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" Magnetic polarons in superconducting RuSr_2(Eu,Gd)Cu_2O_8"

Relatore: Prof. Miguel A. Gusmao, Universita' Federale, Port

Aula Newton
25 Maggio 2005 ore 15.00

In recent years, some ruthenates have attracted much attention because of their interesting magnetic and superconducting properties. Several experiments with RuSr2(Eu,Gd)Cu2O8 demonstrate that superconductivity develops without suppressing magnetic order, suggesting that superconducting CuO2 planes and magnetic RuO2 planes behave as nearly separate entities. Susceptibility measurements yield a positive Curie constant, while neutron diffraction experiments indicate antiferromagnetic ordering. Aiming to understand this puzzling behavior, and focusing on the magnetic properties,we start with Ru ions on a square lattice, taking into account the on-siteCoulomb interactions between t2g orbitals, and a tetragonal crystal-field splitting. Using perturbation theory in the hopping terms, we derive effective Hamiltonians to describe the RuO2 planes, and construct a phase diagram for the undoped case. We also obtain that electron doping yields formation of ferromagnetic polarons in an AF background, possibly explaining the observed magnetic response, and presenting interesting consequences to the superconducting phase.