Abstract
Superconducting onset temperatures T c up to 23-44 K are reported for the new RuCa 2 RCu 2 O 8+δ (R = Pr, Nd, Sm, Eu, Gd) system with a Ru-1212 RuCa 2 PrCu 2 O 8+δ -type structure. Preliminary study indicates that the crystal structure is an orthorhombic distortion of the tetragonal RuSr 2 GdCu 2 O 8+δ -type structure, which has space group P4/mmm, T m ∼ 136 K, and T c (max) ∼ 65 K. With similar RuO 6 octahedral and CuO 5 pyramidal units, superconductivity from the Cu-O bi-layers apparently coexists with the weak ferromagnetic (WFM) order with magnetic ordering temperature T m ∼ 49 K. Low temperature anomalous in magnetic properties observed in both field-cooled (FC) and zero-field-cooled (ZFC) modes are closely related to the complex interplay and coexistence between weak ferromagnetic order and superconductivity as well as from structural distortion and sensitive sample oxygen concentration variation. No superconductivity or Ru-1212 structure can be detected for larger rare earth compounds R = La and Ce. © World Scientific Publishing Company.