Extended Data Fig. 9: Evolution of pseudogap with surface doping.

A doping series of APRES data are taken from black phosphorus whose surface is doped by Na, K, Rb and Cs, marked at the bottom right of each panel. Shown together in number is the dopant density n_d in units of 10¹⁴ cm⁻². It can be clearly seen that the magnitude of the pseudogap progressively decreases to zero with increasing n_d, as summarized in the phase diagram (Fig. 5b). The hole-like states developing inside of the C1 band (pronounced for Na at n_d = 3.3 × 10¹⁴ cm⁻²) are the valence band of black phosphorus²³. The additional feature near E_F and at the Γ point (pronounced for K at n_d = 2.9 × 10¹⁴ cm⁻²) is another conduction band of black phosphorus reproduced in first-principles band calculations²⁴.