Anisotropy of the upper critical fields and the paramagnetic Meissner effect in La_1.85Sr_0.15CuO₄ single crystals

Optimally doped La_1.85Sr_0.15CuO₄ single crystals have been investigated by dc and ac magnetic measurements. These crystals have rectangular needle-like shapes with the long needle axis parallel to the crystallographic c axis (c-crystal) or parallel to the basal planes (a-crystal). In both crystals, the temperature dependence of the upper critical fields (H_C2) and the surface critical field (H_C3) were measured. The H-T phase diagram is presented. Close to T_C = 35 K, for the c-crystal, , whereas for the a-crystal the obtained is much higher than 1.69, predicted by the ideal mathematical model. At low applied dc fields, positive field-cooled branches known as the 'paramagnetic Meissner effect' (PME) are observed; their magnitude is inversely proportional to H. The anisotropic PME is observed in both a- and c-crystals, only when the applied field is along the basal planes. It is speculated that the high γ^a and the PME are connected to each other.