Infrared spectra of the one-dimensional quarter-filled Wigner lattice compoundsδ-(EDT-TTF-CONMe2)2X(X=AsF6, Br): Domain-wall excitations

Optical investigations of quarter-filled charge ordered insulators $\ensuremath{\delta}$-(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${}^{\ensuremath{-}1}$ for the $X=\text{Br}$ compound and about 700 cm${}^{\ensuremath{-}1}$ 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.