Magnon nodal-line semimetals and drumhead surface states in anisotropic pyrochlore ferromagnets

TL;DR

This paper introduces magnonic nodal-line semimetals in anisotropic pyrochlore ferromagnets, revealing topologically protected magnon band degeneracies and surface states, expanding the family of topological magnons.

Contribution

It identifies a new topological phase of magnons with nodal lines in pyrochlore ferromagnets, characterized by specific symmetries and surface state dependencies.

Findings

Magnon spectra feature twofold degeneracies along closed loops in reciprocal space.

Topological invariants confirm the protection of magnon nodal lines.

Surface states depend on surface termination details.

Abstract

We introduce a new type of topological magnon matter: the magnonic pendant to electronic nodal-line semimetals. Magnon spectra of anisotropic pyrochlore ferromagnets feature twofold degeneracies of magnon bands along a closed loop in reciprocal space. These magnon nodal lines are topologically protected by the coexistence of inversion and time-reversal symmetry; they require the absence of spin-orbit interaction (no Dzyaloshinskii-Moriya interaction). We calculate the topological invariants of the nodal lines and show that details of the associated magnon drumhead surface states depend strongly on the termination of the surface. Magnon nodal-line semimetals complete the family of topological magnons in three-dimensional ferromagnetic materials.