Single Pair of Weyl Points in Nonmagnetic Crystals

TL;DR

This paper demonstrates that nonmagnetic spinless crystals can host minimal Weyl semimetal states with a single pair of Weyl points, characterized by large separation and even topological charge, expanding the scope of topological materials.

Contribution

It identifies 32 candidate space groups for single-pair Weyl points in nonmagnetic systems and confirms their presence in phonon spectra of specific materials, revealing new classes of topological states.

Findings

32 candidate space groups identified for single-pair Weyl points

Weyl points have even topological charge and are well separated in momentum space

Extended surface Fermi loops with non-contractible topology observed

Abstract

Topological semimetal states having the minimal number, i.e., only a single pair, of Weyl points are desirable for the study of effects associated with chiral topological charges. So far, the search for such states is focused on magnetic spinful systems. Here, we find that nonmagnetic spinless systems can host a class of single-pair-Weyl-point (SP-WP) states, where the two Weyl points are located at two high-symmetry time-reversal-invariant momenta. We identify 32 candidate space groups that host such states, and we show that the chiral charge of each Weyl point in the SP-WP state must be an even integer. Besides achieving the minimal number, Weyl points in SP-WP states are far separated in momentum space, making the physics of each individual point better exposed. The large separation combined with the even topological charge lead to extended surface Fermi loops with a non-contractible winding topology on the surface Brillouin zone torus, distinct from conventional Weyl semimetals. We confirm our proposal in the phonon spectra of two concrete materials TlBO$_2$ and KNiIO$_6$. Our finding applies to a wide range of systems, including electronic, phononic, and various artificial systems. It offers a new direction for the search of ideal platforms to study chiral particles.