Observation of vortex distribution in samples of Bi-2223 Ag-sheathed tapes with and without uranium doping by means of the high-resolution Bitter method

We examined the distribution of vortices in both Bi-2223 Ag-sheathed tape and U-doped Bi-2223 Ag-sheathed tape applying the high-resolution Bitter method. In the low temperature region (T≪T_c), we found that the vortices in the undoped Bi-2223 Ag-sheathed tape developed into either a triangular arrangement or a heterogeneous arrangement along the grain boundaries of the Ag sheath, which form weak pinning centers. The vortex distribution for U-doped Bi-2223 samples was slightly different from the distribution for the undoped material. We observed some movement of the vortices in both the U-doped and the undoped Bi-2223 Ag-sheathed tapes at 50 K. At higher temperatures, T>75 K, the vortices in the Bi-2223 tapes moved even faster so that no deposition pattern, indicating the location for vortices, formed.