Multiple Supersonic Phase Fronts Launched at a Complex-Oxide Heterointerface

Selective optical excitation of a substrate lattice can drive phase changes across heterointerfaces. This phenomenon is a nonequilibrium analogue of static strain control in heterostructures and may lead to new applications in optically controlled phase change devices. Here, we make use of time-resolved nonresonant and resonant x-ray diffraction to clarify the underlying physics and to separate different microscopic degrees of freedom in space and time. We measure the dynamics of the lattice and that of the charge disproportionation in ${\mathrm{NdNiO}}_3$, when an insulator-metal transition is driven by coherent lattice distortions in the ${\mathrm{LaAlO}}_3$ substrate. We find that charge redistribution propagates at supersonic speeds from the interface into the ${\mathrm{NdNiO}}_3$ film, followed by a sonic lattice wave. When combined with measurements of magnetic disordering and of the metal-insulator transition, these results establish a hierarchy of events for ultrafast control at complex-oxide heterointerfaces.

Figure 1

(a) Temperature-dependent hysteretic peak position of the ${(055)}_{Pbnm}$ diffraction peak, measured from the ${\mathrm{NdNiO}}_3$ thin film at 8 keV photon energy. The inset shows corresponding diffraction peaks ($\theta \text{−}2\theta$ scans) measured below and above the phase transition temperature. Data taken at Diamond Light Source, I16 beam line. (b) Time evolution of the ${\mathrm{NdNiO}}_3$ ${(505)}_{Pbnm}$ reflection ($\theta \text{−}2\theta$ scans) at 8 keV photon energy and at 40 K nominal sample temperature, induced by the direct lattice excitation of the ${\mathrm{LaAlO}}_3$ substrate with $15-\mu \mathrm{m}$ wavelength pulses of $\sim 3.6\mathrm{mJ}/{\mathrm{cm}}^2$ fluence. The diffraction intensity, normalized to the incident x-ray intensity, is color coded on a linear scale. Data were taken at the LCLS free electron laser, XPP beam line. The right panel shows the midinfrared excitation spectrum (black data points, Gaussian fit in red) compared to the resonances of the highest-frequency phonons in bulk ${\mathrm{NdNiO}}_3$ (green) and ${\mathrm{LaAlO}}_3$ (blue). (c) Comparison between the vibrationally induced changes at the ${(505)}_{Pbnm}$ and ${(055)}_{Pbnm}$ reflections, detected $+4\mathrm{ps}$ after the excitation. The solid lines are Gaussian fits to the data.

Figure 2

Static photon energy-dependent intensity of the ${\mathrm{NdNiO}}_3$ ${(505)}_{Pbnm}$ reflection (blue dots), measured at the LCLS free electron laser in the vicinity of the Ni $K$ edge at 40 K nominal sample temperature. These data are normalized to the x-ray intensity incident on the sample and compared to the ${(101)}_{Pbnm}$ reflection of a ${\mathrm{NdNiO}}_3$ thin film grown on a ${\mathrm{SrTiO}}_3$ substrate, measured at 7 K using synchrotron light (red solid line, data taken from Ref. [21]). (b) Transient peak intensity of the ${\mathrm{NdNiO}}_3$ ${(505)}_{Pbnm}$ reflection, measured at x-ray energies resonant (8.346 keV, blue data) and off-resonant (8.329 keV, green data) with the Ni $K$ edge, induced by the same ${\mathrm{LaAlO}}_3$ phonon excitation with a $3.6\text{−}\mathrm{mJ}/{\mathrm{cm}}^{-2}$ fluence. Again, these diffraction intensities are normalized to the incident x-ray intensity. The measured resonant diffraction intensity comprises a charge order contribution (illustrated by the gray shaded region), which disappears on a time scale shorter than that of the lattice dynamics. (c) Time evolution of the charge order parameter, calculated from the data shown in (b) as described in the text.

Figure 3

Transient low-frequency terahertz reflectivity of a ${\mathrm{NdNiO}}_3/{\mathrm{LaAlO}}_3$ heterostructure, induced by the direct lattice excitation of the ${\mathrm{LaAlO}}_3$ substrate at 10 K. The insulating nickelate film is transformed to the metallic state within less than 2 ps.

Figure 4

(a) Short time scale spatiotemporal dynamics of the ${\mathrm{NdNiO}}_3$ lattice, magnetic and insulator-metal dynamics along the thin film out-of-plane direction. The structural phase front [black dots, extracted from data presented in Fig. 1] propagates from the interface across the $\sim 13\mathrm{nm}$ film at about the bulk speed of sound ($4.1\mathrm{nm}/\mathrm{ps}$, gray solid line). The red data points represent the magnetic melt front, copied from Ref. [8]. The insulator-metal front induced by the same excitation in a $\sim 33\mathrm{nm}$ ${\mathrm{NdNiO}}_3$ film (extracted from the terahertz data shown in Fig. 3) is shown as the blue data points, together with the charge order melt front [blue solid line, extracted from the resonant diffraction data presented in Fig. 2]. The gray shaded area indicates that data were taken on samples of different thicknesses. (b) Illustration of the involved phase front propagation. At equilibrium, the material is antiferromagnetically and charge ordered. Following excitation of the substrate lattice, the loss of charge disproportionation, magnetic order melting, and the lattice rearrangement advance at different speeds across the film.