CuO₂ bilayer decoupling and T_c suppression in Pr-substituted YBa₂Cu₃O_7+δ

The mechanism behind superconductivity suppression induced by Pr substitutions in high Tc superconductor YBa₂Cu₃O_7+δ (YBCO) has been a mystery since its discovery: in spite of being isovalent to Y³⁺ with a small magnetic moment, Pr³⁺ is the only rare-earth element that has a dramatic impact on YBCO's superconducting properties. Here, understanding the T_c suppression mechanism may hold key to understanding why it is so high. Using angle-resolved photoemission spectroscopy (ARPES), inelastic x-ray scattering (IXS) and DFT+U calculations, we unveil how Pr substitution modifies the low-energy electronic structure and suppresses T_c. Contrary to the prevailing Fehrenbacher-Rice (FR) and Liechtenstein-Mazin (LM) models, the Fermi surface contains no signature of any 4f-states. Yet, strong electron doping is observed primarily on the CuO₂ plane, with little on the CuO chain. Meanwhile, we reveal a pronounced CuO₂ bilayer decoupling and an enhanced CuO chain hopping with Pr-substitution, 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.