Temperature dependence of the vortex remanent state in high-Tcsuperconductors

Abstract

Temperature and magnetic field dependence of the vortex penetration into a superconductor and the resulting trapped vortex field (the vortex remanent state) were investigated for Bi 2 Sr 2 CaCu 2 O 8+x (BSCCO) and YBa 2 Cu 3 O 6+x (YBCO) single crystals and BSCCO thin films. The experiments revealed changes in the pinning regime (the magnitude and magnetic relaxation) of the trapped vortex field with an increasing temperature. The trapped vortex field, obtained by applying a constant magnetic field, exhibits a maximum at a certain temperature, that separates the partial vortex penetration regime at low temperatures from the complete vortex penetration state at higher temperatures. The corresponding vortex remanent states in these two regimes are characterized by two distinctly different relaxations, the logarithmic and the nonlogarithmic ones at temperatures below and above the maximum, respectively, for both BSCCO and YBCO. At temperatures close to T c surface/geometric barrier affect the relaxation rates. PACS number(s): 74.25. Op, 74.25.Uv, 74.25.Wx According to the Bean's model, 1 when an external magnetic field is applied to a superconductor, the internal magnetic field is not uniform and its local value depends on the position inside a superconductor. 2 When the external magnetic field is removed, a nonuniform vortex field is trapped in a superconductor. Trapping of the internal field inside a superconductor can be realized by using the following procedure: 3 apply an external magnetic field, H a , to a superconductor at different temperatures, which results in the penetration of vortices into the bulk of the superconductor. H a is subsequently reduced to zero and the vortex lines are trapped inside the sample. At a fixed temperature, the trapped magnetic field increases with an increasing H a and finally reaches a saturated (remanent) value. 4 The remanent value of the trapped internal field is proportional to the critical current.However, applying the same constant H a at different temperatures leads to a different situation. In this case, the magnitude of the trapped internal field is determined by the dependence of the penetration and pinning of the vortices on temperature. At low temperatures, the sample is partly penetrated by the field and the internal field is trapped at the sample's edges. It is expected that the trapped internal field increases with an increasing temperature and at a certain temperature, fully penetrates the sample, i.e., reaches a maximum value at the sample's center. The question is, however, what is the magnitude and the temperature dependence of this field at higher temperatures? Does it follow the temperature dependence of the remanent critical state where the current density acquires the critical value J c , or does it have a different temperature dependence? What are the relaxation rates of this field at temperatures corresponding to the partial and complete vortex penetration states? Are they affected by the surface/geometrical barriers? What is their dependence on the ...