"Accidental Degeneracies, Non-analyticity, and Topological Behavior in Electronic Spectra of Crystals"

 

Topological properties of condensed matter emerged as a frontier concept in condensed matter physics around 1980, and garnered the Physics Nobel Prize this year. Related phenomena (topological insulators, topological superconductors, Dirac and Weyl semimetals, and more) have arisen as an extremely active research area in the past decade. After some background including an example of some features of a topological insulator, I will focus on the "nodal loop semimetal."

This phase of matter results when two energy bands in a 3D crystal form a closed line of degeneracies that are partially occupied, in an otherwise gapped system.

 

Two or three examples will be discussed.