Jun Otsuka

A study on the control of the material properties of Bi2Sr2CaCu2Ox thin films for intrinsic Josephson junctions

Abstract We investigated the annealing conditions to observe intrinsic Josephson effects using Bi-cuprate superconductor thin films. We fabricated a mesa structure using as-grown and annealed Bi 2 Sr 2 CaCu 2 O x (Bi-2212) thin films and measured their electrical properties along the c -axis. Consequently, we succeeded in fabricating intrinsic Josephson junctions using Bi-2212 thin films by means of a two-step annealing procedure, which is a combination of a high-temperature annealing in O 2 and a low-temperature annealing in N 2 . By this annealing method, the grain size changed over the range from 0.02 μm to more than 5 μm in diameter and the critical temperature T c could be controlled in the range of 72–85 K. The critical current density along the c -axis, J cc , was also controlled in the range of 10 4 –10 1 A/cm 2 . The temperature dependence of the resistivity of the mesa with reduced J cc was semiconductor-like above T c . The current–voltage ( I – V ) characteristics exhibited clear hystereses and multiple branches, and the observed voltage jump was 3–20 mV.

Surface resistance of Bi-2212 films and influence of intergrowth

Abstract We investigated surface resistance R s of Bi-2212-based films fabricated by the multitarget sputtering technique, focusing on the effect of intergrowth on R s . The Bi-2212 single-phase film had surface resistance R s of about 1 mΩ at 30 K and 20 GHz. Intergrowth with Bi-2201 made the R s worse, while intergrowth with Bi-2223 improved, not only the R s , but also the critical current density J c more than expected from the ratio of Bi-2223. These phenomena were discussed in terms of the change in the carrier density by the intergrowth. The improvement of these properties by the intergrowth with Bi-2223 was possibly due to the decrease in carrier density in Bi-2212, which is naturally overdoped, by the interaction with neighboring Bi-2223 cells.

Preparation of intrinsic Josephson junctions using Bi-2212 thin films

We fabricated intrinsic Josephson junctions 10 /spl mu/m/spl times/10 /spl mu/m in area using Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub x/ (Bi-2212) thin films with large grains employing a two-step annealing procedure. The first annealing step at a high temperature in O/sub 2/ is for the growth of crystal grains and the second step at a lower temperature in N/sub 2/ is for the control of the oxygen content in the film. By this method, Bi-2212 thin films with the c-axis critical current density J/sub c/ of 10/sup 1/-10/sup 4/ A/cm/sup 2/ were obtained. A mesa structure was formed on the surface of the annealed film and we measured its electrical properties along the c-axis. The current-voltage characteristics with clear hystereses and multiple branches were observed for the samples with J/sub c/ of /spl sim/10/sup 3/ A/cm/sup 2/ or less. The gap structure was observed in the current-voltage characteristics for low mesas. J/sub c/ and the voltage jump decreased more rapidly with increasing temperature than those predicted by the BCS theory. We could obtain intrinsic Josephson junctions in Bi-2212 thin films with similar properties as in single crystal samples.

Surface resistance of Bi-2212 films fabricated by multitarget sputtering

We investigated the surface resistance of Bi-2212 films fabricated by the multitarget sputtering method. The surface resistance was not dependent on the product of the grain size and the critical current density, although many high temperature superconductors such as Y-Ba-Cu-O agree with such a weak-link model. Therefore, it is possible that the conduction losses are relatively low at grain boundaries in Bi-Sr-Ca-Cu-O. On the other hand, it was found that the intergrowth of Bi-2212 with Bi-2201 or Bi-2223 influenced the surface resistance badly. This is very interesting, although it has never been clarified as to why the surface resistance is dependent on the intergrowth. Further study will be required in order to interpret the behavior of the surface resistance of BSCCO.

Corrosion Behavior of Silicon Nitride Ceramics in Sulfuric Acid Solution

Corrosion behavior of gas pressure sintered silicon nitride with alumina and yttria additives was investigated in sulfuric acid solution with respect to the initial stage of the corrosion in which drastic strength degradation and fairly large weight change were observed in our previous study. Thickness of the corroded layer, in which glassy grain boundary phase was dissolved in the solution, was affected by the surface finishing of samples. The corroded layer was thicker in as-ground specimen than in specimen whose surface layer of about 30μm or more was removed by polishing. Residual stress introduced by surface machining was determined by an X-ray diffraction analysis for the as-ground specimen and the polished specimen. It was observed that the compressive stress of about 70MPa remained on the as-ground specimen surface and that the residual stress was gradually removed by polishing. To clarify the effect of surface condition on the corrosion behavior, annealing was employed as an alternative method to eliminate the residual stress and the same corrosion test was conducted. The similar result was obtained for the annealed specimens as for the polished specimens. From these results, the strength degradation and weight change at initial stage for the as-ground specimens are considered to be related to the surface damage by machining.

Control of the Material Properties of Bi-2212 Thin Films and Its Application to Intrinsic Josephson Junctions

We succeeded in fabricating intrinsic Josephson junctions using Bi2Sr2CaCu2O8+δ (Bi-2212) thin films by means of a two-step annealing procedure. By this method, the grain size changed over the range of 0.02~5 μ m in diameter and the critical temperature could be controlled in the range from 73K to 85K. We fabricated a mesa structure 10 X 10 μ m2 in area using annealed Bi-2212 thin films and measured electrical properties along the c-axis. The temperature dependence of the resistivity of the mesa above T c was semiconductor-like. The current-voltage (I-V) characteristics exhibited clear hystereses and multiple branches, and the observed voltage jumps were between 3mV and 20mV. Furthermore, the critical current was modulated by a magnetic field.

Microwave Properties of Bi 2 Sr 2 CaCu 2 O x Films Fabricated by Multitarget Sputtering

We investigated the surface resistance of Bi2Sr2CaCu2Ox films by the dielectric resonator method. The surface resistance of the films fabricated by the multitarget sputtering method is the order of 10−3Ω at about 20 GHz at 30 K, while that of the ones annealed at 820°C for 2 hours in O2 is about 10−4 Ω. These values are as low as those of Y-Ba-Cu-O and Tl-Ba-Ca-Cu-O or lower, while it is also necessary to confirm the accuracy of the surface resistance measurement.