Magnetic and electronic structures of superconducting RuSr₂GdCu₂O₈

The coexistence of ferromagnetism and superconductivity in RuSr₂GdCu₂O₈ was reported both from experiments (by Tallon et al.) and first-principles calculations (by Pickett et al.). Here we report that our first-principles full-potential linearized augmented plane wave calculations, employing the precise crystal structure with structural distortions (i.e., RuO₆ rotations) determined by neutron diffraction, demonstrate that antiferromagnetic ordering of the Ru moments is energetically favored over the previously proposed ferromagnetic ordering. Our results are consistent with recently performed magnetic neutron diffraction experiments (Lynn et al.). Ru t_2g states, which are responsible for the magnetism, have only a very small interaction with Cu e_g states, which results in a small exchange splitting of these states. The Fermi surface, characterized by strongly hybridized dpσ orbitals, has nesting features similar to those in the two-dimensional high-T_c cuprate superconductors.