Superconductivity and Magnetism in RuSr2GdCu2O8

The ruthenocuprate RuSr₂GdCu₂O₈ orders magnetically at 135 K and then becomes superconducting at 45 K. These transitions have been observed by several groups, but the intrinsically complex nature of the compound, as well as the unexpected coexistence of magnetism with high-temperature superconductivity, makes it uncertain what the magnetic, electronic, and structural character of its ground state is. We find that density functional-based evaluations give a magnetic and structural character that is consistent with the latest data: the RuO₆ octahedra rotate (by 7° around the c^ axis) causing a doubling of the cell, and an antiferromagnetic structure that has this same cell doubling is favored over a ferromagnetic alignment of Ru spins. The minority Ru d_xy states are partially occupied, leaving a metallic RuO₂ layer that dopes the CuO₂ bilayer and may lead to canting of the Ru spins.