Observation of low-energy 4f electrons in Pr-doped high-T_c cuprate Bi-2212

Over 50 different elements have been used for substitution in high temperature cuprate superconductor Bi₂Sr₂CaCu₂O_8+y (Bi-2212) since 1988. Among the out-of-plane dopants, Pr is one of the most effective T_c suppressant in part due to its electron doping effect. However, the role of Pr’s 4f orbitals and its antiferromagnetic order near 8 K remain less explored. Here we report the structural and electronic evolution in Bi₂Sr₂Ca_1-xPr_xCu₂O_8+y single crystals with resonant ARPES, XRD, and STEM-EDX. Our results show a systematic c-axis contraction and ab plane expansion, driven by the increased interlayer electrostatic interaction, with STEM confirming Pr substitution at the Ca site. In the electronic structure, besides electron doping, we observe a suppression of the quasiparticle peak along the antinodal direction and the emergence of multiple Pr 4f-derived states overlapping with both the ZRS singlet band and the nonbonding oxygen states. These findings suggest possible hybridization between the Pr 4f and the superconducting CuO plane. Together, these results provide a new avenue to investigate the interaction between low-energy f-electrons and high temperature superconductivity.