X-Ray Photoemission and Absorption Studies of Overdoped YBa₂Cu₃O_7+δ

Cuprate superconductors possess the highest critical temperatures of any superconductor at ambient pressure as well as a rich phase diagram that varies with hole and electron doping, with a superconducting dome emerging on the hole doping side whose maximum delineates that region into the overdoped and underdoped side.  Due to its wealth of overlapping complex phases and relative ease of synthesis, the underdoped side has been widely studied, while the overdoped region has been comparatively under-explored.  However, multiple cuprate compounds have been discovered to exhibit superconductivity at dopings well beyond the boundaries of the traditional superconducting dome.  One such material, high pressure oxygenated (HPO) YBa₂Cu₃O_7+δ (YBCO), has a high TC that is maintained as doping is changed from YBa₂Cu₃O₇ to YBa₂Cu₃O₈.  However, HPO YBCO and other compounds beyond the superconducting dome have only been synthesized as polycrystals, making many traditional probes difficult.  Many questions remain about the material’s actual doping, homogeneity, as well as the proper method of surface preparation for UHV studies.  We will present progress on using X-Ray Absorption Spectroscopy (XAS) and microscale X-Ray Photoemission Spectroscopy (XPS) to characterize the doping and chemical variability of HPO YBCO.