Nuclear spin relaxation in high-Tc superconductors

The project was to investigate the electronic states of thallium-containing high-Tc superconductors applying the NMR techniques, the main goal being to find answers to the following questions not yet resolved: 1) What is the valence of the Tl ions in these materials? 2) What anisotropy is there at the various lattice sites? 3) Can metal-type behaviour signs be found? 4) How strong are antiferromagnetic correlations? 5) How does the structure of the flux line lattice look like in the superconducting state? The experiments revealed precise information on both the valence and the anisotropy of interactions, from which the symmetry of the Tl positions could be derived. It was possible to directly observe in the NMR experiment the exchange of Ca and Tl atoms, and from the resulting defect line together with the corresponding knight shift and spin-lattice relaxation, metal-type behaviour was concluded to occur in the superconductors. A comparative analysis of the spin-lattice relaxation at various lattice sites allowed to establish a correlation between antiferromagnetic fluctuations and metal-type charge carriers, and to describe the influence on the critical temperature. (orig./MM)