of 100 mV.

TUNNELING SPECTROSCOPY AND PAIRING SYMMETRY OF THE HIGH-TC SUPERCONDUCTORS

Abstract Influences of d-wave pairing symmetry on superconducting electronic states is studied by tunneling spectroscopy. Detailed experiments on spatial and orientational dependences of the surface electronic states of YBa2Cu3O7−δ thin films and preliminary results of the proximity effect observation in Au layer deposition on YBCO are presented. By comparing these experimental results with recent theoretical works, the existence of anomalous interference effects are verified in the electronic states of the high-Tc superconductor, due to the d-wave symmetry of the pair potential.

Spin-polarized tunneling of La0.67Sr0.33MnO3/YBa2Cu3O7−δ junctions

Abstract The transport properties in contacts between ferromagnets and high- T c superconductors were investigated in La 0.67 Sr 0.33 MnO 3 /YBa 2 Cu 3 O 7− δ cross-strip junctions. The conductance spectra show zero-bias conductance peaks (ZBCPs), reflecting charge transport along the ab plane. When an external magnetic field is applied to the junctions, the conductance spectra show two notable features, i.e., an increase in background conductance and an asymmetric field response of ZBCP whose width responds nonlinearly to the applied field. These features are shown to be qualitatively consistent with a theoretical prediction of tunneling spectroscopy, in which presence of a ferromagnetic insulator layer between a spin-polarized ferromagnet and a d-wave superconductor is assumed. Possible origins for the ferromagnetic insulator 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.

Bound States in Superconductors

Bound states of quasiparticles in a normal metal placed between two different pair potentials are investigated. The energy levels of the bound states become discrete to satisfy a quantum condition, and are strongly affected by the macroscopic phases of pair potentials in the superconductors. The most interesting feature is formation of bound states on the Fermi level. Our model intuitively and systematically explains qualitative features of the bound states in various situations such as vortex cores and surfaces.

Measurement System for Quantum Hall Effect Utilizing a Josephson Potentiometer

In order to improve the measurement accuracy of the quantum Hail effect, a new method has been developed utilizing a Josephson potentiometer and a Superconducting Quantum Interference Device (SQUID) galvanometer. A relative uncertainty of a few parts in 108 can be realized for the measurement of the ratio of the quantized Hall resistance to a reference resistor using this method.

Integrated DC–SQUID Magnetometer

A magnetometer using a refractory material integrated with a pickup coil and a DC superconducting quantum interference device (DC–SQUID) has been developed for biomagnetic applications. It has an overall size of 8 ×9 mm2 and a field resolution of less than 40 fT/ √Hz which is limited by the electronics at frequencies above 20 Hz.

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).

HTS-dcSQUID gradiometer for nondestructive evaluation

We have fabricated a HTS-dcSQUID gradiometer on the basis of a single YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// film for a hand held NDE system. The two loops of the gradiometer are coupled directly to a SQUID arranged at the center of the pickup coil. The gradiometer worked at 77 K and at normal laboratory environment without magnetic shielding. The effect of the small area of the SQUID loop on the performance of the gradiometer was experimentally estimated. The performance of the SQUID gradiometer was investigated by measuring holes in a carbon fiber reinforced plastic plate in a magnetically shielded room.

SCANNING TUNNELLING SPECTROSCOPY OF YBa2Cu3O7−δ THIN FILMS

Abstract Orientational dependence of the tunnelling spectroscopy of YBa2Cu3O7−δ (YBCO) films is investigated by scanning tunnelling spectroscopy. For (110)-oriented films, the surface structures estimated by reflection high energy electron diffraction (RHEED) showed high crystallinity. The tunnelling spectra of the corresponding films showed zero-bias conductance peaks which were uniformly distributed in space. The result is consistent with the theory based on the d 2 x 2 −y -wave symmetry of the pair potential. On the other hand, for (100)-oriented films, the surface structures observed by RHEED showed relatively low crystallinity. The tunnelling spectra of these films showed various types of spectra with strong spatial dependence. These results reflect the high sensitivity of the surface electronic states to the surface qualities of high-Tc superconductors.