Explanation for the low-temperature behavior of Hc1 in YBa2Cu3O7

We present magnetization curves M(H) for YBa₂{Cu}₃O₇ crystals. The onset of deviation from linearity, {δ}M, being analyzed in the framework of the Bean model, yields an ``upturn'' in the temperature dependence of the lower critical field Hc1(T) at low T, similar to that observed by other groups. We propose a generalized Bean model, which takes into account the Bean-Livingston surface barrier. In this model the Abrikosov vortices start to penetrate into the sample at distinct points at the surface where the barrier is suppressed by surface defects. As a result {δ}M appears to be proportional to (H-Hc1{)}³$ rather than to (H-${H}c1)² as in the conventional Bean model. Our analysis shows the conventional BCS saturation of Hc1 at low temperatures.