Microprobing of the chain site in YBa 2Cu 3O 6+x by cobalt-57 as a function of the oxygen content : Superconductor-insulator transition

Abstract

An observation of the direct correlation between superconductivity and isomer shift of a Mössbauer probe is reported. The chain site in YBa 2Cu 3O 6+ x was microprobed by 57Co 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 Mössbauer isomer shift, which is inversely related to the s-electron density at the 57Fe nuclear probe, increases continuously with diminishing oxygen content and undergoes a sharp leveling off just below O 6.4. 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.