Soft pinning and the two-step Tc transition of granular ceramic superconductors

Abstract

In the (Bi_1−xSn_x)Sr₂CaCu₂O_8+δ and (Bi_1−xSn_x)Sr₂CaCu₂O_8+δ superconducting systems prepared via the solid state reaction method with 0 ≤ x ≤ 0.7, a distinct two-step transition was observed in the real part of the complex susceptibility curves. This phenomenon was previously explained in terms of the intragrain at the first transition (T_C1), and then at the second transition (T_C2) intergrain shielding effects in the bulk material, T_C1 > T_C2. This identifies the two-step transition observed as an extrinsic effect. However, it should be noted that for other granular superconductors this two-step transition was not observed. It is therefore important to investigate the character of this two-step transition and know how its presence can further our understanding of high-T_C ceramic superconductors and its applications.
We propose that the use of oxide precursors must aid in the formation of flux pinning centers that keep the intergranular Josephson vortices in place after the first critical temperature transition (T_C1). These pinning centers are temperature dependent, or soft pinning centers, such that when a certain temperature near the second transition (T_C2) is achieved, magnetic flux is easily excluded in the network of linked intergranular material and the Josephson vortices are forced to move out. Moreover, the observed two-step transition is independent of the superconducting properties of the two systems. Therefore, a phenomenon that is not intrinsic to superconducting properties must indeed cause the notable temperature delay between intragrain and intergrain shielding effects.