Can We Achieve High In-Field J_c at 77 K in Bi-Sr-Ca-Cu-O?

Abstract

The critical current density of Bi-Sr-Ca-Cu-O (BSCCO) at 77 K has reached the order of 10⁴ A/cm², but it is rapidly reduced to less than 1000 A/cm² at even small applied field (< 0.5 T). The mechanism responsible for such a behavior has been identified as a 3D to 2D vortex crossover that is directly related to the distance between the conduction Cu-O bilayers. For BSCCO, the bilayer distance is relatively large compared to that of YBa2Cu3O_x, and thus it has a lower crossover temperature and field. Although great attempt has been made to increase flux pinning by introduction of defects, the enhancement is only seen at low temperatures. This study presents a possible approach in flux pining of highly anisotropic high-T_a superconductors. In contrast to the previous effort in pinning of the 2D vortices, it is proposed to increase the Cu-O bilayer coupling strength by a unique processing method. Some experimental evidence suggests that the local lattice distortion can cause a slight reduction in bilayer distance. Since the Josephson coupling is related to the bilayer spacing, the coupling strength is enhanced leading to an increased 3D to 2D crossover temperature and field. As a consequence, the vortex state will remain 3D in a greater T-H region where pinning can be much more effective. Both theoretical discussions and experimental results on the new approach are presented.