Indications of electronic correlations in the $1∕5$-filled two-dimensional conductor ${\beta }^{″}\text{−}{\left(\mathrm{BEDO}\text{−}\mathrm{TTF}\right)}_5{\left[\mathrm{Cs}\mathrm{Hg}{\left(\mathrm{SCN}\right)}_4\right]}_2$

The polarized reflectivity of ${\beta }^{″}\text{−}{\left(\mathrm{BEDO}\text{−}\mathrm{TTF}\right)}_5{\left[\mathrm{Cs}\mathrm{Hg}{\left(\mathrm{SCN}\right)}_4\right]}_2$ is studied in the infrared range between $60{\mathrm{cm}}^{-1}$ and $6000{\mathrm{cm}}^{-1}$ from room temperature down to 6 K. Already at $T=300\mathrm{K}$ a pseudogap in the optical conductivity is present at about $150{\mathrm{cm}}^{-1}$; the corresponding maximum in the spectrum shifts to lower frequencies as the temperature decreases. A robust Drude component is observed in the spectra which is much larger than comparable quarter-filled compounds; we ascribe it 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 $4500{\mathrm{cm}}^{-1}$ which appears for the electric field polarized parallel to the stacks of the BEDO-TTF molecules is associated with structural modulations in the stacks; these modulations lead to a rise of the dc and microwave resistivity in 100 to 30 K temperature range.

Figure 1

Temperature dependence of the normalized dc resistance of ${\beta }^{″}\text{−}{\left(\mathrm{BEDO}\text{−}\mathrm{TTF}\right)}_5{\left[\mathrm{Cs}\mathrm{Hg}{\left(\mathrm{SCN}\right)}_4\right]}_2$ parallel and perpendicular to the stacking direction.

Figure 2

Temperature dependence of microwave resistivity ${\beta }^{″}\text{−}{\left(\mathrm{BEDO}\text{−}\mathrm{TTF}\right)}_5{\left[\mathrm{Cs}\mathrm{Hg}{\left(\mathrm{SCN}\right)}_4\right]}_2$ measured at 24 GHz with the electric field parallel (full symbols) and perpendicular (open circles) to the stacks of the BEDO-TTF molecules.

Figure 3

Reflectivity and optical conductivity spectra of ${\beta }^{″}\text{−}{\left(\mathrm{BEDO}\text{−}\mathrm{TTF}\right)}_5{\left[\mathrm{Cs}\mathrm{Hg}{\left(\mathrm{SCN}\right)}_4\right]}_2$ for polarizations $E\Vert$ stacks and $E\perp$ stacks at temperatures $T=300$, 200, 100, and 10 K.

Figure 4

Optical conductivity spectra of ${\beta }^{″}\text{−}{\left(\mathrm{BEDO}\text{−}\mathrm{TTF}\right)}_5{\left[\mathrm{Cs}\mathrm{Hg}{\left(\mathrm{SCN}\right)}_4\right]}_2$ for the polarizations (a) $E\Vert$ stacks and (b) $E\perp$ stacks for different temperatures as indicated. The curves are shifted for clarity reasons.

Figure 5

Optical conductivity and Drude weight from exact diagonalization calculations of an extended Hubbard model on a square lattice. The system is at $1∕5$-filling, $U=20t$ and the cluster size is $L=20$. A broadening of $\eta =0.2t$ has been used for comparison to experimental data. At $1∕5$-filling, the system remains metallic for any value of $V$ in contrast with the $1∕4$-filled salt, which becomes insulating for $V\gtrsim V_c\approx 2t$. These calculations are consistent with the strong Drude component observed in the optical conductivity of $1∕5$-filled salts in contrast to the weaker one found for their $1∕4$-filled counterparts. Although strongly suppressed with respect to the $1∕4$-filled salt, a low frequency feature is evident for the plotted values of $V∕t$.