Nature of Dynamical Coupling between Polarization and Strain in Nanoscale Ferroelectrics from First Principles

A first-principle-based technique is used to investigate dynamical coupling between polarization and picosecond time-scale strain pulses in ferroelectric nanolayers. Two different dynamical mechanisms are found. The first mechanism concerns homogeneous dipole patterns, is governed by the ultrafast soft-mode dynamics, mostly consists in the modification of the dipoles' magnitude, and leads to a polarization only weakly changing and following the strain pulse via an "usual" coupling law. On the other hand, the second mechanism occurs in highly inhomogeneous dipole patterns, is characterized by a large change in polarization and by a time delay between polarization and strain, and is governed by the "slower breathing" of dipolar inhomogeneities. This second mechanism provides a successful explanation of puzzling experimental data.