Tatsuhiko Fujii

Metal-Insulator-Superconductor Field-Effect-Transistor Using SrTiO3/YBa2Cu3Oy Heteroepitaxial Films

Planar-type metal-insulator-superconductor field-effect-transistors (MISFET) were fabricated and their current modulation characteristics were investigated. The FET had a structure of Al-gate metal/(100)SrTiO3-gate insulator/(001)YBa2Cu3Oy-channel, where the oxide layers were grown by selective heteroepitaxy employing pulsed ArF excimer laser deposition. The FET gate was 10 µm long and 100 µm wide. Apparent field-effect modulation of drain current was seen in both the normal (T>Tc) and superconducting (T<Tc) states. In the superconducting state, not only the critical current but also flux-flow resistance were appreciably changed according to the applied gate voltage.

Dielectric properties of SrTiO3 epitaxial film and their application to measurement of work function of YBa2Cu3Oy epitaxial film

The dielectric properties of SrTiO3 epitaxial film were studied using film grown on oxide superconductor YBa2Cu3Oy. They were, to a certain extent, similar to those of the single-crystal bulk in terms of both temperature and electric field dependence, though the value of the dielectric constant was low, particularly at cryogenic temperatures. The observed dielectric properties were analytically expressed. Using the dramatic field dependence of the dielectric constant of the SrTiO3 film in the Au-SrTiO3-YBa2Cu3Oy metal-insulator-superconductor (MIS) structure, we determined the work function of (001)YBa2Cu3Oy epitaxial film to be ~6.1 eV.

Preparation of CaTiO3 heteroepitaxial thin films by excimer laser deposition

CaTiO3 thin films were prepared by a 193 nm ArF excimer laser deposition method. Epitaxial CaTiO3 thin films were obtained on SrTiO3 (100) substrate and YBa2Cu3Oy (001) deposited on MgO (100) single‐crystal substrate. The grown CaTiO3 thin films orientated to (010) direction and the unit cell of CaTiO3 rotated 45° around the [100] axis of the SrTiO3 substrate. It also happened in CaTiO3/YBa2Cu3Oy/MgO system. The static dielectric constant was 120 at room temperature and reached 175 at lower temperatures.

Epitaxial SrTiO3 thin films grown by ArF excimer laser deposition

SrTiO3 (STO) thin films were prepared by an ArF excimer laser deposition method. At the growth temperature of 630°C, the natural orientation of STO was significantly changed from (111) to (100) and (110). The ratio of X-ray diffraction intensities (100)/(200) was closest to the ideal value when grown at 660°C, resulting in minimized disorder of atomic arrangement. Epitaxial STO films were obtained on STO(100) and MgO(100) substrates, and on an YBaCuO(001) epitaxial film. They showed resistivity in excess of 5×1010 Ωcm and dielectric constant as high as 140e0 at room temperature. The maximum dielectric constant of 440e0 was obtained at 40 K.

Analysis on Electric Field Effect in Al/SrTiO3/YBa2Cu3Oy Structure

Planar-type Al/(100)SrTiO3/(001)YBa2Cu3Oy metal-insulator-superconductor field-effect transistors (MISFETs) were investigated experimentally and theoretically. The applied gate field induces the change of the carrier concentration in YBa2Cu3Oy channel (or conducting layer thickness), which in turn modulates the critical current Jc through the flux-creep model and flux-flow resistivity ρt through the Bardeen-Stephen model. Theoretical results explained, to a certain extent, the observed field effect in the MISFET semiquantitatively.

Growth by self-flux method and post-annealing effect of Bi2Sr2Ca1Cu2Ox single crystals

Abstract Post-annealing effects on superconducting characteristics have been studied in Bi 2 Sr 2 Ca 1 Cu 2 O x single crystals grown by a conventional flux method. Also, favorable growth conditions and the effect of the pre-sintering process on the starting materials for flux growth have been examined. The best superconducting behavior is obtained in post-annealed crystals grown from pre-sintered powder materials. The critical current density J c estimated from magnetization hysteresis in annealed crystals grown with pre-sintered materials is roughly 8×10 5 A/cm 2 ( H ∥ c ) and 5×10 4 A/cm 2 ( H ⊥ c ) at 4.2 K at zero field.

Design and feasibility test of unshielded HTS-SQUID system for magnetocardiogram diagnosis

We designed a composite-type HTS-SQUID gradiometer using four magnetometers. One serves as a detector for both the magnetocardio signal and environmental noise signal, and remaining three are references for the noise canceling. An adaptive filtering algorithm was applied to compensation procedure to an automatic operation. We thus fabricated an unshielded HTS-SQUID gradiometer by installing all components in a specially designed package. With digital filtering and simple integration for only 20 s interval, a smooth magnetocardiogram curve was obtained. In addition, the adaptive filtering algorithm worked well and improved the noise-to-signal ratio.

Optimized NdBa2Cu3Oy thin films deposited by eclipsed pulsed laser ablation

Abstract NdBa 2 Cu 3 O y (Nd123) thin films are deposited on ( 1 0 0 )MgO, ( 1 0 0 )Y-stablized ZrO (YSZ) and ( 1 0 0 )SrTiO 3 (STO) at the low temperature of 650°C by pulsed laser ablation with a shadow mask (eclipsed pulsed laser deposition (PLD)). X-ray diffraction and reflection high-energy electron diffraction patterns show that all thin films are oriented in ( 0 0 1 ) direction. Compared with the conventional PLD samples, the eclipsed PLD samples exhibit a narrower superconducting transition width ( ΔT c ). The suppression of ΔT c is believed to result from the elimination of droplets and the stoichiometric deviation of Nd specie. The film on STO shows better superconducting properties than those on YSZ and MgO. Its onset T c , ΔT c and J c reaches 93.5 K, 3.5 K and 6×10 6 A/cm 2 , respectively. A Nd123/MgO thin film step-edge junction is prepared as well. A typical RSJ-like (Josephson) current–voltage curve is observed.

Heteroepitaxial Growth of SrBi4Ti4O15/Bi2Sr2CuO6+y Structure by ArF Excimer Laser Ablation

Heteroepitaxial SrBi4Ti4O15/Bi2Sr2CuO6+y thin films were grown on (100)MgO substrates by the excimer laser ablation method. X-ray, reflection high-energy electron and low-energy electron diffraction (XRD, RHEED and LEED) revealed the epitaxial ordering of deposited film crystal oriented normal to the substrate surface, though the streak patterns were slightly dim. The permittivity of SrBi4Ti4O15 film was obtained as 120 e0 from C-V measurement for Al/SrBi4Ti4O15/Bi2Sr2CuO6+y parallel plate capacitor.

Effects of partial substitution for constituents of Bi2Sr2Ca1Cu2Ox single crystals

Abstract Effects of partial substitution for constituents of J c and U o were studied in Bi 2 Sr 2 Ca 1 Cu 2 O x single crystals. Pb, Ba and Li substitution was effective in slightly improving T c , but Y degraded it significantly. Although U o was insensitive to or showed a slight increase with Ba and Li substitution, the value of J c determined by the magnetic relaxation tended to decrease. Y substitution caused a considerable degradation of both J c and U o . On the contrary, Pb substitution for Bi at about 17% gave a remarkable enhancement of J c to about 10 6 A/cm 2 at 4.4 K, while U o was rather insensitive to the substitution.