Photo-induced nonlinear band shift and valence transition in SmS

TL;DR

This study investigates how ultrafast laser pulses induce nonlinear changes in the electronic structure of SmS, revealing a photo-induced valence transition and surface effects with potential phase boundary implications.

Contribution

It demonstrates the observation of ultrafast valence transition and surface photovoltage effects in SmS using time-resolved photoelectron spectroscopy, highlighting photo-induced phase boundary proximity.

Findings

Ultrafast valence transition from divalent to trivalent Sm ions.

Surface photovoltage shift of approximately 93 meV.

Photo-induced energy shift of about 58 meV consistent with pressure-induced phase change.

Abstract

The photo-induced band structure variation of a rare-earth-based semiconductor, samarium monosulfide (SmS), was investigated using high-harmonic-generation laser-based time-resolved photoelectron spectroscopy. A nonlinear photo-induced band shift of the Sm 4f multiplets was observed. The first one is a shift to the high-binding-energy side due to a large surface photovoltage (SPV) effect of approximately 93 meV, comparable to the size of the bulk band gap, with a much longer relaxation time than 0.1 ms. The second one is an ultrafast band shift to the low binding energy side, which is in the opposite direction to the SPV shift, suggesting an ultrafast valence transition from divalent to trivalent Sm ions due to photo-excitation. The latter energy shift was approximately 58 meV, which is consistent with the energy gap shift from ambient pressure to the boundary between the black insulator and golden metallic phase with the application of pressure. This suggests that the photo-induced valence transition can reach the phase boundary, but other effects are necessary to realize the golden metallic phase.