Twofold degenerate quadruple Weyl nodes in chiral cubic crystals

Unlike conventional Weyl nodes, unconventional ones carry a quantized monopole charge C>1, and their existence needs the protection of crystalline symmetries in addition to translation symmetry. There have been many studies on unconventional Weyl nodes, yet we have so far missed one, which is the twofold Weyl node with C=4^[1]. In this talk, I will first explain why researchers missed the twofold degenerate quadruple Weyl node (TQW) by introducing the research history of unconventional Weyl nodes. Then, I will explain how to obtain the TQW in crystals. Afterward, a series of LaIrSi-type materials that have the TQW in both their spinless electronic band structure and the phonon spectra will be discussed. In the electronic band structure, the TQW will evolve into a fourfold quadruple Weyl node and change both the chirality and the monopole charge after considering spin-orbit coupling, which is uncommon in the known Weyl semimetals. At last, I will discuss the relationship between the winding number and pseudospin and offer an intuitive way to understand the complex pseudospin texture of the TQW. If there is enough time, I also would like to make a brief introduction to experimentally detecting the TQW in BaPtGe^[2].

[1] PHYSICAL REVIEW B 102, 125148 (2020)
[2] arXiv:2010.00764 (2020).