Symmetry-enforced ideal lantern-like phonons in ternary nitride Li$_6$WN$_4$

TL;DR

This study predicts that the ternary nitride Li$_6$WN$_4$ hosts ideal lantern-like topological phonons with nodal surfaces and lines, shaped by nonsymmorphic symmetry, expanding the understanding of topological quantum phases in real materials.

Contribution

The paper reveals the existence of ideal lantern-like topological phonons in Li$_6$WN$_4$, demonstrating their symmetry-enforced degeneracies and surface states through first-principles calculations.

Findings

Presence of nearly flat nodal-surface and nodal-line phonon structures

Lantern-shaped degeneracies guaranteed by nonsymmorphic symmetry

Topological surface states covering half the surface BZ

Abstract

Condensed matter systems contain both fermionic and bosonic quasiparticles. Owing to the constraint imposed by the Fermi level, ideal material candidate for the emergent particles with higher-dimensional degeneracy manifold (i.e., nodal lines and nodal surfaces) has not been found in electronic systems. This paper demonstrates that according to the first-principle calculations and symmetry analysis, realistic ternary nitride Li$_6$WN$_4$ features ideal (nearly flat) nodal-surface and nodal-line structures in its phonon spectra. These nodal degeneracies are shaped like lanterns, and their existence is guaranteed by nonsymmorphic symmetry. The corresponding topological phonon surface state covers exactly half the surface Brillouin zone (BZ) and can thereby be distinguished from those of conventional nodal-line and nodal-surface semimetals. The results of our study demonstrate the existence of ideal lantern-like phonons in realistic materials, which enriches the classification of topological quantum phases and provides a good basis for investigating the interaction between nodal-line and nodal-surface phonons in a single material.