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) 2 I 3 , 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 m a * =1.5 m e and m c * =3.0 m e . 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.