The electronic structure changes of SmS surfaces under potassium (K) doping are elucidated using 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 are trapped by isolated excitons, 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.
@article{arxiv.2205.15522,
title = {Surface valence transition in SmS by alkali metal adsorption},
author = {Takuto Nakamura and Toru Nakaya and Yoshiyuki Ohtsubo and Hiroki Sugihara and Kiyohisa Tanaka and Ryu Yukawa and Miho Kitamura and Hiroshi Kumigashira and Keiichiro Imura and Hiroyuki S. Suzuki and Noriaki K. Sato and Shin-ichi Kimura},
journal= {arXiv preprint arXiv:2205.15522},
year = {2023}
}