SURFACE-BARRIER AND LOWER CRITICAL-FIELD IN YBA2CU3O7-DELTA SUPERCONDUCTORS

The fields for first vortex entry and last vortex exit, H-1 and H-2, and the lower critical field H(c1) for a grain-aligned, sintered YBa2Cu3O7-delta superconductor have been determined from saturated magnetic-hysteresis loops using an extended critical-state model. For fields oriented along the grain c axis, H-1 increases with decreasing temperature, showing an upturn below 50 K, whereas H-2 remains small and positive, in general agreement with the theory of Bean-Livingston surface barriers. H(c1) has a Bardeen-Cooper-Schrieffer temperature dependence above 50 K, but it rises at low temperatures. For fields oriented in the ab plane, H(c1) has a similar temperature dependence, but surface barriers are not evident in the magnetization.