Electronic structure of some beta -(C10H8S8)2X compounds as studied by infrared spectroscopy.

Abstract

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}$)]: \ensuremath{\beta}-(ET${)}_{2}$${\mathrm{AuI}}_{2}$ and \ensuremath{\beta}-(ET${)}_{2}$${\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 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (0.01 eV) through 33 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (\ensuremath{\sim}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 ${t}_{?}$\ensuremath{\approxeq}0.22 eV and ${t}_{\ensuremath{\perp}}$\ensuremath{\approxeq}0.09 eV for \ensuremath{\beta}-(ET${)}_{2}$${\mathrm{AuI}}_{2}$; ${t}_{?}$\ensuremath{\approxeq}0.18 eV and ${t}_{\ensuremath{\perp}}$\ensuremath{\approxeq}0.07 eV for \ensuremath{\beta}-(ET${)}_{2}$${\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.