# Current-Induced Metallization and Valence Transition in Black SmS

## TL;DR

This study investigates how applying an electric current induces a transition from insulator to metal in SmS, revealing that increased hybridization between Sm 4f and 5d orbitals causes the valence change and metallization.

## Contribution

It provides direct experimental evidence linking current-induced metallization in SmS to enhanced Sm 4f-5d hybridization and valence transition, clarifying the origin of CIMT.

## Key findings

- Carrier density rapidly increases with current at low temperatures.
- The direct gap size increases with current.
- Mean valence shifts from Sm2+ to mixed valence with current.

## Abstract

A strongly-correlated insulator, samarium mono-sulfide (SmS), presents not only the pressure-induced insulator-to-metal transition (IMT) with the color change from black to golden-yellow but also current-induced IMT (CIMT) with negative resistance. To clarify the origin of the CIMT of SmS, the electronic structure change has been investigated by optical reflectivity and angle-integrated photoelectron spectra by applying an electric current. At lower temperatures than about 100 K, where the nonlinear $V$-$I$ curve has been observed, the carrier density rapidly increases, accompanied by decreasing relaxation time of carriers with increasing current. Then, the direct gap size increases, and the mean valence changes from Sm$^{2+}$-dominant SmS to the mixed-valent one with increasing current. These results suggest that the CIMT originates from increasing the Sm $4f$-$5d$ hybridization intensity induced by the applied current.