Temperature-Dependent Optical Constants of Nanometer-thin Flakes of Fe(Te,Se) Superconductor in the Visible and Near-Infrared Regime

Iron chalcogenides superconductors, such as Fe(Te,Se) have recently garnered significant attention due to their simple crystal structure with a relatively easy synthesis process, high-temperature superconductivity, intrinsic topological band structure, and an unconventional pairing of superconductivity with ferromagnetism. Here, we report the complex in-plane refractive index measurement of nanometer-thin Fe(Te,Se) flake exfoliated from a single crystal FeTe$_{\text{0.6}}$Se$_{\text{0.4}}$ for photon wavelengths from 450 to 1100 nm over a temperature range from 4 K to 295 K. The results were obtained by employing a two-Drude model for the dielectric function of Fe(Te,Se), a multiband superconductor, and fitting the absolute optical reflection spectra using the transfer matrix method. A high extinction coefficient in the visible to near-infrared range makes nanometer-thin Fe(Te,Se) flakes a promising material for photodetection applications.