BROADENING PHENOMENA OF THE RESISTIVE TRANSITION IN SINGLE-CRYSTALLINE YBA2CU3O7 IN MAGNETIC-FIELDS

The superconducting transition of single-crystalline YBa2Cu3O7 has been studied by resistivity measurements in magnetic fields up to 8 T in various current and magnetic field configurations. The results show that for magnetic fields applied parallel to the crystallographic c-axis, the broadening is most significant compared with any other configuration, irrespective of the relative orientation of the current and the magnetic field. This fact implies that the flux motion due to the Lorentz force is not the dominant factor for the broadening, and suggests that the broadening mechanism is closely related to a more intrinsic property of the superconducting state in this compound. In addition, a clear "shoulder" at the foot of the transition that appears only for magnetic fields applied perpendicular to the current direction manifests the existence of a Lorentz-force driven mechanism. The "shoulder" can be reasonably well interpreted as a crossover effect from flux-creep to flux-flow regime. Well above the "shoulder" level, the resistivity seems to depend only on the magnetic field orientation with respect to the crystallographic axes. For this reason, the appearance of the resistivity in this latter region is attributed to "giant" superconducting fluctuations induced by a magnetic field in the disordered flux-line state. © 1990.