  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.
