Temperature dependence of the lower critical field of high-T-c superconducting crystals near Tc

We have measured the nonlinear susceptibility of YBa2Cu4O8, Bi2Sr2CaCu2O8, and untwinned YBa2Cu3O7 crystals near the superconducting critical temperat ure T-c in order to determine the lower critical field H-c1(T). The ac fiel d amplitude dependence of the susceptibility for homogeneous YBa2Cu4O8 and Bi2Sr2CaCu2O8 crystals is in agreement with the geometrical barrier model a llowing us to determine the penetration field H-p(T)proportional to H-c1(T) . We clearly show that in these crystals the previously reported breakdown of H-c1 near T-c coincides with the low-temperature ac field amplitude inde pendent onset of the superconducting transition. We show that this anomalou s behavior of H-c1(T) is due to the influence of inhomogeneities and it is not an intrinsic property. We have also investigated the influence of the e arth field on the nonlinear susceptibility, and we demonstrate the importan ce of its shielding for careful measurements of H-c1 near T-c. [S0163-1829( 99)10029-8].