Seminar: Observation of Unpaired Weyl Points

Speaker: Prof. Dr. Ming Shi

Paul Scherrer Institute, Switzerland

Time: 4:30pm, Tuesday, Dec. 28, 2021

Location: Room 242, East 4, Ziingang Campus

Abstract:

Constrained by the Nielsen-Ninomiya no-go theorem, in all so-far experimentally determined Weyl semimetals (WSMs) the Weyl points (WPs) always appear in pairs in the momentum space with no exception. As a consequence, Fermi arcs occur on surfaces which connect the projections of the WPs with opposite chiral charges. However, this situation can be circumvented in the case of unpaired WP. In this presentation I will show that a singular Weyl point is allowed to emerge at the center of the Brillouin zone (BZ), which is surrounded by closed Weyl nodal walls located at the BZ boundaries and there is no Fermi arc connecting its surface projection. This observation indicates that nontrivial band crossings of different dimensionalities can emerge concomitantly in condensed matter, while their coexistence ensures the net topological charge of different dimensional topological objects to be zero, which extends the applicable range of the original Nielsen-Ninomiya no-go theorem which was derived from zero dimensional paired WPs with opposite chirality.