Explanation for the low-temperature behavior of Hc1 in YBa2Cu3O7

We present magnetization curves M(H) for ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ crystals. The onset of deviation from linearity, δM, being analyzed in the framework of the Bean model, yields an ‘‘upturn’’ in the temperature dependence of the lower critical field ${\mathit{H}}_{\mathit{c}1}$(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 δM appears to be proportional to (H-${\mathit{H}}_{\mathit{c}1}$ ${)}^3$ rather than to (H-${\mathit{H}}_{\mathit{c}1}$ ${)}^2$ as in the conventional Bean model. Our analysis shows the conventional BCS saturation of ${\mathit{H}}_{\mathit{c}1}$ at low temperatures.