Symmetry-Protected Topological Triangular Weyl Complex

TL;DR

This paper predicts a novel triangular Weyl complex in phononic systems, featuring unique surface arcs connecting different Weyl points, with potential implications for experimental detection and applications in phononics and photonics.

Contribution

It introduces the concept of a triangular Weyl complex with single and double Weyl phonons in realistic materials, expanding the understanding of Weyl physics beyond conventional pairs.

Findings

Presence of single and double Weyl phonons in specific materials.

Unique surface arcs connecting Weyl points across the entire Brillouin zone.

Surface states are nontrivial and detectable in experiments.

Abstract

Weyl points are often believed to appear in pairs with opposite chirality. In this work, we show by first-principles calculations and symmetry analysis that single Weyl phonons with linear dispersion and double Weyl phonons with quadratic dispersion are simultaneously present between two specific phonon branches in realistic materials with trigonal or hexagonal lattices. These phonon Weyl points are guaranteed to locate at high-symmetry points due to the screw rotational symmetry, forming a unique triangular Weyl complex. In sharp contrast to conventional Weyl systems with surface arcs terminated at the projections of a pair of Weyl points with opposite chirality, the phonon surface arcs of the unconventional triangular Weyl complex connect the projections of one double Weyl point and two single Weyl points. Importantly, the phonon surface arcs originating from the triangular Weyl complex are extremely long and span the entire surface Brillouin-zone. Furthermore, there are only nontrivial phonon surface states across the iso-frequency surface, which facilitates their detection in experiments and further applications. Our work not only offers the promising triangular phonon Weyl complex but also provides guidance for exploring triangular Weyl bosons in both phononic and photonic systems.