Broadband terahertz spectroscopy of the insulator-metal transition driven by coherent lattice deformation at theSmNiO3/LaAlO3interface

Abstract

We investigate the nonequilibrium insulator-metal transition driven in a $\mathrm{SmNi}{\mathrm{O}}_{3}$ thin film by coherent optical excitation of the $\mathrm{LaAl}{\mathrm{O}}_{3}$ substrate lattice. By probing the transient optical properties over a broad frequency range $(100\ensuremath{-}800\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1})$, we analyze both the time-dependent metallic plasma and the infrared optical phonon line shapes. We show that the light-induced metallic phase in $\mathrm{SmNi}{\mathrm{O}}_{3}$ has the same carrier density as the equilibrium metallic phase. We also report that the $\mathrm{LaAl}{\mathrm{O}}_{3}$ substrate acts as a transducer only at the earlier time delays, as the vibrations are driven coherently. No long-lived structural rearrangement takes place in the substrate. Finally, we show that the transient insulator-metal transition occurs both below and above the N\'eel temperature. We conclude that the supersonic melting of magnetic order measured with ultrafast x rays is not the driving force of the formation of the metallic phase. We posit that the insulator-metal transition may origin from the rearrangement of ordered charges at the interface propagating into the film.