First-principle study of the electronic structure and magnetism in RuSr<Subscript>2</Subscript>GdCu<Subscript>2</Subscript>O<Subscript>8</Subscript> under pressure

We have performed a first-principle calculation of the structural, electronic and high pressure properties of RuSr<Subscript>2</Subscript>GdCu<Subscript>2</Subscript>O<Subscript>8</Sub script>, 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 RuO<Subscript>6</Subscript> octahedra has been attributed to the residual forces on O<Subscript>Ru</Subscript> which results in shear strain in the RuO<Subscript>2</Subscript> layer. Partial charge analysis shows that applying pressure up to 6 GPa leads to hole creation in the CuO<Subscript>2</Subscript> sheets which causes increase in the superconducting transition temperature. We have estimated the Curie temperature T <Subscript> M </Subscript> 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 RuO<Subscript>2</Subscript> 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 d <Subscript> x </Subscript> <Subscript>2</Subscript> <Subscript>-</Subscript> <Subscript> y </Subscript> <Subscript>2</Subscript> states increases slightly with pressure but it is still very small that it does not affect superconductivity, and the hole doping mechanism is dominant. Copyright EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg 2010