Infrared spectra of the one-dimensional quarter-filled Wigner lattice compounds $\delta$-(EDT-TTF-CONMe${}_2$)${}_{2}X$ ($X$ $=$ AsF${}_6$, Br): Domain-wall excitations

Optical investigations of quarter-filled charge ordered insulators $\delta$-(EDT-TTF-CONMe${}_2$)${}_{2}X$, $X={\text{AsF}}_6$, Br are reported. The spectra evidence the one-dimensional nature of the systems at all temperatures. The charge order exists already at room temperature, the optical gap is estimated to be of the order of 550 cm${}^{-1}$ for the $X=\text{Br}$ compound and about 700 cm${}^{-1}$ for the $X={\mathrm{AsF}}_6$ material. The optical response in the direction of the stacks is discussed in terms of domain-wall excitations of one-dimensional quarter-filled Wigner lattice at moderate values of nearest-neighbor electronic repulsion. A similarity between the effects of temperature contraction and chemical pressure is found.

Figure 1

Room-temperature reflectivity spectra of $\delta$-(EDT-TTF-CONMe${}_2$)${}_2$AsF${}_6$ in the three principal directions. The insert shows crystal structure EDT${}_2-X$[2] with crystal axes ($a,b,c$) marked. Spheres represent AsF${}_6$ or Br anions. Charge rich and charge poor EDT-TTF-CONMe${}_2$ molecules alternate within the stacks, they are represented by blue and red, respectively. The strongest overlap and the highest reflectivity are observed in the a direction, while spectra parallel to b and c are typical for an insulator.

Figure 2

$\delta$-(EDT-TTF-CONMe${}_2$)${}_2$AsF${}_6$ temper-ature-dependent reflectivity in the $\mathbf{E}\parallel \mathbf{a}$ direction and selected temperatures for $\mathbf{E}\parallel \mathbf{b}$ (insert).

Figure 3

$\delta$-(EDT-TTF-CONMe${}_2$)${}_2$Br temperature-dependent reflectivity in the $\mathbf{E}\parallel \mathbf{a}$ direction and selected temperatures for $\mathbf{E}\parallel \mathbf{b}$ (insert).

Figure 4

$\delta$-(EDT-TTF-CONMe${}_2$)${}_2$AsF${}_6$ temper-ature-dependent conductivity in the a direction.

Figure 5

$\delta$-(EDT-TTF-CONMe${}_2$)${}_2$Br temperature-dependent conductivity in the a direction.

Figure 6

The plot compares conductivity spectra of $\delta$-(EDT-TTF-CONMe${}_2$)${}_2$Br at 300 K and $\delta$-(EDT-TTF-CONMe${}_2$)${}_2$AsF${}_6$ at 100 K. The insert shows an example of a fit of $\delta$-(EDT-TTF-CONMe${}_2$)${}_2$Br spectra at 300 K to Lorentz-Fano formula. The fit is made simultaneously for reflectivity and conductivity.

Figure 7

Dependence of the position of the electronic band for $\delta$-(EDT-TTF-CONMe${}_2$)${}_2$Br (black squares) and $\delta$-(EDT-TTF-CONMe${}_2$)${}_2$AsF${}_6$ (red circles) on temperature, received from the Fano-Lorentz fit, compared to the temperature variation of the lattice parameter $a$.[2] The insert shows a schematic view of an optical excitation from a charge-rich to a charge-poor site, which leads to a creation of two domain walls in a charge-ordered chain.

Figure 8

Dependence of the spectral weight of the electronic band for $\delta$-(EDT-TTF-CONMe${}_2$)${}_2$Br and $\delta$-(EDT-TTF-CONMe${}_2$)${}_2$AsF${}_6$ on the position of the maximum, showing that both compounds follow the same dependence. The insert shows the dependence of the spectral weight on temperature for both compounds.