Critical Field Anisotropy of a Single Crystal GdBa₂Cu₃Ox and HoBa₂Cu₃Ox

A study of anisotropic critical field has been carried out on single crystal samples of GdBa 2 Cu 3 O x and HoBa 2 Cu 3 O x . The angular dependence of the upper critical field can be fitted to the anisotropic Ginzburg-Landau model. The anisotropy ratio H c2 ⊥ / H c2 // depends somewhat on the definition of H c2 and is also temperature dependent, but it generally falls between 3 and 7. The superconducting properties of GdBa 2 Cu 3 O x and HoBa 2 Cu 3 O x are almost identical with that of YBa 2 Cu 3 O x which we reported earlier, except for small differences in T c .