Magnetic order driven ultrafast phase transition in NdNiO3

Ultrashort x-ray pulses can be used to disentangle magnetic and structural dynamics and are accordingly utilized here to study the photoexcitation of NdNiO3 (NNO), a model nickelate exhibiting structural and magnetic dynamics that conspire to induce an insulator-to-metal transition (IMT). During the course of the photoinduced IMT with above-gap excitation, we observe an ultrafast (<180 fs) quenching of magnetic order followed by a time delayed collapse of the insulating phase probed by x-ray absorption and THz transmission (450 fs) that correlates with the slowest optical phonon mode involved in the structural transition. A simultaneous order-disorder response at the Ni site and displacive response at the Nd site coexist in the ultrafast magnetic response. Crucially, we observe the optical phonon through its coherent coupling with Nd magnetic order, demonstrating that the magnetic and structural degrees of freedom both contribute in driving the IMT. Density functional theory calculations reveal a consistent scenario where optically driven intersite charge transfer drives a collapse of antiferromagnetic order that in turn destabilizes the charge-ordered phase resulting in an IMT. These experiments provide different modalities for the control of electronic phase transitions in quantum materials based on the ultrafast interplay between structural and magnetic orders created by femtosecond photoexcitation. © 2022 American Physical Society.