Broadband terahertz spectroscopy of the insulator-metal transition driven by coherent lattice deformation at the $\mathrm{SmNi}{\mathrm{O}}_3/\mathrm{LaAl}{\mathrm{O}}_3$ interface

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 $\left(100-800\mathrm{c}{\mathrm{m}}^{-1}\right)$, 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 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.

Figure 1

Equilibrium optical reflectivity of (a) the bare $\mathrm{LaAl}{\mathrm{O}}_3$ substrate and (b) the nickelate heterostructure. For both materials, the optical response is dominated by three infrared-active phonons of $\mathrm{LaAl}{\mathrm{O}}_3$ at around 200, 450, and $700\mathrm{c}{\mathrm{m}}^{-1}$. (c) Temperature dependence of the resistivity (black solid line) and the magnetic diffraction peak intensity (blue circles) [2] for the $\mathrm{SmNi}{\mathrm{O}}_3$ thin film. $\mathrm{SmNi}{\mathrm{O}}_3$ is a paramagnetic (PM) metal above 370 K, a charge-ordered (CO) paramagnetic insulator at 200 K < $T$ < 370 K, and a charge-ordered antiferromagnetic (AFM) insulator below 200 K.

Figure 2

Transient optical reflectivity of the bare substrate (a–c), and the nickelate heterostructure measured with perpendicular (d–f), and parallel (g–i) pump-probe geometries at $T=10\mathrm{K}$. Upper panel: transient reflectivity at selected time delays. Middle and lower panels: transient reflectivity change from 0 to 30 ps. The 0-ps time delay is defined as the peak of the pump pulse.

Figure 3

(a) Equilibrium reflectivity (gray), transient reflectivity at 1 ps (dark blue), and the fit (orange) of $\mathrm{SmNi}{\mathrm{O}}_3/\mathrm{LaAl}{\mathrm{O}}_3$. (b) The real part of the optical conductivity extracted from the fit. (c) Within experimental error, the same maximum transient conductivity is reached for all measured temperatures. (d) The lifetime of the transient state tends to diverge approaching the metal-insulator transition temperature $T_{MI}$.