MICROPROBING OF THE CHAIN SITE IN YBA2CU3O6+X BY CO-57 AS A FUNCTION OF THE OXYGEN-CONTENT - SUPERCONDUCTOR INSULATOR TRANSITION

An observation of the direct correlation between superconductivity and isomer shift of a Mossbauer probe is reported. The chain site in YBa2Cu3O6+x was microprobed by Co-57 as a function of oxygen content. The dominant four-fold oxygen-coordinated species A serves uniquely to sense the electronic environment at the chain site. The magnitude of the Mossbauer isomer shift, which is inversely related to the s-electron density at the Fe-57 nuclear probe, increases continuously with diminishing oxygen content and undergoes a sharp leveling off just below O6.4. This corresponds to superconductor (metal) to insulator transition observed by Cava and coworkers for a variety of parameters versus oxygen content. It has been attributed to an electronic charge transfer from the chain to the sheet site. The increase in isomer shift with diminishing oxygen content is indicative of a decrease in itinerant electron population in 4s orbitals and may reflect the charge transfer from the chain site.