Electronic structure of some β-(${\mathrm{C}}_{10}$${\mathrm{H}}_8$${\mathrm{S}}_8$${)}_2$X compounds as studied by infrared spectroscopy

Polarized reflectance measurements have been made on two isostructural conducting compounds of bis(ethylenedithio)tetrathiafulvalene [BEDT-TTF or ET, (${\mathrm{C}}_{10}$${\mathrm{H}}_8$${\mathrm{S}}_8$)]: β-(ET${)}_2$${\mathrm{AuI}}_2$ and β-(ET${)}_2$${\mathrm{I}}_2$Br. The former is superconducting at ambient pressure with $T_c$=5 K, whereas the latter retains normal-metal conductivity to low temperatures. The reflectance measurements, made at temperatures of approximately 30 and 300 K, spanned 80 ${\mathrm{cm}}^{\mathrm{-}1}$ (0.01 eV) through 33 000 ${\mathrm{cm}}^{\mathrm{-}1}$ (∼4 eV); they were made for polarization along the ET molecular stacking axis and transverse to it in the sheets or layers of ET molecules. Band-structure parameters determined from the plasmon frequencies imply that the anisotropy is rather low for organic conductors, with $t_{?}$≊0.22 eV and $t_{\perp }$≊0.09 eV for β-(ET${)}_2$${\mathrm{AuI}}_2$; $t_{?}$≊0.18 eV and $t_{\perp }$≊0.07 eV for β-(ET${)}_2$${\mathrm{I}}_2$Br. At 300 K, the spectra differ substantially from the expectations of simple one-electron models, suggesting that both electron-phonon and electron-electron interactions play important roles in the electronic structure of these materials. At low temperatures, the materials display basically metallic characteristics, yet deviate significantly from simple Drude-model behavior. At 30 K, no effect attributable to superconducting fluctuations could be observed.