The effect of Ge substitution on optical properties of FeGa₃ single crystals

Studied initially for its large thermopower effect, the intermetallic semiconductor FeGa₃ has recently been shown to provide a suitable platform for studying electron correlations, such as metal-insulator transition, non Fermi liquid behavior and ferromagnetism. In particular, with Ge-doping at Ga site the ground state is tuned from a Kondo insulator to a paramagnetic and further to a ferromagnetic metal. Here we present optical reflectance measurements on single crystals of FeGa_3−xGe_x, for different values of x. The temperature was varied between 300 K and 5 K, and the frequency ranged from 80 cm⁻¹ to 50 000 cm⁻¹. From Kramers-Kronig transformation we obtained various optical functions and dis- cuss here the effect of Ge substitution on optical conductivity across all three proposed ground states.