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

The discovery of iron-based superconductors (IBS) in 2008 introduced a new class of materials for high-temperature superconductors beyond copper-based materials. Among IBS, iron- chalcogenides such as Fe(Te, Se) have attracted great attention recently due to many enticing properties such as simple crystal structure, high-temperature superconductivity, intrinsic topological band structure, and an unconventional pairing of superconductivity with ferromagnetism. Most of the studies on Fe(Te,Se) have been focused on its magnetic properties, and the optical properties of nanometer- thin films or flakes of Fe(Te,Se) for photonic applications in visible to near-IR wavelengths over a wide range of temperatures remain relatively unexplored.

In this work, we report measurement of the complex-valued in-plane refractive index of nanometer-thin Fe(Te,Se) flake for photon wavelengths from 450 to 1100 nm over a temperature range from 4 K to 295 K. To obtain the complex refractive index, we employed a two-Drude model for the dielectric function of Fe(Te,Se) and the transfer matrix method to account for interference effects. In the superconducting state (4 K), the refractive index and the extinction coefficient vary in the range of 2.81 - 4.26 and 2.31 - 3.21, respectively. In the normal state (295 K), they vary between 2.79 - 4.38 and 2.14 - 3.04. A higher extinction coefficient of Fe(Te,Se) is advantageous for photodetection applications.