Symmetry-enforced planar nodal chain phonons in non-symmorphic materials

TL;DR

This paper discovers a new class of planar nodal chain phonons in non-symmorphic materials, protected by symmetry, with unique degeneracies, confirmed by first-principles calculations, expanding the understanding of topological phononic states.

Contribution

It introduces a novel class of planar nodal chain phonons in non-symmorphic systems, identifying specific space groups and demonstrating their properties through first-principles calculations.

Findings

Identified 8 space groups hosting planar nodal chain phonons.

Confirmed the existence of these chains in wurtzite GaN via first-principles.

Discovered two classes of drumhead surface states associated with these chains.

Abstract

Topological semimetal states which are constrained by symmetries and give birth to innovative excitations are the frontiers of topological quantum matter. Nodal chains in which two nodal rings connect at one point were first discovered in non-symmorphic electronic systems and then generalized to symmorphic phononic systems. In this work, we identify a new class of planar nodal chains in non-symmorphic phononic systems, where the connecting rings lie in the same plane. The constituting nodal rings are protected by mirror symmetry, their intersection is guaranteed by the combination of time-reversal and non-symmorphic two-fold screw symmetry. In addition, the connecting points are four-fold degenerate while those in previous works are two-fold degenerate. We searched all 230 space groups and found 8 space groups that can host the proposed planar nodal chain phonons. Taking wurtzite GaN (space group No.186) as an example, the planar nodal chain is confirmed by first-principles calculations. The planar nodal chains result in two distinct classes of drumhead surface. The first category lies on the [10(-1)0] surface Brillouin zone and the second lies on the [0001] surface Brillouin zone. Our finding reveals a class of planar nodal chains in non-symmorphic phononic systems, expands the catalog of topological nodal chains, and enriches the family of topological surface states.