Optical investigations of anisotropy of the conducting $\pi$-electron system in crystals of the organic superconductor (EDT-TTF)$_4$[Hg$_3$I$_8$]$_{1-x}$

The polarized reflectance spectra of microcrystals of the new organic superconductor (EDT-TTF)$_4$[Hg$_3$I$_8$]$_{1-x}$ ($T_c$ = 8.1 K for $x$ = 0.027; $T_c$ = 7 K at 0.3 kbar for $x$ = 0.019) have been investigated in the spectral region of 700–6000 cm$^{-1}$ (0.087–0.740 eV) at temperatures ranging from 10 to 300 K. A quantitative analysis of the spectra has been performed in the framework of the phenomenological Drude and Drude–Lorentz dispersion models, as well as in the framework of the theoretical “phase phonon” model that takes into account the interaction of free electrons with intramolecular vibrations. The effective mass of charge carriers $m^*$, the width of the initial metallic $\pi$-electron band W, and the integral $t$ of the electron transfer between the molecular $\pi$-orbitals of neighboring molecules have been determined. In the low-frequency range (700–1600 cm$^{-1}$), the vibrational structure associated with intramolecular vibrations of the EDT-TTF molecule has been revealed. It has been shown that the most intense feature ($\omega$ = 1330 cm$^{-1}$) of this structure is caused by the interaction of “quasi-free” electrons with intramolecular vibrations.