Optical Properties of Organic Superconductor $\kappa$-(BETS)$_2$FeBr$_4$

The optical response to far- and mid-infrared radiation has been measured for quasi two-dimensional plate-shaped crystals of $\kappa$-(BETS)$_2$FeBr$_4$ [where BETS = bis(ethylenedithio)-tetraselenafulvalene]. $\kappa$-(BETS)$_2$FeBr$_4$ is the first antiferromagnetic organic superconductor at ambient pressure with N\'{e}el temperature T$_N=2.5$ K and superconducting transition temperature T$_C=1.1$ K. Polarized thermal reflectance measurements were performed to compare the reflectance above and below T$_C$ and T$_N$ using a Martin-Puplett-type polarizing interferometer and $^3$He cryostat. In addition polarized absolute reflectance measurements in the far- and mid-infrared were carried out at temperatures in the normal state between 4 K and 300 K using a Michelson interferometer and cold finger cryostat. Kramers-Kronig analysis was then used to determine the optical conductivity of $\kappa$-(BETS)$_2$FeBr$_4$ at these temperatures.