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_cexp(-T/T₀), consistent with a model where two-dimensional pancakes penetrate the Bean-Livingston barrier by thermal activation. The low critical field H_c1^⊥ determined from the bulk penetration of vortices is also presented.