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arXiv 0909.4153cond-mat.str-el
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(TMTTF)$_2$ X中电荷有序转变的热膨胀研究

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

Mariano de Souza, Daniel Hofmann, Pascale Foury-Leylekian, Alec Moradpour, Jean-Paul Pouget, Michael Lang

摘要:

报道了准一维(TMTTF)$_{2}$ X化合物(X = SbF $_6$和Br)在电荷有序(CO)跃迁时的\emph{c$^*$}轴膨胀率($α_{c^{*}}$)的高分辨测量结果,并与X = PF $_6$和AsF $_6$盐的结果进行了比较.对于X = SbF $_6$,由于长程库仑力的屏蔽,在$T_{CO}$观察到一个尖锐的$λ$型异常,这与PF $_6$和AsF $_6$在$T_{CO}$的阶梯状平均场异常形成对比,其中CO发生在Mott-Hubbard电荷局域区。对于后两种盐,观察到对$α_{c^{*}}$的负贡献高于$T_{CO}$。该特征被分配给阴离子的刚性单元模式,其对于$T$ $<$ $T_{CO}$变得不活跃。我们对X = Br盐的$α_{c^{*}}$结果(其中不存在这种刚性单元模式)显示没有这种负贡献的痕迹,证实了基于X = PF $_6$和AsF $_6$盐的阴离子刚性单元模式的模型。

英文摘要:

We report results of high-resolution measurements of the \emph{c$^*$}-axis expansivity ($α_{c^{*}}$) 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 $λ$-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 $α_{c^{*}}$ is observed above $T_{CO}$. This feature is assigned to the anions' rigid-unit modes, which become inactive for $T$ $<$ $T_{CO}$. Our $α_{c^{*}}$ 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.

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