Low-Frequency Optical Properties of β′′-(BEDO- TTF)₅[CsHg(SCN)₄]₂: Indications of Electronic Correlations in a 1/5-Filled Two-Dimensional Conductor

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The polarized reflectivity of β′′-(BEDO-TTF)₅[CsHg(SCN)₄]₂ is studied in the infrared range between 60 cm-1 and 6000 cm-1 from room temperature down to 10 K. Already at T=300 K a pseudogap in the optical conductivity is present of about 300 cm-1; the corresponding maximum in the spectrum shifts to lower frequencies as the temperature decreases. In contrast to quarter-filled BEDT-TTF-based conductors of the β′′-phase a robust Drude component in the conductivity spectra is observed which we ascribe to the larger fraction of charge carriers associated with the 1/5-filling of the conduction band. This observation is corroborated by exact diagonalization calculations on an extended Hubbard model on a square lattice for different fillings. A broad band at 4000 cm-1 which appears for the electric field polarized parallel to the stacks of the BEDO-TTF molecules is associated to structural modulations in the stacks; these modulations lead to a rise of the dc and microwave resistivity in 100 to 30 K temperature range.