Fig. 1: Fermi surface evolution of Ca₃Cu₂O₄Cl₂ with Rb deposition.

a Schematic electronic structure picture of the cuprate parent compound. It consists of the lower Hubbard band (LHB) and upper Hubbard band (UHB) that are separated by the on-site Coulomb repulsion U. The charge-transfer band (CTB) lies in between the LHB and UHB bands, giving rise to a charge-transfer gap Δ. The chemical potential μ is located inside the gapped region. b Crystal structure of CCOC326 consisting of double CuO₂ planes in one structural unit. The crystal cleaves between two adjacent CaCl layers, as marked by the pink sheet. c Schematic illustration of in situ Rb deposition onto the sample surface. Electrons are transferred into the top layers due to the physical absorption of Rb on the sample surface. d Fermi surface mapping of CCOC326 at different Rb-deposition stages. For convenience, we use 0–13 to denote the deposition sequence where 0 represents the original sample before Rb deposition. The images for sequences 0–9 are obtained on one sample while sequences 10–13 are obtained from another sample that complements each other. The Fermi surface images are obtained by integrating the spectral weight over an energy window of [−0.03, 0.03] eV with respect to the Fermi level. e Estimated electron doping level after each Rb-deposition stage. For the deposition stages from 3 to 11, the electron doping level is calculated from the area of the electron-like Fermi surface centered at (−π, 0). For the deposition sequences 12 and 13, it is estimated from the area of the hole-like Fermi surface centered at (−π, π). The doping level for deposition stages 1 and 2 are obtained by linear extrapolation in the low doping region according to the deposition procedures. Error bars reflect the uncertainty in determining the Fermi momentum.