Exploring the Charge-Ordering Transition in (TMTTF)$_2$X via Thermal Expansion Measurements

TL;DR

This study investigates the charge-ordering transition in (TMTTF)$_2$X compounds using high-resolution thermal expansion measurements, revealing how different anions influence the nature of the transition and associated lattice dynamics.

Contribution

It provides the first detailed comparison of thermal expansion anomalies across various (TMTTF)$_2$X salts, highlighting the role of long-range Coulomb screening and anion rigid-unit modes.

Findings

Sharp lambda anomaly for X=SbF6 indicating long-range Coulomb screening effects.

Step-like anomaly for X=PF6 and AsF6 with negative contributions from anion modes.

Absence of negative contributions in X=Br confirming the role of rigid-unit modes.

Abstract

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 results 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.