The quasi-two-dimensional bismuth layer-like AMnBi₂ (A= alkaline as well as rare earth atom) lately advanced as an arena for the investigation of low-energy quasiparticle excitations in topological materials. The A= Sr or Ca compositions have attracted special attention because of the coexistence of the anisotropic Dirac cones with antiferromagnetic order. In a broader context, the title compound also provides an opportunity to study low-dimensional magnetism and its putative relationship to the electronic properties. This work describes novel results of reflectivity measurements from the far-infrared up to the ultraviolet that probe the optical response as a function of temperature. This gives access to the optical conductivity which captures the relevant energy scales shaping the electronic structure. We discover [1] a reshuffling of spectral weight, defined as the integral of the real part of the optical conductivity, for energy scales up to 0.2 eV. This occurs at the onset of the spin reorientation transition which also manifests as an anomaly in the dc transport data of the title compound. This may reveal the inclination towards a Fermi surface instability in topological materials.
[1] M. Corasaniti et al., Phys. Rev. B100, 041107(R) (2019).