Flux-Pinning Properties for CVD Processed YBa2Cu3O7 Films

Abstract

Since the discovery of high-temperature superconductors (HTSC), their practical applications at high temperatures such as at liquid nitrogen temperature have been widely expected. Many groups have successfully developed Ag-sheathed Bi-Sr-Ca-Cu-O (BSCCO) and recently a magnetic field of 7T was generated by a superconducting magnet using Bi2Sr2Ca2Cu3O10 tapes under a cryocooling condition [1]. However, the BSCCO system shows a poor magnetic field dependence of the critical current density J c in a high-temperature region above 30 K [2]. The YBa2Cu3O7 (YBCO) system is, on the other hand, expected to see practical use as a superconducting wire of HTSC because of its high J c properties even at high temperature and high magnetic field. This means that strong pinning centers exist in the YBCO system. Fine precipitates such as CuO [3] and Y2O3 [4], oxygen deficiency [5], Y211 large particles [6] and so on were pointed out as effective pinning centers in the YBCO system. Moreover, Tachiki and Takahasi have proposed a new concept for the flux pinning in HTSCs [7]. Superconducting layers can work as strong pinning centers intrinsically, due to a layer structure and a short coherence length along the c-axis of HTSCs. Therefore, this model is called an intrinsic pinning model and is characterized by a two-dimensional feature due to the large structure. Since the intrinsic pinning is closely related to the modulation of the superconducting energy along the c-axis, the relevant pinning strength depends on the anisotropy of the system.