Studies on Anisotropic Upper Critical Fields of HoBa2Cu3O7-δ and ErBa2Cu3O7-δ Using Preferentially Oriented Pellets

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Abstract
Anisotropic behaviors of the upper critical fields H c2 studied in as-sintered disks of HoBa2Cu3O7-δ and ErBa2Cu3O7-δ are described. Highly oriented samples with c-axis perpendicular to the disk plane, which are characterized by X-ray diffraction analysis, show anisotropic behavior of H c2 for midpoints and almost isotropic behavior for the offset field. For HoBa2Cu3O7-δ, the slopes (-dH c2/dT) for midpoints are 5.2 T/K and 2.5 T/K for the magnetic field parallel and perpendicular to the disk plane, respectively, and are 3.9 T/K and 1.9 T/K for ErBa2Cu3O7-δ. An analysis based on a simplified model of preferentially oriented disk of a quasi-two-dimensional anisotropic superconductor suggests that the anisotropy factor ε between the c-axis and the a-b plane is about 0.2 for both materials.