EVIDENCE OF SURFACE BARRIERS IN SINGLE-CRYSTAL TL2BA2CUO6 SUPERCONDUCTORS

We report direct evidence of surface barriers in single crystals of Tl2Ba2CuO6 superconductors. Magnetization is measured as a function of temperature with magnetic field applied parallel to the c axis. For small field, a characteristic linear increase in M is observed at an onset temperature T1. For higher magnetic field, a new feature at low T is observed in the M(T) data. The flux penetration temperature is retarded to a higher temperature. The penetration field H(p) at temperature below 50 K determined by the first linear rise in M(T) can be fitted with the relation' H(p)(T)=H(c)exp(-T/T0), consistent with a model where two-dimensional pancakes penetrate the Bean-Livingston barrier by thermal activation. The low critical field H(c1)perpendicular-to determined from the bulk penetration of vortices is also presented.