Abstract
Comprehensive measurements of the pressure- and temperature-dependent dc-transport are combined with dielectric spectroscopy and structural considerations in order to elucidate the charge and anion orderings in the quasi-one-dimensional charge-transfer salts (TMTTF) 2 X with non-centrosymmetric anions X = BF 4 , ClO 4 and ReO 4 . Upon applying hydrostatic pressure, the charge-order transition is suppressed in all three compounds, whereas the influence on the anion order clearly depends on the particular compound. A review of the structural properties paves the way for understanding the effect of the anions in their methyl cavities on the ordering. By determining the complex dielectric constant ϵ ^ ( ω , T ) in different directions we obtain valuable information on the contribution of the anions to the dielectric properties. For (TMTTF) 2 ClO 4 and (TMTTF) 2 ReO 4 , ϵ b ′ exhibits an activated behavior of the relaxation time with activation energies similar to the gap measured in transport, indicating that the relaxation dynamics are determined by free charge carriers.
Highlights
Electronic correlations play a decisive role in the quasi-one-dimensional molecular charge-transfer salts (TMTTF)2 X, where TMTTF means tetramethyltetrathiafulvalene
In addition to charge order, the (TMTTF)2 X salts with non-centrosymmetric anions X = SCN, SFO3, NO3, BF4, ClO4, and ReO4 undergo an ordering of the anions at TAO ; here we focus on the tetrahedral counter ions
In the case of (TMTTF)2 BF4, this corresponds to a peak at TCO ≈ 80 K, which is reduced in amplitude as frequency increases
Summary
Electronic correlations play a decisive role in the quasi-one-dimensional molecular charge-transfer salts (TMTTF) X, where TMTTF means tetramethyltetrathiafulvalene. As a result of the 2:1 stoichiometry with positively charged (TMTTF)ρ0 , ρ0 = 0.5, and monovalent anions X − , the (TMTTF) X salts should have a 3/4-filled conduction band. The slight dimerization of the molecular stacks leads to a splitting into a completely filled lower and a half-filled upper band; metallic properties are expected. This behavior is observed in the sibling TMTSF compounds, known as Bechgaard salts, here TMTSF stands for tetramethyltetraselenafulvalene. In the case of the Fabre-salts TMTTF, a minimum in resistivity ρ( T ) is observed at Due to the reduced dimension, Fermi liquid theory breaks down separating spin and charge degrees of freedom [1,2].
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