EXPLANATION FOR THE LOW-TEMPERATURE BEHAVIOR OF HC1 IN YBA2CU3O7

We present magnetization curves M(H) for YBa2Cu3O7 crystals. The onset of deviation from linearity, delta-M, being analyzed in the framework of the Bean model, yields an "upturn" in the temperature dependence of the lower critical field H(c1)(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 delta-M appears to be proportional to (H-H(c1))3 rather than to (H-H(c1))3 as in the conventional Bean model. Our analysis shows the conventional BCS saturation of H(c1) at low temperatures.