The exceptional electronic and spintronic properties of magnetic Heusler alloys, which include half-metals and Weyl semimetals, are strongly sensitive to deviations from the ideal atomic structure. To ensure that these materials have been produced with the desired properties, it is necessary to determine both the structural ordering and the electronic structure, which can be challenging. Here, we present the results of a far-infrared-to-visible optical spectroscopy study of films of room-temperature ferromagnetic Weyl semimetal

C o_{2} MnGa

. Combined with a determination of the level of ordering from x-ray diffraction, we have investigated near Fermi energy valence and conduction band intra- and interband transitions and their dependence on the atomic order. Motivated by band structure calculations, we have modeled our optical spectra with two Drude terms and two Lorentz oscillators, where the latter are assigned to interband transitions. The scattering rate of the itinerant carriers, determined from the width of the Drude term, increases threefold with increasing disorder, while the carrier density to effective mass ratio is unchanged. Based on our band structure and the joint density of states calculations, we have assigned the oscillator that dominates the interband spectral region near 1 eV to transitions across the minority spin gap along the Γ-

X

direction. It is found that the energy of this transition is strongly sensitive to the degree of order and decreases rapidly with increasing disorder as states fill a decreasing minority spin gap. Our results demonstrate optical spectroscopy is a sensitive way to fingerprint structural order in the technologically relevant near Fermi level electronic states in Heusler alloys.

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