Vortex dynamics and supercurrents in bismuth-2212 superconducting single crystals and films

The magnetic properties of Bi2Sr 2CaCu2O 8+x (BSCCO) single crystals and thin films have been studied systematically with a contactless Hall probe system. The BSCCO single crystals of Tc ≃ 92 K were grown using a crucible-free optical floating-zone technique. The thin film disks of Tc ≃ 82 K were grown using magnetron sputtering technique. Our findings are as follows: (1) The vortex penetration process is strongly time dependent. The penetration time scale varies and depends on the temperature, the applied field and the geometry of the sample. (2) The persistent current relaxations and the effective energy barrier against vortex motion is investigated in a ring-shaped Bi2Sr2CaCu 2Ox single crystal. The results show a transition from a strongly non-logarithmic regime to a logarithmic one with increasing temperature. The vortex dynamics in the non-logarithmic regime are insensitive to changes in the microstructure (decreasing oxygen content), but the relaxations in the logarithmic regime are strongly affected by the microstructure. (3) A peak in the temperature dependence of the trapped vortex field is observed at a temperature below the vortex melting line. Logarithmic and non-logarithmic dependence on time were observed in the magnetic relaxations at temperatures below and above the peak, respectively. The results suggest the presence of the vortex-depinning transition at temperatures below the melting line of BSCCO. (4) Vortex penetration, vortex relaxation and vortex-depinning transition in YBCO single crystal are found qualitively similar to that of BSCCO superconductor, but quantitively different. These difference are supposed to be caused by the strong anisotropy in BSCCO superconductor.