Temperature dependent infrared spectroscopy of PrGe_2-x

Similar to the Weyl semimetals PrAlGe or PrAlSi, Pr³⁺ ions undergo ferromagnetic ordering below 20 K in the PrGe_2-x compound, as well. To gain insight into the origins of ferromagnetic coupling in these systems, we performed broadband reflectance measurements on single crystals of PrGe_2-x, from 80 cm^-1 to 50,000 cm^-1, and temperatures between 300 K and 5 K. Kramers-Kroning transformation and Lorentz-Drude fits were used to calculate various optical functions and we discuss in particular optical conductivity. We found that PrGe_2-x has metallic behavior, with a residual-resistance ratio RRR≅3, in very good agreement with direct current resistivity measurements. We further discuss the effect of magnetic order on optical conductivity, by comparison with the Weyl semimetals PrAlSi and PrAlGe.