Gaps in Topological Magnon Spectra: Intrinsic vs. Extrinsic Effects

Revealing and understanding the presence of a gap at a putative magnon crossing point is an essential part of characterizing the magnetic topological properties of a material. An important experimental approach in this regard is an inelastic neutron scattering study of a single crystal to probe the momentum and energy dependence of the spin excitation spectrum. Here we discuss this approach for studies of topological magnons and other cases where the scattering intensity rapidly disperses in the vicinity of a singularity or band crossing in the spin excitation spectrum. In particular, we show the apparent difference in energies between modes at the Dirac wave-vector, i.e. gap size, is extremely sensitive to experimental conditions including sample mosaic, resolution, and data processing. Accounting for instrumental resolution effects is required to accurately compare measurements to analytic models. As a test case, we examine CrCl₃ which orders antiferromagnetically at 14 K with ferromagnetic alignment of spins within honeycomb lattice planes which are in turn stacked antiferromagnetically along the c-axis. Our inelastic neutron scattering measurements provide a comprehensive map of the excitation spectrum near the Dirac point and establish CrCl₃ as an ideal quasi-two-dimensional (2D) Heisenberg ferromagnet with a gapless Dirac magnon. However, we also find that the size of a topological magnon gap to be considerably overestimated if “typical” Q-integration ranges are used. This result provides an explanation of the apparent discrepancies between first-principles calculations and spectroscopic measurements of Dirac magnon gap sizes of recently studied materials and further provides guidelines for accurate measurements of topological magnon gaps.