Observation of a geometrical barrier in YBa2Cu3O6.95 single crystals

The remnant and in-field magnetic field distributions of two crystals of YBa2Cu3O6.95, one with a single twin boundary and one completely untwinned, were observed using a nondestructive magneto-optic technique. Geometrical barrier effects on the flux penetration of both samples were found to be present. Two distinct penetrating field patterns were observed resulting from the different pinning characteristics of the samples. A concentration of penetrating vortices in the center of one sample indicated an extremely low degree of pinning. A comparison was made between experimental flux penetration profiles from both samples and those calculated using a theoretical model for a thin strip superconductor. A single twin boundary in the extremely low pinning crystal was also observed to be a barrier to transverse vortex motion.