Lower critical fields of Bi2Sr2CaCu2O8+d and YBa2Cu3O7-d single crystals

Temperature dependence of the lower critical fields H (c1)(T) of Bi2Sr2CaCu 2O8+d and YBa2Cu3O7-d single crystals has been investigated for both orient ations of the applied fields, parallel and perpendicular to the c axis. The magnetization curves of the samples were measured using a superconducting quantum interference device, and H (c1,c)(T) (H parallel to c axis) and H ( c1,ab)(T) (H perpendicular to c axis) were obtained using the modified Kim- Anderson critical-state model. From the results, we found both H (c1,c)(T) and H (c1,ab)(T) for a Bi2Sr2CaCu2O8+d single crystal had an anomalous uptu rn below about 40 K, while a YBa2Cu3O7-d single crystal did not show any an omaly down to about 10 K. Discussion on a remarkable anomaly H (c1,ab)(T) a t low temperatures is made using the formulas for anisotropic H (c1) in the Ginzburg-Landau theory. The result suggests that the temperature dependenc e of the coherence length in the ab plane plays an essential role. We may i nsist that the anomalous upturn of H (c1,ab) for a Bi2Sr2CaCu2O8+d single c rystal at low temperatures is attributed to the enhancement of the supercon ducting order parameter in the domain between the CuO2 layers, as theoretic ally predicted in terms of a proximity effect. Penetration-depth ratios lam bda(c)(2)(0)/lambda(c)(2)(T) and lambda(ab)(2)(0)/lambda(ab)(2)(T) were als o calculated using a theory based on incoherent quasiparticle hopping. Plot s of the ratios as a function of the reduced temperature qualitatively repr oduce the prediction by the hopping model, and suggest that both Bi2Sr2CaCu 2O8+d and YBa2Cu3O7-d single crystals seem to possess the nature of d-wave pairing. (C) 1999 American Institute of Physics. [S0021-8979(99)06012-0].