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Anisotropy of the upper critical fields and the paramagnetic Meissner effect in La1.85Sr0.15CuO4 single Crystals

Optimally-doped La1.85Sr0.15CuO4 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 (HC2) and the surface critical field (HC3) were measured. The H-T phase diagram is presented. Close to TC =35 K, for the c-crystal, γc = / = 1.80(2), whereas for the a-crystal the γa = / =4.0(2) obtained, is much higher than the theoretical value 1.69. 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 γa and the PME are connected to each other.