Extended Data Fig. 4: Determination of K coverage.

a, Core-level spectra of K 3p with different K coverage on the cleaved [Gd₂C]²⁺·2e⁻ surface. The K 3p core-level peak starts to grow with K deposition near the binding energy of 19 eV and eventually saturates, where we estimate the coverage as 1 ML (thick green curve). Above 1 ML, chemically shifted additional peaks emerge at lower binding energy close to 18 eV. b, A 3D representation of the band evolution with increasing K coverage. c, Extracted band dispersion at several different K coverages indicated by red arrows in b. Surface 2D electron band (1) evolves via K deposition (2) and disappears (3). The complete disappearance of the electron band implies that the surface electron density is actually reduced by K deposition. In the rightmost panel (4), corresponding to K 1 ML, which we estimate with core-level spectra in a, K band appears near Fermi level, which well agrees with the core level estimation.