Unconventional topological Weyl-dipole phonon

TL;DR

This paper predicts a new topological phase called Weyl-dipole in phonon spectra of Y(OH)$_3$, characterized by unconventional Weyl points protected by a quadrupole moment, leading to unique surface and hinge states.

Contribution

First-principles calculations demonstrate the existence of a Weyl-dipole phase in Y(OH)$_3$, revealing unconventional Weyl points with higher Chern numbers and associated topological boundary states.

Findings

Weyl dipole protected by a quantized quadrupole moment.

Presence of unconventional charge-3 Weyl point with Chern number -3.

Unique 2D Fermi-arc and 1D hinge states in Y(OH)$_3$.

Abstract

A pair of Weyl points (WPs) with opposite Chern numbers ${\cal{C}}$ can exhibit an additional higher-order $Z_2$ topological charge, giving rise to the formation of a $Z_2$ Weyl dipole. Owing to the nontrivial topological charge, $Z_2$ Weyl dipoles should also appear in pairs, and the WPs within each $Z_2$ Weyl dipole can not be annihilated when meeting together. As a novel topological state, the topological Weyl-dipole phase (TWDP) has garnered significant attention, yet its realization in crystalline materials remains a challenge. Here, through first-principles calculations and theoretical analysis, we demonstrate the existence of the Weyl-dipole phase in the phonon spectra of the $P6_3$ type Y(OH)$_3$. Particularly, the Weyl dipole in this system is protected by a quantized quadrupole moment, and it distinguished from conventional Weyl dipole, as it comprises an unconventional charge-3 WP with ${\cal{C}}=-3$ and three conventional charge-1 WPs with ${\cal{C}}=1$. Consequently, the Weyl-dipole phase in Y(OH)$_3$ features unique two-dimensional (2D) sextuple-helicoid Fermi-arc states on the top and bottom surfaces, protected by the Chern number, as well as one-dimensional (1D) hinge states that connect the two Weyl dipoles along the side hinges, guaranteed by the quantized quadrupole moment. Our findings not only introduce a novel higher-order topological phase, but also promote Y(OH)$_3$ as a promising platform for exploring multi-dimensional boundaries and the interaction between first-order and second-order topologies.