Temperature Dependence of the Polarized Reflectance Spectra of the θ-Type of Bis(ethylenedithio)tetrathiafulvalenium Triiodide θ-(BEDT-TTF)₂I₃: Estimation of Band Parameters

Polarized reflectance spectra in the region from 720 cm^–1 to 25000 cm^–1 were measured on the single crystal of θ-(BEDT-TTF)₂I₃, an organic superconductor, at temperatures over 16–295 K. A Drude-like spectrum was found for the first time in metallic BEDT-TTF salts. This characteristic spectrum was attributed to the unique structure of the θ-type crystal. The curve-fitting analysis of the spectra gave the plasma frequencies (ω_p)_a = 1.05 eV and (ω_p)_c = 0.73 eV at 16 K, from which the anisotropic effective masses were evaluated to be and. The transfer integrals and other band parameters of the tight-binding band were directly derived from the analysis of the optical data. The transfer integrals thus obtained are t_a = 0.080 eV and t_c = 0.046 eV. These results are to be compared with the calculated band structure.