Surface barrier and lower critical field in YBa2Cu3O7-δ superconductors

The fields for the first vortex entry and last vortex exit, H1 and H2, and the lower critical field Hc1 for a grain-aligned, sintered YBa2Cu3O7-δ superconductor have been determined from saturated magnetic-hysteresis loops using an extended critical-state model. For fields oriented along the grain c axis, H1 increases with decreasing temperature, showing an upturn below 50 K, whereas H2 remains small and positive, in general agreement with the theory of Bean-Livingston surface barriers. Hc1 has a Bardeen-Cooper-Schrieffer temperature dependence above 50 K, but it rises at low temperatures. For fields oriented in the ab plane, Hc1 has a similar temperature dependence, but surface barriers are not evident in the magnetization. © 1993 The American Physical Society.