Optical Signature of Dirac Fermions in High-Entropy Alloys Based on BaMnSb₂

Recent work has shown that the Dirac (linear) band dispersion and spin valley electronic states can be engineered through high-entropy alloying of Dirac semimetals, such as BaMnSb2 (Laha, A., Yoshida, S., Marques dos Santos Vieira, F. et al. Nat Commun 15, 3532 (2024)). Optical conductivity provides a reliable tool for probing electronic band structure and in this work, we demonstrate the existence of linear band dispersion in BaMnSb₂ alloyed with Ca, Eu, Yb at the Ca site. We measured broadband reflectance between 100 cm-1 and 50,000 cm-1 (12 meV to 6 eV), at temperatures from 300 K to 80K. Similar to the pure BaMnSb2, optical conductivity (σ1(ω)) determined from Kramers-Kronig transformation of reflectance in the alloyed, i.e. high entropy system, shows large regions of linear frequency dependence σ1(ω) ∝ ω, characteristic to interband transitions involving Dirac electronic branches. The absorption spectrum is in good qualitative agreement with previous band structure calculations. Moreover, we show that the temperature dependence of the plasma edge and the loss function peak are also very similar between the pure compound and the heavily alloyed phase.