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

TL;DR

This study investigates the anisotropic upper critical fields and the paramagnetic Meissner effect in La1.85Sr0.15CuO4 single crystals, revealing significant anisotropy and unusual magnetic behavior near the superconducting transition.

Contribution

It provides new measurements of the anisotropic upper critical fields and surface critical fields, and reports the observation of the paramagnetic Meissner effect in different crystal orientations.

Findings

The anisotropy ratio γc ≈ 1.80, γa ≈ 4.0, exceeds theoretical predictions.

Positive field-cooled branches (PME) are observed when the field is along the basal planes.

The PME magnitude is inversely proportional to the applied magnetic field.

Abstract

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, {\gamma}c = / = 1.80(2), whereas for the a-crystal the {\gamma}a = / =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 {\gamma}a and the PME are connected to each other.