Mid-infrared response of charge-ordered quasi-1D organic conductors (TMTTF)₂X

Abstract
We investigated the temperature dependence of the charge disproportionation in the charge ordered quasi one-dimensional conductors (TMTTF)₂AsF₆ and (TMTTF)₂PF₆ by infrared spectroscopy. In the mid-infrared, we observed a splitting of characteristic vibronic modes below the charge-order transition which can be directly linked to the charge disproportionation on the molecules. From our analysis of these modes we extracted the temperature dependence of the charge disproportionation which is much stronger in (TMTTF)₂AsF₆ compared to (TMTTF)₂PF₆. The charge disproportionation is only weakly affected by the spin-Peierls transition in the AsF₆ salts and remains basically unchanged in the PF₆ salts. Therefore, our data confirm that the spin-Peierls and charge-ordered ground state coexist at low temperatures.