Bechgaard salts (TMTSF)2X
Based on electron counting arguments, the TMTSF-salts are metals with a formally 3/4 filled conduction band. The actual band filling, however, is 1/2 due to a weak dimerization along the chains; further modifications may be caused by electronic correlations. While (TMTSF)2ClO4 is the only compound which at ambient pressure stays metallic down to 1 K where it becomes superconducting, most of the other Bechgaard salts undergo a metal-to-insulator transition (at temperatures around 10 K) which in some cases like (TMTSF)2PF6 can be suppressed by external pressure ( Dimensional Crossover ). Due to the larger anisotropy, stronger dimerization, and larger on-site Coulomb repulsion the TMTTF salts are closer to the Mott-Hubbard insulating state. (TMTTF)2PF6 is known to be the most correlated compound of the sulfur series. In the phase diagram (TMTTF)2Br lies between (TMTSF)2PF6 and (TMTTF)2PF6 since superconductivity has been observed only under very high pressure.

Phase diagram of (TMTSF)2X and (TMTTF)2X (according to [5] and S. Brown). loc denotes charge localization, CO charge order, SP spin-Peierls, AFM antiferromagnetic, SDW spin-density wave, and SC superconducting.

In 1979 D. Jerome found superconductivity in (TMTSF)2PF6 at temperatures of about 1 K and external pressure; however a first step was done. The Bechgaard salts (TMTSF)2X, where TMTSF stand for tetramethyltetraselenafulvalene, and X is one of the monovalent anions like PF6, AsF6, ClO4, ReO4 etc., are also interesting for other reasons:

(TMTSF)2PF6-Structure: The planar organic molecules are stacked along the a-axis; in the c-direction they are separated by the PF6-anions.

View along the stack of (TMTSF)2PF6: the TMTSF molecules are oriented along the c-axis; in the b-direction the selenium atoms develop interstack contacts and thus form sheets in the ab-plane with the tendency toward two-dimensionality.

Due to the one-dimensional nature of the Bechgaard salts, the low-energy excitations cannot simply be described by Landau's theory of a Fermi-liquid, instead the Tomonaga-Luttinger model ( one-dimensional metals ) has to be applied. Of high importance is the influence of the interchain interaction on the physical properties since it has to lead to modification of the theoretical desciption.

Optical conductivity of (TMTSF)2PF6

The optical conductvity of (TMTSF)2PF6 shows a stong anisotropy. At low temperatures strong deviations from a simple Drude behavior is seen in the chain direction. There is a finite-energy excitation around 200 cm-1 which develops for temperatures below 200 K and thus is not related to the SDW gap. And a zero-energy mode builds up as the temperatures is lowered with an extremely small relaxation rate. Perpendicular to the stacks the conductivity is lower, but still shows a Drude-like behavior at low temperatues.

At around 12 K (TMTSF)2PF6 undergoes a transition to a spin-density-wave ground state, i.e. a periodic modulation of the electronic spins which is not accompanied by a charge modulation or a lattice distortion. However a gap in the density of states opens at the Fermi-surface and causes a drastic change in most of the physical properties. Due to spin-phonon coupling there is also a significant change in the acoustic properties, like sound velocity and attenuation. The susceptibility vanishes rapidly, the dc resistivity increases many orders of magnitude. The optical properties, however, still show appreciable contributions in the low-energy range below the single-particle gap. Some can be identified as collective excitations of the SDW as a whole.
Most of the transport measurements have been performed along the chain direction where non-linear conductivity was observed due to collective transport of the SDW. Much less is know about the properties in the perpendicular direction.

The nature of the superconductivity is not fully understood yet. Slowly cooled (TMTSF)2ClO4 becomes superconducting at 1.2 K; for (TMTSF)2PF6 on the other hand an external pressure of 6.5 kbar is needed to induce superconductivity. Early NMR experiments show no Hebel-Slichter maximum and may indicate the influence of antiferromagnetic fluctuations and p-wave pairing. Recently this idea was supported by measurements of the c-axis resistance in an externel field of 10 Tesla and more. Experiments on the thermal transport rule out the existance of nodes in the gap. Investigations of the electrodynamic properties might clear this controversy since the optical conductivity is sensitive to low-energy excitations.

    Literature:
  1. M. Dressel et al., Phys. Rev. Lett. 77, 398 (1996) .
  2. L. Degiorgi et al., Phys. Rev. Lett. 76, 3838 (1996) .
  3. A. Schwartz et al., Phys. Rev. B 58, 1261 (1998) .
  4. M. Dressel, Physica C 317-318, 89 (1999) .
  5. J. Moser et al., Eur. Phys. J. D 1, 39 (1998) .

The investigations on the Bechgaard salts are performed in close collaboration with the University of California Los Angeles, the Eidgenössische Technische Hochschule Zürich, the Universität Frankfurt, and many other groups all around the world.

TMTTF salts
The TMTTF-salts where TMTTF denotes tetramethyltetrathiafulvalene are even more one-dimensional compared to their TMTSF analogs. 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 sulfer for selenium, these compounds to localized electrons and spins.

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).

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.

    Literature:
  1. M. Dumm et al., Phys. Rev. B 62, 6512 (2000) .

The investigations on the TMTTF-salts are a close collaboration with the Indiana University Bloomington, the Universität Augsburg, and many other groups all around the world.

Contact: M. Dumm, M. Dressel