Abstract
In the quasi-one-dimensional (TMTTF)2X compounds with effectively quarter-filled bands, electronic charge order is stabilized from the delicate interplay of Coulomb repulsion and electronic bandwidth. The correlation strength is commonly tuned by physical pressure or chemical substitution with stoichiometric ratios of anions and cations. Here, we investigate the charge-ordered state through partial substitution of the anions in (TMTTF)2[AsF6]1−x[SbF6]x with x≈0.3, determined from the intensity of infrared vibrations, which is sufficient to suppress the spin-Peierls state. Our dc transport experiments reveal a transition temperature TCO = 120 K and charge gap ΔCO=430 K between the values of the two parent compounds (TMTTF)2AsF6 and (TMTTF)2SbF6. Upon plotting the two parameters for different (TMTTF)2X, we find a universal relationship between TCO and ΔCO yielding that the energy gap vanishes for transition temperatures TCO≤60 K. While these quantities indicate that the macroscopic correlation strength is continuously tuned, our vibrational spectroscopy results probing the local charge disproportionation suggest that 2δ is modulated on a microscopic level.
Highlights
IntroductionOrganic charge-transfer salts are model systems realizing electronic correlations and
Organic charge-transfer salts are model systems realizing electronic correlations andMott–Hubbard physics, yielding a plethora of metal–insulator transitions in many different compounds
The transition temperature TCO = 120 K agrees with our expectations based on the stoichiometry, placing (TMTTF)2 [AsF6 ]1− x [SbF6 ]x with x = 0.3 closer to (TMTTF)2 AsF6 than to (TMTTF)2 SbF6
Summary
Organic charge-transfer salts are model systems realizing electronic correlations and. Mott–Hubbard physics, yielding a plethora of metal–insulator transitions in many different compounds. Owing to their effectively quarter-filled bands with nominally one electron per two organic molecules, the quasi-one-dimensional Fabre salts (TMTTF) X are prone to charge-order instabilities [1]. More complex phase transitions to anion-ordered and spin-Peierls phases result in modifications of the magnetic and structural degrees of freedom, such as the formation of a spin gap and tetramerization of the TMTTF molecules.
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