IT System

Abstract

Abstracl. We have carried out extensive NMR measurements in a Tl-Sr-Ca-Cu-0 system as a function of y. the yttrium/calcium concentration, over the stoichiometry range encompassing the metal-superconductor-(antiferromagnetic)semiconductor transition. The powder samples were partially aligned, and the measurements included a systematic study of "Y shifts, and some relaxation data, in the temperature range 160 to 300 K. as well as room temperature 63Cu and '05TI NMR at 1.5 K. We have also carried out ~csusceptibiiity and x-ray measurements to provide information on the phase purity and electronic structure of the cuprates. Our main conciusions are (i) that the yttrium is coupled more weakly to the copper oxide planes in this system than it is in YBCO. (ii) that the small variations of shift and width in the TI resonance over the alloy range are probably diamagnetic in origin and therefore provide demagnetization and penetration depth information, and (iii) that there appears to be some correlation between the superconducting fraction as measured by Acsusceptibility, and the observability, or lack of it, of Cu NMR and of low temperature TI NMR. We develop a model of local defects associated with the yttrium doping to explain these data; the basic characteristic of this model is that yttrium substitution for calcium engenders eiectronically disordered regions in the crystal. These disordered regions are eliminated from the superconducting fraction, through disruption of the CuO planes, and, at the same time, the disorder is almost certainly sufficient to eliminate copper nuclei from Observability in NMR, probably through major changes in the associated quadrupole interactions. The disruption of the electronically active planes not only quenches electrical conduction in the same area, but also disrupts the antiferromagnetic fluctuations in these areas in the planes. Under these circumstances low temperature TI NMR becomes unobservable; we attribute this to a freezing of the copper-oxygen based moments in the area of the disorder