Emergent Picosecond-Scale Dynamics of Nanoscale Ferroelectric Polarization Configurations

Ferroelectric thin films and superlattices with nanoscale dimensions exhibit a competition of energetic phenomena that produce configurations of the polarization that are distinct from bulk materials. These configurations have a profound effect on the equilibrium and driven dynamics of the nanoscale material. These dynamics are coupled to the structure of these layers and can be observed in synchrotron radiation or free-electron-laser structural studies. For example, the nanoscale polarization configuration of PbTiO₃/SrTiO₃ (PTO/STO) superlattices, for example, exhibits equilibrium fluctuations that arise from the existence of many configurations with nearly identical free energy. The optically driven dynamics of the PTO/STO superlattice include a picosecond-scale distortion of the polarization in response to the optically induced acoustic pulse and a change in the magnitude of the polarization of the PTO due to polarization screening. The effect of polarization screening is comparatively larger in a strongly BaTiO₃/CaTiO₃ (BTO/CTO) superlattice. In BTO/CTO superlattice screening effects following above-bandgap excitation lead to a reduction in the polarization of the CaTiO₃ layer, evident as a contraction of the out-of-plane lattice parameter of the CaTiO₃component. The characteristic timescale for the development of these effects is on the scale of several picoseconds to tens of picoseconds, set by the propagation of acoustic pulses through the system. Further picosecond-scale diffraction studies reveal that the nanoscale structure affects the longitudinal acoustic sound velocity. There are prospects to employ these effects to affect structural parameters such as the oxygen octahedral tilt that affect other properties of ferroelectrics as well as the magnetism of multiferroics.