PHYSICS PHD DISSERTATION DEFENSE: Xiaoqi Sun

Ph.D. Candidate:  Xiaoqi Sun
Research Advisor:  Xiaoliang Qi

Date:   Wednesday, April 29, 2020
Time:   10:00 am

Zoom Meeting Link: https://stanford.zoom.us/j/112410428

Zoom Meeting Password: (Email mariaf67 [at] stanford.edu (mariaf67[at]stanford[dot]edu) for password)

Title: Conversion and braiding rules of band-structure nodes

Abstract: 

Topological semimetals are characterized by topologically protected band-structure nodes, such as Weyl points. These band nodes can be understood as momentum-space analogs of topological defects. However, although notions associated with topological defects have been systematically studied since 1970s, many of the phenomena have not been considered in the context of band-structure nodes. In this presentation, I will further elaborate on the analogy between band nodes and topological defects by two examples: momentum-space analogs of boojums and of Alice strings.

The talk consists of two parts. In the first part we show that band nodes occurring on high-symmetry planes are analogous to boojums, i.e. topological defects bound to the surface of an ordered medium. By exploiting this similarity, we identify rules for converting generic nodes to nodes in high-symmetry planes (and vice versa). In the second part of the talk, we show that exceptional lines in non-Hermitian band structures act as Alice strings. This is manifested by the reversal of the topological charge of a node, if it is braided around an exceptional line.