Periodicity of the Spin Structure Observed in the Optical Response of CeBi Single Crystals

We report a study of the optical conductivity of CeBi in different magnetic phases. Going from the paramagnetic phase to the various antiferromagnetic and ferrimagnetic phases, different new interband transitions appear below the absorption edge of the $p\to d$ transitions. These new interband transitions disappear in a magnetic field that is sufficiently strong to align the spins parallel. The appearance of new interband transitions is indubitably associated with the folding of the Brillouin zone, which is a consequence of the change of the periodicity of the spin structure.