Evidence for two distinct orthorhombic structures associated with different T_c regimes in LnBa₂Cu₃O_7−δ (Ln = Nd, Eu, Gd and Dy): a study of the dependence of superconductivity on oxygen stoichiometry

Abstract

Superconductivity in LnBa₂Cu₃O_7−δ with Ln = Nd, Eu, Gdand Dy has been investigated as a function of δ, closely following the accompanying changes in crystal structure. Orthorhombic GdBa₂Cu₃O_7−δ and DyBa₂Cu₃O_7−δ show a T_c of ≈ 90 K up to δ = 0.2 and a lower T_c plateau (40-50 K) in the δ range 02 to 0.4, similar to that found in YBa₂Cu₃O_7−δ. The orthorhombic structure II in the lower T_c regions is different from the structure I in the 90 K T_c (low δ) region. The unit cell parameters of the orthorhombic I structure in the high T_c region bear the relationship of a a ≠ b~c/3. This relationship is not seen in the low T_c plateau. The low T_c plateau region does not distinctly manifest itself in NdBa₂Cu₃O_7−δ just as in LaBa₂Cu₃O_7−δ.