First-principle study of the electronic structure and magnetism in RuSr2GdCu2O8 under pressure

We have performed a first-principle calculation of the structural, electronic and high pressure properties of RuSr2GdCu2O8, a ferromagnetic superconductor, by employing a full-potential linearized augmented plane-wave method within the density-functional theory. The effect of pressure was achieved by varying the volume of the unit cell with constant a:b:c ratio. The experimentally observed anti-phase rotation of RuO6 octahedra has been attributed to the residual forces on ORu which results in shear strain in the RuO2 layer. Partial charge analysis shows that applying pressure up to 6 GPa leads to hole creation in the CuO2 sheets which causes increase in the superconducting transition temperature. We have estimated the Curie temperature TM of this compound in the mean-field approximation using Heisenberg model with first-nearest neighbor exchange interactions determined from DFT calculations for parallel and anti-parallel spin configurations of Ru moment in RuO2 planes. The effect of pressure causes the magnetic moment of Ru atoms to decrease due to the increase of hybridization between the adjacent Ru atoms. The calculated exchange splitting in Cu dx2-y2 states increases slightly with pressure but it is still very small that it does not affect superconductivity, and the hole doping mechanism is dominant.