Doping effects in high-Tc superconductors

The purpose of the project has been to study how the superconducting and magnetic properties of the high temperature superconductors change as function of oxygen stoichiometry and cation doping. The primary system of investigation has been YBa2Cu3O6+x, which has been studied as function of oxygen stoichiometry, 0 < x < 1, and cation doping with Al, Co and Fe on the Cu-sites and Nd and Pr on the Y-site. In these materials the hole doping into the CuO2 planes, that is necessary for superconductivity, is strongly depending on structural ordering. The static properties and the kinetics of the structural ordering process have been studied experimentally by neutron and high energy synchrotron x-ray diffraction, by Raman scattering, and by computer simulation technique. Not only the oxygen stoichiometry but also the cation doping has been shown to influence the magnetic phases, in some cases in an unexpected manner. Thus, by neutron diffraction experiments it has been shown that doping with non-magnetic Al gives rise to a new magnetic phase. A theoretical model, has been developed. The magnetic phases of the Cu and Nd ordering in NdBa2Cu3O6+x, and of the Cu and Pr ordering in PrBa2Cu3O6+x have been studied by neutron diffraction with the main purpose of understanding why PrBa2Cu3O6+x is magnetic and non-superconducting for all oxygen stoichiometries. In NdBa:2Cu3O6+x studies of the magnetic flux lattice have been carried out by Small Angle Neutron Scattering. Additional structural studies of the superconducting and magnetic phases of related materials, of RENi2B2C (RE = rare earth), and of oxidized and cation doped materials based on La2CuO4+δ have been carried out. Methods for structural studies and analyses, and equipment for electrical and magnetic characterization have been developed. (EG) 5 tabs., 46 ills., 35 refs