（6.13）：Topological nodal-loop and nodal-surface metals;Magnetic quadrupole moment and its applications in solid state physics
报告一：Topological nodal-loop and nodal-surface metals （报告人：新加坡科技大学杨声远助理教授）
报告摘要：Topological states of matter have been attracting great interest. Following the studies on topological insulators, the recent focus shifts towards the various topological metals (or semimetals), in which nontrivial band crossings appear near the Fermi level, endowing the low-energy electrons with novel emergent properties. In this talk, I discuss a class of metals where the band crossings form 1D loops in the Brillouin zone. I show that the loops can be characterized by several properties. Depending on how the loop winds around the Brillouin zone, it has a characterization. The type of dispersion around the loop classifies the loop into type-I, type-II, or the hybrid type. The type-II loop shows suppressed low-frequency optical absorption, and the hybrid loop shows zero-field magnetic breakdown and distinct features in the magnetic quantum oscillations. Under spin-orbit coupling, some loops are gapped out, but some are robust owing to certain nonsymmorphic symmetries. Under proper conditions, multiple loops can exist, connect each other, and form a nodal chain. If time permits, I will also discuss nodal-surface semimetals, where 2D surface of band crossings can be stabilized by symmetry in the absence/presence of spin-orbit coupling and also in magnetic materials.
报告人简介：杨声远，江苏省南京人。2001-2002年就读于清华大学电气自动化专业。2002-2005在香港大学修读数学物理专业，获理学学士。2005-2011年在得克萨斯大学奥斯汀分校修读凝聚态物理研究生，获博士学位。2011-2013年在休斯敦CGG US Services公司工作，任地质结构分析成像研究员。2013年起在新加坡科技设计大学担任助理教授。以往的研究课题包括磁性材料的磁动力学，二次谐波的发射理论，反常霍尔效应，二维材料/拓扑材料的物理性质。目前的研究方向包括拓扑半金属，拓扑绝缘体，二维材料和纳米结构的物理性质，电子晶体，自旋和谷电子学，量子输运等。
报告二：Magnetic quadrupole moment and its applications in solid state physics （报告人：美国卡内基梅隆大学肖笛副教授）
报告摘要：The essence of an in-medium formulation of electromagnetism is the use of electric and magnetic multipole moment to characterize the intrinsic charge and current densities. For example, at first order the curl of the magnetization gives the magnetization current. The next order contribution comes from the magnetic quadrupole moment density. The quadrupole moment is also intimately related to the ferrotoroidal states, which breaks individual time-reversal and space inversion symmetry but retain their combined symmetry. It has been proposed that the antisymmetric part of the quadrupole moment, called toroidization, can be used as an order parameter to characterize such states.
In this talk, I will present a microscopic theory of magnetic quadrupole moment in crystalline solids. Our theory provides a gauge-invariant expression of the quadrupole moment, and allows us to establish a concrete relation between the quadrupole moment and magnetoelectric polarizability. I will then demonstrate the application of our theory in two examples: (1) temperature gradient induced magnetization, and (2) nonlinear anomalous thermoelectric transport.
报告人简介：Prof. Di Xiao received his B.S. degree in physics fromPekingUniversityin 2001, and a Ph.D. at theUniversityofTexasatAustinin 2007. He then worked at Oak Ridge National Laboratory, first as a postdoc, then a staff member in the Materials Science and Technology Division. In 2012, he joinedCarnegieMellonUniversityand is now an associate professor in the Physics Department. Prof. Xiao is the recipient of the Cottrell Scholar Award by the Research Corporation for Science Advancement in 2016. He has published more than 90 papers in journals including Science, Nature, Physical Review Letters and Review of Modern Physics. His papers have been cited over 15,000 times with an h-index of 41. More information about his research can be found at https://www.andrew.cmu.edu/user/dixiao