Selective optical excitation of a substrate lattice can drive phase changes across heterointerfaces. This phenomenon is a non-equilibrium 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 non-resonant 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 NdNiO 3 , when an insulator-metal transition is driven by coherent lattice distortions in the LaAlO 3 substrate. We find that charge redistribution propagates at supersonic speeds from the interface into the 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 hetero-interfaces.
2/16Complex oxide heterostructures have emerged as a versatile means to engineer functional materials at equilibrium [1,2,3]. In nickelate thin films, for example, electronic and magnetic properties have been controlled statically by producing interfacial strain through the substrate [4,5,6].Insulator-metal, magnetic and structural transitions can also be induced dynamically by controlling these hetero-interfaces with light, particularly when the crystal lattice of the substrate is deformed coherently with mid-infrared radiation. In the case of LaAlO 3 /NdNiO 3 heterostructures, a long-lived metallic phase can be triggered in the NdNiO 3 film by exciting large amplitude vibrations in the LaAlO 3 substrate [7]. This new type of optically controlled phase transition is very appealing as a basis for fast phase change devices, in which the optical control is spatially separated from the functional material. One can envisage device architectures, in which phase fronts transport and manipulate information in new ways.The underlying physics and the role of different orders in space and time has been investigated in some detail in recent experiments. Ultrafast resonant soft x-ray diffraction experiments have, for example, revealed that a supersonic paramagnetic/antiferromagnetic front accompanies the insulator metal transition (IMT), propagating from the hetero-interface into the material [8]. Additional insight was provided by optical experiments in a related LaAlO 3 /SmNiO 3 heterostructure, in which the same substrate excitation was shown to induce an IMT in the nickelate film also above the Néel temperature [9]. This observation was taken as evidence that melting of magnetic order may follow the IMT and may not be its cause. Loss of charge 3/16 disproportionation alone, launched at the interface and propagating into the material, may in fact be the root cause for the IMT.Here, we complete this physical picture with femtosecond non-resonant and resonant hard x-ray diffraction (XRD) experiments, which are used to measure the rearr...