Surface valence transition in SmS by alkali metal adsorption

TL;DR

This study investigates how potassium adsorption induces a valence change in SmS surfaces, revealing a carrier-induced transition from Sm$^{2+}$ to Sm$^{3+}$ that differs from pressure-induced transitions, and clarifies the underlying Mott transition mechanism.

Contribution

It demonstrates a surface valence transition in SmS caused by alkali metal adsorption, providing new insights into the electronic structure changes and phase transition mechanisms.

Findings

K doping increases Sm surface valence from divalent to trivalent

Carrier-induced valence transition opposes pressure-induced behavior

Transition involves exciton trapping and Mott transition

Abstract

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 Sm$^{2+}$ to Sm$^{3+}$ 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 Sm$^{2+}$ to the gold metal with Sm$^{3+}$ 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.