Exchange gap in GdPtBi probed by magneto-optics

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 Γ8 band inversion in GdPtBi by the means of infrared spectroscopy.