Dirac magnons, nodal lines, and nodal plane in elemental gadolinium

Abstract

We investigate the magnetic excitations of elemental gadolinium (Gd) using inelastic neutron scattering, showing that Gd is a Dirac magnon material with nodal lines at K and nodal planes at half integer . We find an anisotropic intensity winding around the K-point Dirac magnon cone, which is interpreted to indicate Berry phase physics. Using linear spin wave theory calculations, we show the nodal lines have non-trivial Berry phases, and topological surface modes. Together, these results indicate a highly nontrivial topology, which is generic to hexagonal close packed ferromagnets. We discuss potential implications for other such systems.

Spin exchange Hamiltonian and topological degeneracies in elemental gadolinium

“…In this study, we use inelastic neutron scattering to determine the magnetic exchange Hamiltonian of elemental Gd to the 30th neighbor exchange, showing the exchange approximately follows an Ruderman-Kittel-Kasuya-Yoshida (RKKY) model. In a separate paper [5] we also show that these spin waves yield topological features; in this paper we extend this topology discussion with further details about anisotropic HCP rare earths.…”

“…As discussed in Ref. [5], the Gd neutron spectrum shows a nodal line degeneracy at K = (1/3, 1/3, ) which has a π Berry phase around it, and a nodal plane degeneracy at half-integer . An example of a K linear band crossing is shown in Fig.…”