We measured the magneto-reflectivity spectra (4–90 meV, 0–16 T) of the triple-point semimetal GdPtBi and found them to demonstrate two unusual broad features emerging in field. The electronic bands of GdPtBi are expected to experience large exchange-mediated shifts, which lends itself to a description via effective Zeeman splittings with a large $g$ factor. Based on this approach, along with an ab initio band structure analysis, we propose a model Hamiltonian that describes our observations well and allows us to estimate the effective $g$ factor, $g^{*}=95$. We conclude that we directly observe the exchange-induced ${\mathrm{\Gamma }}_8$ band inversion in GdPtBi by means of infrared spectroscopy.

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• Magneto-optics
• Topological materials
• Weyl semimetal
• Density functional theory
• Infrared spectroscopy