Doping effects in high-T_c 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. It is well-known that the optimal superconducting properties are obtained at a specific doping level, and at too low or no doping competing magnetic phases develop.

There are no final conclusions about the relations between these properties, but the studies carried out in the present program on a number of superconducting materials have supplied a significant improvement of our basic understanding. The primary system of investigation has been YBa_2Cu_3O_6_+_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 CuO_2 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 NdBa_2Cu_3O_6_+_x, and of the Cu and Pr ordering in PrBa_2Cu_3O_6_+_x have been studied by neutron diffraction with the main purpose of understanding why PrBa_2Cu_3O_6_+_x is magnetic and non-superconducting for all oxygen stoichiometries. In NdBa:2Cu_3O_6_+_x studies of the magnetic flux lattice have been carried out by Small Angle Neutron Scattering.