Missing f-electrons in rare earth doped superconducting cuprate Pr-YBCO

Among all rare earth dopants, Pr is known for its rapid suppression of high-Tc superconductivity in YBa2Cu3O7 (YBCO). This supposedly isovalent Y-site substitution was also shown to violate Abrikosov–Gor'kov magnetic pair-breaking theory, posing a longstanding enigma to the field. To explain such a formidable suppressor of high-Tc superconductivity, Fehrenbacher-Rice (FR) model and Liechtenstein-Mazin (LM) model postulate that holes are depleted from CuO2 plane through Pr 4fz(x2-y2)-O2p orbital hybridization, which elevates additional heavy f-bands to the Fermi level. However, surface charging issue has long precluded direct electronic structure investigations of possible interplay between f-states and d-wave superconductivity. In this work, we combine single crystal x-ray diffraction, angle resolved photoemission spectroscopy (ARPES) and first principles calculation to examine the electronic and crystal structure of Pr-doped YBCO. We find exceptionally strong Tc suppression with, surprisingly, Ba-site substitution. In these systems, highly effective electron doping occurs on the CuO2 plane, and to a lesser extent the CuO chain. Moreover, a shrinking CuO2-bilayer splitting energy suggests rapid decoupling of CuO2 bilayer with less than 10% Pr doping. Meanwhile, electron hopping along the CuO chain, which has long been considered unalterable, is substantially enhanced. Our first principles results show that the prevailing FR/LM models dwell on a meta-stable structure, and the true ground state has fully filled/ionized f-states eVs away from the Fermi level. These results not only reveal electron doping as the crucial mechanism for the exceptionally rapid Tc suppression in Pr-doped YBCO, but also underscore the highly effective electronic structure control by dopants that couple the CuO chain to the CuO2 plane.