Zhongmin Wen

Tunnel gap structure and tunneling model of the anisotropic YBaCuO/I/Pb junctions

Abstract Measurements on the YBCO/I/Pb and YBCO/I/Au planar tunnel junctions (YBCO:YBA2Cu3O7−x, I: insulating barrier) with various artificial and native barriers are reported. The junctions with an artificial barrier of a few nm thick, grown by in situ deposition, exhibit the YBCO and Pb gap structures, and zero bias anomaly (conductance peaking behavior around zero bias). These tunnel characteristics are similar among various artificial barriers (MgO, AlOx, Y2O3) used. On the other hand, the junctions with a native barrier grown ex situ by exposing the YBCO film to the ambient atmosphere exhibit the YBCO and Pb gap structures and the proximity effect like conductance dip around zero bias, consistent with the results reported previously by other groups. The anisotropic gap values are identified as Δab=16–20 meV in the ab-plane and Δc∼5 meV along the c-ax is. The results are interpreted consistently in terms of the model proposed taking the contributions from both the ab-plane and c-axis tunneling processes and the effect of surface degradation of the YBCO films into account.

Preparation of Y−Ba−Cu−O thin films by rf-magnetron sputtering

Thin films of Y-Ba-Cu-O compound are prepared by rf-magnetron sputtering technique. Superconductivity appears for those films annealed at 900-930°C. The best thin films exhibit superconducting onset at 86 K and complete zero resistance at 76 K. From the X-ray diffraction analysis, it is found that the films annealed at 920°C have a strong c-axis orientation perpendicular to the film surface. The observed X-ray intensities are in good qualitative agreement with the calculated ones. The results of SEM and Auger electron spectroscopy are also presented.

Josephson tunneling and gap structure in Y1Ba2Cu3O7-y/oxide barrier/Pb planar junctions on tetragonal LaSrGaO4 (100) substrates

The Y1Ba2Cu3O7−y/oxide barrier/Pb thin film tunnel junctions are fabricated on the (100) plane of new tetragonal LaSrGaO4 single‐crystal substrates by an in situ electron‐beam deposition technique. Reproducible Josephson behavior is observed for all fabricated junctions. Above a Josephson critical current, a premature switching voltage of about 1 mV appears. The I‐V curve under an applied magnetic field exhibits low‐leakage quasiparticle tunneling characteristic. Modulation of Josephson critical current by the applied magnetic field yields a reversible Fraunhofer‐like pattern against flux direction.

Effect of Substrate Temperature on the Crystalline State of Ba2ErCu3O7-x Superconducting Thin Films by rf Magnetron Sputtering

The crystalline state of the Ba2ErCu3O7-x thin films deposited at various substrate temperatures by rf magnetron sputtering are investigated. The as-sputtered films deposited above 650°C have a layered-perovskite structure which exhibits superconductivity by an annealing process in an oxygen flow. The annealed films have a better crystalline state and better superconducting properies than the films annealed after the deposition at lower substrate temperatures. The spatial distribution of the film composition across the thickness is also studied by Rutherford backscattering spectrometry.

Preparation and properties of Bi(Pb)-Sr-Ca-Cu-O films above 100 K by RF magnetron sputtering

Abstract Fabrication of Bi(Pb)-Sr-Ca-Cu-O thin films above 100 K by RF magnetron sputtering using a target containing Pb is reported. The superconducting and material properties of films under various sputtering and annealing conditions are studied. Thin films deposited at 700°C and annealed at 886°C in air exhibit a zero-resistivity temperature of 104 K. Dependence of high- T c phase growth on substrate temperature and annealing temperature is also presented.

Magnetic Properties and Current-Voltage Characteristics of Anisotropic Y1Ba2Cu3O7-y / Native Barrier / Pb Josephson Planar Tunnel Junctions

The magnetic properties and the current-voltage characteristics of Y1Ba2Cu3O7-y(YBCO) / native barrier / Pb Josephson tunnel junctions employing both the c-axis- and a, b-axes- oriented YBCO films are reported. The stable Fraunhofer-like patterns of Josephson critical current under the applied magnetic field are observed. The current-voltage curves of the junctions made of a, b-axes-oriented YBCO films exhibit hysteretic behavior typical of conventional Josephson tunnel junctions. The anomalous temperature dependence of Josephson critical current is also reported.

Fabrication and physical properties of DC squids using Bi(Pb)SrCaCuO thin films

Abstract The fabrication and physical properties of DC SQUIDs using Bi(Pb)SrCaCuO thin films are reported. The total Josephson critical current of DC SQUIDs with different microbridge widths (0.5–12μm) ranges from 8 μA to 5 mA. The observed quantum interference pattern under an applied magnetic filed is largely deformed and a discussion based on the multiple interference effect is given. The measured voltage modulation depth is found to be smaller than the calculated value. The Josephson critical current exhibits a (1− T / T rmc ) n dependence on temperature where n =1−2.5.

In Situ Fabrication of Reproducible Josephson Tunnel Junctions Employing Co-evaporated YBaCuO Films

The in-situ fabrication of reproducible and controllable YBaCuO(YBCO)/barrier/Pb planar Josephson tunnel junctions utilizing an electron-beam and thermal heater coevaporation chamber equipped with a shadow mask system is presented. As an insulating barrier of tunnel junction, both the artificial and native barriers are used. The tunnel resistance exhibited an exponential behavior against the barrier thickness. The results on the quasiparticle tunneling and Josephson tunneling are shown. The observed Josephson characteristics were appreciably different between the junctions with the c-axis and a(b)-axis oriented YBCO films. The anomalous temperature dependence of Josephson critical current is also reported.

Proximity Effect in YBaCuO/Al/AlOx/Pb Planar Tunnel Junctions

We report the proximity effect of high Tc YBCO(YBaCuO) superconductors using an YBCO/AI/AIOx/Pb(or Au) tunnel-junction geometry. The junctions were fabricated by an in situ deposition technique on YBCO epitaxial films utilizing an electron-beam coevaporation system. The insulating layer was formed by the oxidation of Al surface in oxygen gas atomosphere. The observed characteristics strongly depended on the Al thickness. Without an Al film, a broad YBCO gap structure at about 20mV and a pronounced Pb gap structure around zero bias were observed. The Pb gap structure was broadened as the Al thickness was increased. For the Al thickness above 15nm, the Pb gap structure became completely disappeared.