Studies on the Control of the Critical Current Density of High-Quality Bi-System Superconductor Thin Films And Its Application

Bi_2Sr_2Ca_<n-1>Cu_n0_x（BSCCO）超导体所需的临界电流密度J_C取决于应用。因此，在广泛的范围内控制J_C对于这种材料的实际应用是必要的。本研究采用2212相（n=2）.1研究了退火对BSCCO高质量薄膜J_C的控制，以及J_C与其他材料性能的关系。通过退火控制临界电流密度：由于J_C不能反映薄膜小晶粒时材料的固有特性，因此尝试了高温氧退火，得到了大到台面尺寸的晶粒。通过低温氮退火控制BSCCO中的氧含量，随着退火时间的延长，沿c轴方向的J_C在10^4 ~ 10^1A/cm^2范围内减小。临界温度随退火时间的变化先急剧上升后缓慢下降。这些现象反映了载流子密度的变化。临界电流密度与其他性能的关系：主要研究了沿ab方向的J_C与表面电阻的关系。在y系超导体中，表面电阻与晶粒尺寸和J_C的乘积的关系尚未得到证实，而与J_C的关系则被提出。临界电流密度控制方法的应用可能性：作为J_C控制方法的应用，制备了本征结，并研究了J_C与器件性能的相关性。它们的电流-电压（I-V）特性分为两种类型：一种是通量流型，另一种是多分支型，典型的本征结具有电压跳变和滞后。对于J_C小于等于10^3A/cm^2的样品，观察到具有多个分支的I-V曲线。

The critical current density J_C required for Bi_2Sr_2Ca_<n-1>Cu_n0_x(BSCCO) superconductors depends on the application. Therefore, the control of J_C over a wide range is necessary for the practical application of this material. In this research, the control of J_C of BSCCO high-quality films by annealing and the relationship between J_C, and other material properties were investigated using the 2212 phase (n=2).1. Control of the Critical Current Density by Annealing :Since J_C does not reflect the intrinsic material properties when the film is composed of small grains, annealing in oxygen at high temperatures was tried and grains as large as the mesa size could be obtained. By controlling the oxygen content in BSCCO by annealing in nitrogen at low temperatures, J_C along the c-axis direction decreased over the range of 10^4-10^1A/cm^2 with increasing annealing time. The critical temperature increased steeply and then decreased slowly as a function of the annealing time. It was suggested that these phenomena reflect the change in the carrier density.2. Relationship between the Critical Current Density and Other Properties :Relationship between J_C along the ab-direction and the surface resistance was mainly investigated. The dependence of the surface resistance on the product of the grain size and J_C, which is reported for Y-system superconductors, was not confirmed, while its dependence on J_C, was suggested.3. Application Possibility of the Control Method of the Critical Current Density :As an application of the control method of J_C, intrinsic junctions were fabricated and the correlation between J_C and the device properties was investigated. Their current-voltage (I-V) characteristics were classified into two types : one was the flux-flow type and the other was the multiple-branching type typical for intrinsic junctions showing voltage jumps and hystereses. The I-V curve with multiple branches was observed for samples with J_C of 10^3A/cm^2 or less.