Bechgards salts

Bechgaard salts (TMTSF)2X

 

 

Prof. Dr. Dr. h.c. Denis Jerome (right) discussion Prof. Dr. Martin Dressel (center) and Prof. Dr. Dieter Schweitzer (left) on occasion of the celebration of the honorary doctorate of the Universität Stuttgart bestowed upon Prof. Jerome in November 2005.

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, ReO4etc., are also interesting for other reasons:

  • Many of the systems are good metals down to low temperature before they become superconducting.
  • Although they are basically one-dimensional, the interchain coupling may be changed by applying pressure or replacing the anions, thus allowing to tune the dimensionality.
  • some of the salts show transition to a spin-density-wave (SDW) ground state or a charge-density wave ground state.
  • Anion ordering may cause structural changes.
  • Charge ordering represents an entirely electronic ferroelectricity, albeit small effects on the anions may occur and the magnetic symmetry is changed.

 

Fig 3: (a) represents (TMTSF)2PF6-Structure: The planar organic molecules are stacked along the a-axis; in the c-direction they are separated by the PF6 anions; (b) illustrates the 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 (indicated by the red dashed lines).

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 description.

Fig 4: Temperature dependence of the electrical resistivity of (TMTSF)2PF6 along all three directions. Below TSDW = 12 K (displayed in part (1) of (a)), the material undergoes a spin-density wave transition that leads to an opening of a gap in all directions of the Fermi surface. In the intermediate temperature range (part (2) of (a)), the system is metallic in all three directions with a strong anisotropy; it can be described by a Fermi-liquid. At temperature higher than the interstack coupling (represented by part (3) of (a) zoomed and illustrated in (b)) , the system behaves like a Luttinger liquid with the corresponding power-law dependences in the a- and c- directions.

The optical conductivity of (TMTSF)2PF6 shows a strong 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 (corresponding to excitations across the Mott gap) 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 temperature 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 temperatures

 

Fig 5: Reflectivity spectra of (TMTSF)2PF6 measured at different temperatures along the stacking axis a (solid black line) and perpendicular to it (solid green line). The filled symbols are obtained by a coherent THz source , the open symbols are calculated from microwave experiments. The dashed lines represent a Drude fit, respectively. The inset of panel (b) shows the schematic phase diagram of the deconfinement transition for a system of weakly coupled conducting chains as suggested by Giamarchi and collaborators. The transition from a Mott insulator to a two- or three-dimensional metallic state occurs at T=0 K when t reaches a critical value t*. At high enough temperature, the increase in t leads to a transition from a Mott insulating to a one-dimensional Luttinger liquid and further to a dimensional crossover into a metallic state. The development of the SDW gap at 70 cm-1 is seen from the low-temperature conductivity E a plotted in the inset of panel (c).

Fig 6: Optical conductivity of (TMTSF)2PF6 for two different directions parallel and perpendicular to the stacks. In the longitudinal direction two contributions can be observed at low temperatures: a rather broad peak (green) centered around 200 cm-1that can be assigned to excitations across the Mott-Hubbard gap. The narrow zero-frequency mode corresponds to coherent transport along the chain due to self-doping of the Mott-insulator by a finite coupling between the chains.

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 and 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 nonlinear conductivity was observed due to collective transport of the SDW. Much less is known 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 external field of 10 Tesla and more. However, alternative explanations by Fulde-Ferrell-Larkin-Ovchinnikov phase have been suggested. Experiments on the thermal transport rule out the existence 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. J. Moser et al., Eur. Phys. J. D 1, 39 (1998).
  5. M. Dressel
    Spin-charge separation in quasi one-dimensional organic conductors
    Naturwissenschaften 90, 337 - 344 (2003).
  6. M. Dressel, K. Petukhov, B. Salameh, P. Zornoza und T. Giamarchi
    Scaling behavior of the longitudinal and transverse transport in quasi-one-dimensional organic conductors
    Phys. Rev. B 71, 075104 (2005).
  7. M. Dressel
    Ordering Phenomena in Quasi One-Dimensional Organic Conductors
    Naturwissenschaften 94, 527 (2007).
  8. B. Köhler, E. Rose, M. Dumm, G. Untereiner und M. Dressel
    Comprehensive transport study of anisotropy and ordering phenomena in quasi-one-dimensional (TMTTF)2X salts (X = PF6,AsF6,SbF6,BF4,ClO4,ReO4)
    Phys. Rev. B 84, 035124 (2011).
  9. M. Dressel
    Quantum criticality in organic conductors? Fermi-liquid versus non-Fermi-liquid behavior
    J. Phys.: Condens. Matter 23, 293201 (2011).
  10. M. Dressel
    Electrodynamics of Bechgaards Salts: Optical Properties of One-Dimensional Metals
    International Scholarly Research Network (ISRN)
    Condensed Matter Physics 732973 (2012).