Charge- and anion-orderings in the quasi-one-dimensional organic conductor (TMTTF)${}_2$NO${}_3$

Abstract

The quasi-one-dimensional organic conductors (TMTTF)${}_{2}X$ with non-centrosymmetric anions commonly undergo charge- and anion-order transitions upon cooling. While for compounds with tetrahedral anions ($X$ = BF${}_4^{-}$, ReO${}_4^{-}$, and ClO${}_4^{-}$) the charge-ordered phase is rather well understood, the situation is less clear in the case of planar triangular anions, such as (TMTTF)${}_2$NO${}_3$. Here we explore the electronic and structural transitions by transport experiments, optical and magnetic spectroscopy. This way we analyze the temperature dependence of the charge imbalance 2$\delta$ and an activated behavior of $\rho (T)$ with ${\Delta }_{\mathrm{CO}}\approx 530$~K below $T_{\mathrm{CO}}=250$~K. Since (TMTTF)${}_2$NO${}_3$ follows the universal relation between charge imbalance 2$\delta$ and size of the gap ${\Delta }_{\mathrm{CO}}$, our findings suggest that charge order is determined by TMTTF stacks with little influence of the anions. Clear signatures of anion ordering are detected at $T_{\mathrm{AO}}=50$~K. The tetramerization affects the dc transport, the vibrational features of donors and acceptors, and leads to formation of spin singlets.

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