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 Hp at temperature below 50 K determined by the first linear rise in M(T) can be fitted with the relation Hp(T)=Hcexp(-T/T0), consistent with a model where two-dimensional pancakes penetrate the Bean-Livingston barrier by thermal activation. The low critical field Hc1 determined from the bulk penetration of vortices is also presented.