Superconductivity suppression and bilayer decoupling in Pr-substituted YBa₂Cu₃O_7-δ

The mechanism behind superconductivity suppression induced by Pr substitutions in YBa₂Cu₃O_7-δ (YBCO) has been a mystery since its discovery: in spite of being isovalent to Y³⁺ with a small magnetic moment, it is the only rare-earth element that has a dramatic impact on YBCO's superconducting properties. Using angle-resolved photoemission spectroscopy (ARPES) and DFT+[Formula: see text] calculations, we uncover how Pr substitution modifies the low-energy electronic structure of YBCO. Contrary to the prevailing Fehrenbacher-Rice (FR) and Liechtenstein-Mazin (LM) models, the low-energy electronic structure contains no signature of any f-electron hybridization or additional f-state Fermi surface sheets. Yet, strong electron doping is observed primarily on the antibonding Fermi surface. Meanwhile, we reveal major electronic structure modifications to Cu-derived states with increasing Pr substitution: a pronounced CuO₂ bilayer decoupling and enhanced hopping along the CuO chain, implying indirect electron-release pathways beyond simple 4f state ionization. Our results challenge the long-standing FR/LM mechanism, and establish Pr substituted YBCO as a potential platform for exploring correlation-driven phenomena in coupled 1D-2D systems.