Angular Dependence of the Magnetic-Field-Induced Resistive Transition in Single-Crystal Bi2.2Sr2Ca0.8Cu2O8+

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We report here detailed measurements of the angular and temperature dependence of the magnetic-field-induced resistive transition in single-crystal Bi2.2Sr2Ca0.8Cu2O8+α. Resistance is observed in the sample for small values of applied field (100 Oe) even at low current densities (< 10 A/cm2). This suggests that the resistive transition does not reflect the upper-critical field of the superconductor in a simple manner, but is complicated by flux-flow effects. Consequently, the angular dependence of the resistive transition cannot be fit by any standard model for anisotropic critical fields in superconductors. We find anistropic ratios as large as 50 for fields applied perpendicular and parallel to the c^ axis of the crystal, indicating large differences in the viscosity for flux motion in the two directions.



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