2017120915:00

报告人：李千

Advanced Photon Source (APS), Argonne National Laboratory, USA

Ferroelectric polar vortices: fine structures and terahertz ultrafast dynamics

Abstract:

<span style=";padding: 0px;line-height: 2;font-family: times new roman;font-size: 16px" font-size:16px;color:#000000;"="">　　The recent discovery of polar vortices in PbTiO3/SrTiO3(PTO/STO) superlattices opens up exciting opportunities to explore the structure and dynamics of complex topological states of ferroelectric dipoles and potentially leads to novel electronic function alities.1To obtain deep insights into the vortex formation mechanisms, we applied machine learning analysis to theatomically resolvedmicroscopy images of the PTO/STO system, revealing theground-truthvortex structure which suggests the contribution offlexoelectricity. Phase-field modeling was performed to systematically examine the flexocoupling effects on the system. We further matched the experiment and modeling results and thereby quantified the flexocoupling coefficients of PTO and STO. This study has identified the nontrivial role of flexoelectricity in the vortex formation process and demonstrated a novel approach for extracting material parameters.2On the other hand, ultrafast electric fields in the form of THz laser pulses can couple with the polar vortices via dipole interaction, providing a well-defined, easy-to-model external control knob for studying the ultrafast structural dynamics of this system. We have performed preliminary THz-pump, X-ray diffraction probe studies based on the APS synchrotron, and observed a temporal-resolution (100 ps) limited ultrafast dynamics in the vortex phase, distinct from conventional domain structure. These observations suggest the likely existence of low-energy collective excitations in the polar vortices responsible for the collective THz-driven ionic motions, and have motivated our next exploration into the sub-ps regimes using hard X-ray free electron laser