Fabre salts

Fabre salts (TMTTF)2X

The TMTTF-salts where TMTTF denotes tetramethyltetrathiafulvalene are even more one-dimensional compared to their TMTSF analogs because the selenium atoms are replaced by smaller sulfur atoms which reduces the overlap within the stack and more important between the stacks. Therefore the ratio of Coulomb repulsion U and bandwidth W (transfer integral t) is larger and the systems become at low temperatures a Mott insulators.

Fig 7: Temperature dependence of the dc resistivity of several Fabre and Bechgaard salts. As the temperature is reduced, the charges become increasingly localized in (TMTTF)2AsF6 and (TMTTF)2PF6, before the charge-ordered state is entered below 100 K. (TMTTF)2SbF6 shows a transition from a metal-like state directly into the charge-ordered state at TCO = 157 K. (TMTSF)2PF6 undergoes a SDW transition at TSDW = 12 K. Only (TMTSF)2ClO4 remains metallic all the way down to approximately Tc = 1.2 K.

Only at elevated temperatures the charges are delocalized and rather poor metallic conductivity is observed. Detailed studies of the temperature and pressure dependence of the crystal structure and band structure help to understand the transition from the rather one-dimensional behavior to the more two-dimensional properties exhibit at low temperatures and high pressure. Optical experiments can identify the crossover in the electronic properties by looking at the coherent transport.

Fig 8: The coherence parameters of the interstack charge transport k = wp/2Γ as a function of temperature and pressure for (TMTTF)2PF6 and (TMTSF)2PF6 . The solid blue lines corresponds to kb = 0.85.

Upon cooling a charge order transition occurs with a sharp increase in resistivity. Due to electronic interaction, the charge per molecule is varied from a homogeneous distribution of half an electron per molecule to an alternation of charge rich and charge poor molecules.
Structural considerations are of superior importance for the understanding of the differences between the various TMTSF and TMTTF salts. The organic molecules are stacked along zig-zag chains in the a-direction, separated in the c-direction by the anions. The dimerization decreases by going from the selenium compound to the sulfur counterparts. In contrast to the selenium analogs which in general are metallic down to low temperatures, the TMTTF salts discussed here are Mott-Hubbard insulators due to the small transfer integrals. Consequently they show a broad, but distinct resistivity minimum at high temperatures attributed to the continuous opening of a charge gap which is closely connected to the increased Coulomb interaction and dimerization.
By applying pressure on (TMTTF)2PF6 the temperature resistivity minimum decreases and at 13 kbar the salt is fully metallic and undergoes a SDW phase transition similar to (TMTSF)2PF6. Applying pressure also enhances the interchain coupling, in agreement with the fact that it is more one-dimensional than the selenium analog. (TMTTF)2Br is close to the borderline between itinerant and localized carriers: only below 100 K the resistivity increases due to charge localization. As a consequence, the transition to an antiferromagnetic ground state at 13 K does not lead to a SDW, as observed in (TMTSF)2PF6 which stays metallic down to 13 K, but to a localized antiferromagnet (AFM).

The reduced dimensionality leads to instabilities (one-dimensional metals) of the electronic system and has a distinct influence on the transport as well as on the magnetic properties. By applying external pressure or magnetic field, changing the anions or substituting sulfur for selenium, these compounds can be varied from itinerant to localized electrons and spins. It is very interesting to compare the charge and the spin dynamics by changing the anions in these systems in order to vary the dimensionality and to change the on-site Coulomb repulsion. In this way we can tune the system from a one-dimensional Tomonaga-Luttinger liquid (one-dimensional metals) to a more two-dimensional Fermi liquid. Similar impacts are expected for the application of external pressure.
Charge order affects most of the electronic properties, but for many years no change in the spin arrangement could be observed: the magnetic susceptibility remains unchanged. Only recently electron-spin-resonance experiments on quasi-one-dimensional (TMTTF)2X salts (X = PF6, AsF6, and SbF6) could reveal that the magnetic properties are modified below TCO when electronic ferroelectricity sets in. The coupling of anions and organic molecules rotates the g-tensor out of the molecular plane creating magnetically nonequivalent sites on neighboring chains at the domain walls. Due to anisotropic Zeeman interaction a novel magnetic interaction mechanism in the charge-ordered state is observed as a doubling of the rotational periodicity of ΔH.


Literature:

  1. M. Dumm et al., Phys. Rev. B 62, 6512 (2000).
  2. B. Salameh, S. Yasin, M. Dumm, G. Untereiner, L.K. Montgomery und M. Dressel
    Spin dynamics of the organic linear chain compounds (TMTTF)2X (X = SbF6, AsF6, BF4, ReO4, and SCN)
    Phys. Rev. B 83, 205126 (2011).
  3. S. Yasin, B. Salameh, E. Rose, M. Dumm, H.-A. Krug von Nidda, A. Loidl, M. Ozerov, G. Untereiner, L.K. Montgomery und M. Dressel
    Broken magnetic symmetry due to charge-order ferroelectricity discovered in (TMTTF)2X salts by multifrequency ESR
    Phys. Rev. B 85, 144428 (2012).
  4. M. Dressel, M. Dumm, T. Knoblauch und M. Masino
    Comprehensive Optical Investigations of Charge Order in Organic Chain Compounds (TMTTF)2X
    Crystals 2, 528 - 578 (2012).
  5. M. Dressel, M. Dumm, T. Knoblauch, B. Köhler, B. Salameh, S. Yasin Charge Order Breaks Magnetic Symmetry in Molecular Quantum Spin Chains Advances in Condensed Matter Physics 2012, 398721 (2012).