Optical investigations of the metal-insulator transition in a low-dimensional organic conductor (EDT-TTF)$_4$[Hg$_3$I$_8$]

The polarized reflectance spectra of single crystals of the low-dimensional organic conductor (EDT-TTF)$_4$[Hg$_3$I$_8$] undergoing a metal-insulator phase transition at a temperature $T <$ 35 K have been presented. The spectral region of the study is 700–6000 cm$^{-1}$ (0.087–0.74 eV), and the temperature range is 300–9 K. It has been shown that the reflectance spectra are determined by a system of quasi-free electrons of the upper half-occupied molecular $\pi$-orbitals, which form a half-filled metallic band in the crystals. A high anisotropy of the spectra and their temperature dependences have been found. For two polarizations, the quantitative analysis of the spectra at 100 and 25 K has been performed in the framework of the phenomenological Drude model, the effective mass and the width of the initial metallic $\pi$-electron band have been deter-mined, and it has been found that the conducting system in the crystals has a quasi-one-dimensional character. As temperature decreases, the spectra demonstrate substantial changes indicating the formation of the energy gap (or pseudogap) in the spectrum of electronic states in the range of $\sim$ 1500–2500 cm$^{-1}$. In the low-frequency region (700–1600 cm$^{-1}$), a vibrational structure has been observed, and the most intense feature of the structure ($\omega$ = 1340 cm$^{-1}$) is caused by the interaction of electrons with intramolecular vibrations of the C = C bonds of the EDT-TTF molecule. For temperatures of 15 and 9 K, the analysis of the spectra has been performed in the framework of the theoretical “phase phonon” model taking into account the interaction of electrons with the intramolecular vibrations. It has been concluded that the metal-insulator transition observed in the reflectance spectra of the crystals is similar to the Peierls dielectric transition that occurs in a system of electrons coupled with the intramolecular vibrations of the molecules forming the crystal.