The Nature of the Superconducting State in Rutheno-Cuprates

Abstract

The magnetism and superconductivity (SC) of rutheno-cuprates RuSr₂LCu₂O_8−δ (Ru1212) and RuSr₂(L,Ce)₂Cu₂O_10+δ (Ru1222) were investigated, where L = Gd or Eu. The normal state carrier concentration p, the T _c, the intragrain penetration depth λ, and the diamagnetic field-cooled magnetization were measured in various annealed rutheno-cuprate samples. The p varies with annealing only slightly (from 0.09 to 0.12 holes/CuO₂), but the intragrain T _c by a factor of 2.3 (from 17 to 40 K). The 1/λ ₂, on the other hand, was enhanced more than tenfold (from 0.3 to 6 µm⁻²). The data are in disagreement with both the universal T _c vs. p and T _c vs. 1/λ ₂ proposed for bulk cuprates. These, together with the unusually large dT _c/dH ≈ 100 K/T observed in both Ru1212 and Ru1222, suggest that even the intragrain SC of rutheno-cuprates is granular. A Josephson-junction-array model was then proposed to interpret the data. The memory effect observed in Ru1222 far above the main magnetic transition temperature further suggests that the root of the granularity may be a phase separation, resulting in the mesoscopic ferromagnetic and antiferromagnetic species in these rutheno-cuprates.