Beyond the Pure Hubbard Model in Sr_14-xCa_xCu₂₄O₄₁ Ladders

Since the discovery of superconductivity in the hybrid ladder-chain family Sr_14-xCa_xCu₂₄O₄₁ (SCCO), these materials have been viewed as the prototypical realizations of doped ladder models. While not superconducting, the parent compound Sr₁₄Cu₂₄O₄₁ is itself self-doped and has a robust static charge density wave (CDW) whose wavevector is at odds with the doping estimates from x-ray absorption (XAS) if understood from a Hubbard ladder perspective. We investigate this using state-of-the-art density matrix renormalization group (DMRG) calculations compared to high-resolution resonant inelastic x-ray scattering (RIXS) of the ladders in Sr₁₄Cu₂₄O_41. We extract the low-energy parameters of a pure single-band Hubbard ladder constrained via Bayesian Optimization and demonstrate the need for corrections to the model, including nearest-neighbor attractive interactions that enhance the pairing of doped holes. We show that this model produces results consistent with the experimentally observed CDW and RIXS spectra. Our results support the growing evidence against pure Hubbard model descriptions of strongly correlated cuprates.