Ienari Iguchi

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.

Effective suppression and enhancement of critical current in a nonequilibrium YBa2Cu3O7−x film due to tunnel injection of quasiparticles

Abstract Nonequilibrium phenomena of a high- T c YBa 2 Cu 3 O 7− x (YBCO) film under tunnel current injection from a Pb(or Au)/ MgO/YBCO junction are studied. A clear Cooper-pair breaking effect due to tunnel injection of quasiparticles into the YBCO film is observed at various temperature belows T c of the YBCO film. For a Pb/MgO/YBCO junction which exhibits pronounced Pb and YBCO gap structures and whose junction resistance ( R j ) lies within a certain range, the critical-current enhancement due to tunnel injection of quasiparticles can be observed until the injection current reaches a certain value. Such an effect is not identified in an Au/MgO/YBCO junction. The energy of the maximum injected quasiparticles corresponding to the enhancement state is found to range from Δ to as high as 600 meV. A possible explanation is given for the above experimental results.

Direct observation of Josephson self‐radiation in Bi2Sr2Ca2Cu3Oy break junctions above 77 K

We have directly observed the Josephson self‐radiation emitted from a Bi2Sr2Ca2Cu3Oy break junction at receiver frequencies fREC=11 and 47 GHz above liquid‐nitrogen temperature. The results on the current‐voltage characteristics, the dV/dI vs I curves, the harmonic Josephson self‐radiation, and the dependence of critical current on microwave self‐radiation power are well interpreted in terms of the current biased resistively shunted junction model. The maximum first‐harmonic self‐radiation powers at receiver frequencies fREC=11 and 47 GHz were P(I) =3×10−13 and P(I)=1×10−13 W at 77 K, respectively.

Evaluation of the performance of superconducting nonequilibrium devices

The performance of a superconducting three‐terminal nonequilibrium device utilizing quasiparticle‐injection phenomena is investigated. The switching time is derived from the transient solution of the rate equations, whereas the gains are calculated from the steady‐state solution. For Nb and Pb, the switching times are estimated to be order of 10 ps for sufficiently thin films provided that only the first‐half rapid rise part of the transient behavior is used. The gains are weakly dependent on film thickness because of a high phonon trapping factor. For an appropriate magnitude of load resistance, the current and power gains are calculated to be ∼30 and ∼0.5, respectively. These values are considerably increased by taking the double‐junction‐acceptor geometry.

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 oxygen pressure on the synthesis of Bi4Sr3Ca3Cu4Oy oxide compound

Abstract The effect of oxygen pressure during the sintering and cooling processes on the synthesis of Bi 4 Sr 3 Ca 3 Cu 4 O y oxide compound is investigated. The sintering and cooling pressures below 1 atm are independently controlled. It is found that there exists a certain range of pressure in which the single phase Bi 4 Sr 3 Ca 3 Cu 4 O y is formed. There is also a critical pressure below which the si sintered pellet becomes melted and a superperovskite is no longer formed. Dependence of critical temperature and lattice parameters on oxygen pressure is shown.

Microwave response of YBaCuO/MgO/Pb planar Josephson tunnel junctions

Abstract Microwave response of YBaCuO (YBCO) /MgO/Pb planar Josephson tunnel junctions fabricated by an in situ deposition technique is presented. The observed Josephson critical current is 20–400μA.The current-voltage characterisitcs under microwave irradiation of 9GHz yield the Shapiro steps at integer multiples of hf/2e where f is the microwave frequency. Dependence of the Shapiro step height on the microwave voltage amplitude is consistent with that of the conventional theory. A series of subharmonic steps at half integer multiples of hf/2e are also observable, although their structures are weak.

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.

Behavior of high-Tc Y1Ba2Cu3Oy superconducting bridges in a microwave field

Abstract The external microwave-field dependence of harmonic and subharmonic Shapiro steps and the microwave self-radiation due to the coherent motion of vortices in high- T c superconducting granular Y 1 Ba 2 Cu 3 O y bridges are observed and compared with those of the Josephson Nb-Si-Nb edge-junctions in the centimeter-wave to millimeter-wave range. The harmonic and subharmonic steps of Y 1 Ba 2 Cu 3 O y bridges are related to the synchronization between the applied microwave signal and the local supercurrent oscillations generated by the moving hypervortices in accordance with the Josephson condition V = ( n / m ) ( h /2 e ) f . The microwave-induced subharmonic steps ( n = 1, 2 and m = 1, 2, 3) are discussed in connection with the coherent synchronizing motion of hypervortices in bridges.

Physical aspects of high Tc tunneling junctions

Abstract The tunnel characteristics of high T c superconducting tunnel junctions are briefly reviewed and discussed. A new tunneling model based on the specific features of high T c superconductors is given to interpret the observed results. The temperature dependence of the gap parameter in terms of the tunnel conductance is calculated and compared with the observed data.