Effects of 1-GeV uranium ion irradiation on vortex pinning in single crystals of the high-temperature superconductor YBa2Cu3O7-delta

We report magnetoresistance measurements of the effects of 1-GeV uranium ion irradiation on a clean, untwinned single crystal of YBa2Cu3O7-delta. In order to isolate the effects of irradiation, we masked one region of the crystal from the radiation so that it would retain its original characteristics. The columnar defects created by the irradiation dramatically alter the shape of the irreversibility line. In addition, the pinning anisotropy of the vortices is reversed by the irradiation. We estimate the pinning energy from the depinning angle and compare it to the pinning energy measured in twinned single crystals. We show that the onset of pinning from the columnar defects occurs very close to the zero-field superconducting transition temperature, well into the vortex liquid state. In addition, measurements across the boundary between the irradiated and unirradiated regions directly demonstrate that the dissipative behavior of Vortex flow due to the Lorentz force is similar to the dissipation of two resistors in series for this geometry.