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Charge-Ordering Transition in (TMTTF)$_2$X Explored via Dilatometry

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 نشر من قبل Mariano de Souza Prof. Dr.
 تاريخ النشر 2013
  مجال البحث فيزياء
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Charge-ordering phenomena have been highly topical over the last few years. A phase transition towards a charge ordered state has been observed experimentally in several classes of materials. Among them, many studies have been devoted to the family of quasi-one dimensional organic charge-transfer salts (TMTTF)$_2$X, where (TMTTF) stands for tetramethyltetrathiafulvalene and X for a monovalent anion (X = PF$_6$, AsF$_6$ and SbF$_6$). However, the relationship between the electron localization phenomena and the role of the lattice distortion in stabilizing the charge-ordering pattern is poorly documented in the literature. Here we present a brief overview of selected literature results with emphasis placed on recent thermal expansion experiments probing the charge-ordering transition of these salts.



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We report results of high-resolution measurements of the emph{c$^*$}-axis expansivity ($alpha_{c^{*}}$) at the charge-ordering (CO) transition for the quasi-1D (TMTTF)$_{2}$X compounds with X = SbF$_6$ and Br and make a comparison with previous resul ts for the X = PF$_6$ and AsF$_6$ salts. For X = SbF$_6$, due to the screening of the long-range Coulomb forces, a sharp $lambda$-type anomaly is observed at $T_{CO}$, which contrasts with the step-like mean-field anomaly at $T_{CO}$ for PF$_6$ and AsF$_6$, where CO occurs in the Mott-Hubbard charge-localized regime. For the latter two salts, a negative contribution to $alpha_{c^{*}}$ is observed above $T_{CO}$. This feature is assigned to the anions rigid-unit modes, which become inactive for $T$ $<$ $T_{CO}$. Our $alpha_{c^{*}}$ results for the X = Br salt, where such rigid-unit modes are absent, reveal no traces of such negative contribution, confirming the model based on the anions rigid-unit modes for the X = PF$_6$ and AsF$_6$ salts.
High-resolution thermal expansion measurements have been performed for exploring the mysterious structureless transition in (TMTTF)$_{2}$X (X = PF$_{6}$ and AsF$_{6}$), where charge ordering at $T_{CO}$ coincides with the onset of ferroelectric order . Particularly distinct lattice effects are found at $T_{CO}$ in the uniaxial expansivity along the interstack $textbf{textit{c*}}$-direction. We propose a scheme involving a charge modulation along the TMTTF stacks and its coupling to displacements of the counteranions X$^{-}$. These anion shifts, which lift the inversion symmetry enabling ferroelectric order to develop, determine the 3D charge pattern without ambiguity. Evidence is found for another anomaly for both materials at $T_{int}$ $simeq$ 0.6 $cdot$ $T_{CO}$ indicative of a phase transition related to the charge ordering.
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