Abstract:

　　The recent discovery of polar vortex 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 functionalities. To obtain deep insights into the vortex formation mechanisms, we applied machine learning analysis to the atomically resolved microscopy images of PTO/STO, revealing the ground-truth vortex fine structure which suggests the contribution of flexoelectricity. Phase-field modeling was performed to systematically survey the flexocoupling effect on the system. We matched the experiment and modeling results and thereby quantified the flexocoupling coefficients of PTO and STO. On the other hand, ultrafast electric field in the form of THz laser pulses can couple with the vortices via coherent dipole interaction, providing a well-defined, easy-to-model external control knob for probing the ultrafast structural dynamics of this system. We performed a THz pulse pump, ultrafast X-ray scattering probe study of PTO/STO based on the hard X-ray free-electron laser at the Linac Coherent Light Source. We observed a set of subterahertz collective excitations of the polar vortices stemming from the lattice dipoles, and further identified a unique soft vortex mode with marked strain and electric-field tunabilities. These observations were well corroborated by multiscale simulations combining the newly developed dynamical phase-field modeling and atomistic modeling. 

Reference:

1. Q. Li et al, Nature 2021, 592, 376

2. Q. Li et al, Nature Commun., 2017, 8, 1468

报告人简介：

　　李千，清华大学材料学院助理教授、博士生导师，入选海外高层次青年人才计划。2014年于澳大利亚国立大学获得博士学位，后曾在美国橡树岭国家实验室、阿贡国家实验室从事博士后研究。研究主要围绕同步辐射X射线与中子散射、扫描探针与近场光学显微镜、相场模拟等方法手段，综合探究铁电介电类功能材料在多外场激励下的显微结构与动力学性质。迄今以第一、通讯作者身份在Nature、Nature Commun.、Nano Letters等国际高水平期刊上发表论文21篇，其他合作研究论文50余篇。目前正主持国家自然科学基金面上项目、科技部重点研发计划子课题等科研项目。