Ultrafast Complex Oxide Electronic Materials: Picosecond-Scale Oxygen Octahedral Rotations in Multiferroic BiFeO₃

The structure, electrical polarization, and magnetism of complex oxide electronic materials rely on a balance of competing effects that readily be altered by external perturbations. At ultrafast picosecond-scale times this balance is particularly sensitive to perturbations of the electronic screening introduced by short-duration optical pulses. In the multiferroic oxide BiFeO₃ the result of the optically induced screening includes the rapid development of an elastic stress and, on an acoustic timescale, the expansion of the lattice and a shift in the rotation angle of oxygen octahedra. Experiments with femtosecond time-resolution using optical-pump/x-ray-probe experiments at free electron laser x-ray light sources these features reveal the interaction of structural degrees of freedom with the ultrafast screening. The distortions are consistent with density functional theory structural calculations, leading to the possibility that the structure and properties excited states reached by screening can be systematically explored. Ultrafast diffraction studies of ferroelectric/dielectric superlattices show that the screening-induced phenomena can include the generation of oscillatory modes in the THz frequency regime.