Superconductivity suppression and bilayer decoupling in Pr substituted YBa$_2$Cu$_3$O$_{7-δ}$

TL;DR

Praseodymium substitution in YBa2Cu3O7-δ rapidly suppresses superconductivity, and the mechanism has been debated due to proposed f-electron hybridization. By combining ARPES, DFT+U, and non-resonant inelastic x-ray scattering, the study finds no evidence for low-energy Pr f states or f-derived hybridization near the Fermi level; instead, Pr substitution causes substantial electron doping of the CuO2 antibonding band and a pronounced bilayer decoupling, accompanied by enhanced CuO chain hopping. These results challenge the Fehrenbacher-Rice and Liechtenstein-Mazin hybridization scenarios as the sole explanation for Tc suppression and highlight alternative, correlation-driven pathways in coupled 1D-2D CuO networks. Overall, Pr-substituted YBCO emerges as a versatile platform to explore how site-specific electronic-structure engineering in CuO chains and CuO2 planes governs high-Tc superconductivity and related phenomena.

Abstract

The mechanism behind superconductivity suppression induced by Pr substitutions in YBa$_2$Cu$_3$O$_{7-δ}$ (YBCO) has been a mystery since its discovery: in spite of being isovalent to Y$^{3+}$ 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+$U$ 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 new states. 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 CuO$_2$ bilayer decoupling and an enhanced CuO chain hopping, implying indirect electron-release pathways beyond simple 4$f$ 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.

Figures (20)

• Figure 1: Pr substituted YBCO and REBa$_2$Cu$_3$O$_{7-\delta}$ (RBCO) superconducting transition and crystal structure of Pr substituted YBCO. (A) Rare earth substitution effects in YBCO. Blue circles: RBCO superconducting transition temperature (adapted from YANG1987515). Grey circles (adapted from NEUMEIER1992158), orange stars (Type 1), and triangles (Type 2) : Pr substitution dependence of $T_c$. (B) Crystal structure of Pr substituted YBCO.
• Figure 2: ARPES measured electronic structure of Pr substituted YBCO. (A-E) Fermi surfaces with extracted Fermi momentum . (F) Hole doping level evolution for AB, BB and chain with Pr substitution. Circles (triangles) represent type 1 (type 2) samples. (G-K) Nodal cuts at 51 eV highlighting plane bands with fitted dispersions. (L) Bilayer splitting energy evolution with Pr substitution. (M-Q) Nodal cuts at 43 eV highlighting chain band with fitted dispersions. (R) Band bottom position evolution with Pr substitution. Circles (triangles) represent type 1 (type 2) samples. Orange curves are extracted directly from EDC fitting. Blue dashed lines are band bottom position evolution expected from pure charge doping effects.
• Figure 3: Cu 3$d$ orbital imaging with non-resonance inelastic x-ray scattering. (A) Energy loss spectrum measured with an incident energy of 9690 eV on nominally 80% Pr substituted YBa$_2$Cu$_3$O$_{7-\delta}$ (non-superconducting) at 20 K along [100] direction. (B) In-plane angular dependence of the Cu M$_1$ transition intensity as a direct measure of the in-plane angular profile of Cu 3$d_{x^2-y^2}$ orbital. Orbital lobe profile along the $xz$ plane for (C) 80% Pr substituted YBa$_2$Cu$_3$O$_{7-\delta}$ and (D) fully oxygenated pristine YBa$_2$Cu$_3$O$_{7-\delta}$ ($T_c$=90 K). Black dotted line denotes the orbital shape of Pr-doped sample for ease of comparison.
• Figure 4: Electron doping and superconductivity suppression in Pr substituted YBCO. (A) Surface hole doping level dependence on the bulk doping level for Pr substituted YBCO (markers) and YBCO with hole doping controlled by oxygen content (gray dashed line from PhysRevB.76.064519). The bulk hole doping level for Pr substituted YBCO is obtained by comparing $T_c$ with the oxygen content controlled YBCO phase diagram in PhysRevB.73.180505. (B) Superconducting transition temperature against surface hole doping level derived in this work.
• Figure 5: Pr $f$-orbitals predicted by DFT calculations. (A) The ground-state projected density of state (DOS) of Pr $f$-orbitals on the Y site. Fermi energy is set to be the reference energy on the horizontal axis. (B-C) Corresponding Wannier function isosurfaces of the occupied orbitals (B) $f_{z^3}$ and (C) $f_{y(3x^2-y^2)}$, where blue and yellow represent positive and negative values, separately. The isosurface level is chosen at 20% of the maximum absolute value. The $xyz$ coordinates represent the local coordinates used to define the orbitals of Pr. (D-F) DOS and Wannier functions of Pr $f$-orbitals on the Ba site. The local coordinates are rotated compared to (A-C).