A comparison of twin boundary pinning in nearly fully stoichiometric and oxygen deficient YBa/sub 2/Cu/sub 3/O/sub 7- delta /

The angular dependence of the magnetoresistance was measured on the same single crystal of YBa/sub 2/Cu/sub 3/O/sub 7- delta / prepared initially with a superconducting transition temperature of 93 K and then annealed to obtain a transition temperature of 56.9 K. A second oxygen deficient sample was produced with a transition temperature of 57.0 K and nearly identical transport characteristics. Twin boundary pinning superimposed on flux flow behavior was observed in both the 56.9 K and 93.0 K states for magnetic field rotation in the ab plane. For magnetic field rotations off the c-axis, twin boundary pinning is observed only for the nearly stoichiometric state. The broadening of the resistive transition in magnetic field (H//c) is observed to occur over a much larger interval of reduced temperature in the deoxygenated state. The enhanced broadening of the resistive transition in magnetic field and absence of twin boundary pinning may indicate that the vortex system is two-dimensional, and, in general, defects are rendered ineffective in pinning due to the absence of three-dimensional collective elastic effects. >