Surface valence transition in SmS by alkali metal adsorption

The electronic structure changes of SmS surfaces under potassium (K) doping are elucidated us-ing synchrotron-based core-level photoelectron spectroscopy and angle-resolved photoelectron spectroscopy (ARPES). The Sm core-level and ARPES spectra indicate that the Sm mean valence of the surface increased from the nearly divalent to trivalent states, with increasing K deposition. Carrier-induced valence transition (CIVT) from Sm2+ to Sm3+ exhibits a behavior opposite to that under conventional electron doping. Excess electrons from K atoms are transferred to S sites and the liberated electrons from Sm3+ ions due to CIVT at the surface are trapped like local excitons around the Sm3+ ions, which is inconsistent with the phase transition from the black insulator with Sm2+ to the gold metal with Sm3+ under pressure. This CIVT helps to clarify the pressure-induced black-to-golden phase transition in this material, which originates from the Mott transition of excitons.