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Abstract

We investigate the nonequilibrium insulator-metal transition driven in a SmNiO₃ thin film by coherent optical excitation of the LaAlO₃ substrate lattice. By probing the transient optical properties over a broad frequency range (100 - 800 cm^-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 SmNiO₃ has the same carrier density as the equilibrium metallic phase. We also report that the LaAlO₃ 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 Néel 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.