报告人：
李千，2003至2010年就读于南京航空航天大学材料系获得本科、硕士学位，2010年至2014年就读于澳大利亚国立大学化学研究院获得博士学位，其中于2013年获得国家优秀自费留学生奖学金。2014年至2017年在美国橡树岭国家实验室纳米相材料科学中心任职博士后研究助理。现在美国阿贡国家实验室先进光子源继续从事博士后研究。主要的研究经历和兴趣方向有: 同步辐射X射线、自由电子激光与中子散射，扫描探针与近场光学显微术，铁电压电材料与超声器件，有限元相场模拟以及大数据分析方法等。目前以在Nature Communications, Advanced Materials，ACS Nano等国际期刊上发表论文40余篇，总引用近570次。   

报告摘要：
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 functionalities.1 To obtain deep insights into the vortex formation mechanisms, we applied machine learning analysis to the atomically resolved microscopy images of the PTO/STO system, revealing the ground-truth vortex structure which suggests the contribution of flexoelectricity. 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.2 On 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 coherent THz-driven ionic motions, and have motivated our next exploration into the sub-ps regimes using hard X-ray free electron laser. 
Ref: 1. A. K. Yadav et al, Nature 530, 198 (2016)
2. Q. Li et al, Nature Communications 8, 1468 (2017)