Bean-Livingston barriers and first field for flux penetration in high-T_c crystals

We present evidence for the importance of Bean-Livingston (BL) barriers for field penetration into high-Tc crystals. The magnetization curves M(H) and the first field Hp for flux penetration were measured near the transition temperature Tc of untwinned Y-Ba-Cu-O crystal by using a miniature Hall probe. There are three observations that serve as evidence for the efficiency of BL barriers: (1) the magnetization was found to be almost zero on the descending branch of the magnetization loop; (2) The slope of H p(T) exhibits a clear change close to Tc, being largest at Tc; (3) after introducing damage by irradiating the sample, both the field Hp and the width of the M(H) loops reduce significantly, showing almost reversible behavior for the sample. We explain these observations in terms of BL barriers which are shown to be especially important in high-Tc superconductors, and these could be responsible for the controversy of the Hc1 values reported previously in the literature.