For topological magnon spectra, determining and explaining the presence of a gap at a magnon crossing point is a key to characterize the topological properties of the system. An inelastic neutron scattering study of a single crystal is a powerful experimental technique that is widely employed to probe the magnetic excitation spectra of topological materials. Here, we show that when the scattering intensity rapidly disperses in the vicinity of a crossing point, such as a Dirac point, the apparent topological gap size is extremely sensitive to experimental conditions including sample mosaic, resolution, and momentum integration range. We demonstrate these effects using comprehensive neutron-scattering measurements of CrCl3. Our measurements confirm the gapless nature of the Dirac magnon in CrCl3, but also reveal an artificial, i.e. extrinsic, magnon gap unless the momentum integration range is carefully controlled. Our study provides an explanation of the apparent discrepancies between spectroscopic and first-principles estimates of Dirac magnon gap sizes, and provides guidelines for accurate spectroscopic measurement of topological magnon gaps.
@article{arxiv.2204.03720,
title = {Gaps in Topological Magnon Spectra: Intrinsic vs. Extrinsic Effects},
author = {Seung-Hwan Do and Joseph A. M. Paddison and Gabriele Sala and Travis J. Williams and Koji Kaneko and Keitaro Kuwahara and A. F. May and Jiaqiang Yan and Michael A. McGuire and Matthew B. Stone and Mark D. Lumsden and Andrew D. Christianson},
journal= {arXiv preprint arXiv:2204.03720},
year = {2022}
}