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

Bi_2Sr_2Ca_Cu_n0_x（BSCCO）超导体所需的临界电流密度J_C<n-1>取决于应用。因此，在较宽的范围内控制J_C值是该材料实际应用的必要条件。本研究采用2212相（n=2）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：当薄膜由小晶粒组成时，J_C不能反映材料的本征性质，因此尝试在高温下在氧气中退火，可以获得与梅萨尺寸一样大的晶粒。通过在氮气中低温退火控制BSCCO中的氧含量，随着退火时间的增加，C轴方向的J_C沿着减小，减小范围为10^4-10^1A/cm ^2。临界温度随退火时间的变化先急剧上升后缓慢下降。有人认为，这些现象反映了载流子密度的变化.临界电流密度与其它性能的关系：主要研究了ab方向沿着J_C与表面电阻的关系。Y系超导体的表面电阻与晶粒尺寸和J_C的乘积的关系没有得到证实，而与J_C的关系得到证实.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与器件性能的关系。它们的电流-电压（I-V）特性可分为两种类型：一种是磁通-流动型，另一种是本征结的多分支型，表现出电压跳变和电压突变。J_C小于等于10^3A/cm ^2的样品的I-V曲线呈多分支。

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.