Extended Data Fig. 9: Phase transition via decreasing temperature.

a, Core-level spectra of K 3p with different K coverage at 40 K. b, Surface electron band dispersion with respect to K deposition. Corresponding deposition steps to the core-level spectra in a are indicated in the panel (#0–#2). Despite the broadening of the spectra and the slight reduction of the band minimum energy after the K deposition, the entire band dispersion still preserves nearly parabolic close to the pristine case. c, Enlarged phase diagram of pure 2D electron phase taken from Fig. 1a. ARPES measurements were performed by following the process marked in the phase diagram by black and blue arrows. d, Core-level spectra of K 3p observed at different temperatures. The preserved intensity of the K 3p core level indicates the absence of K desorption during the cooling process. e, Temperature-dependent surface electron band dispersion observed after K deposition (deposition step #2). Temperatures were set to 40, 25, and 10 K. f, Peak positions (markers) obtained from e by fitting EDCs. Overlaid solid lines are guides to the eyes for the band dispersion. This exhibits the band deformation from parabola (red, 40 K) to W-shape (blue, 10 K) with decreasing the temperature.