Organic Conductors

Introduction

Organic conductors have been developed since the 1970's and by now exhibit an electrical conductivity comparable to normal metals and even become superconducting at Tc = 13 K. They constitute a class of solids with no metal atoms present (or relevant); instead the p-electrons distributed over the organic molecule form the orbitals, which overlap and lead to band-like electrical conductivity. Organic solids have an enormous potential for fundamental studies as well as for applications using functional materials. The building blocks can be varied by tailoring the molecules. In addition the structural arrangement in the crystal can be changed in order to fine-tune the competing interactions from the various degrees of freedom. This makes organic materials superior model systems for studying low-dimensional physics and ordering phenomena in solids. While applications using their electrical and optical properties are already wide spread, novel aspects of molecular spintronics are under development and exploration.

Organic conductors serve as model systems for a number of fundamental issues, such as one-dimensional metallic (Luttinger liquid) behavior, charge- and spin-density wave systems, charge order, spin chains, Mott insulator, etc. Molecular Materials also have enormous potential for applications as functional matter: electronic ferroelectricity, switching by light, spin-charge interaction, etc.

Examples

In general these synthetic metals consist of piles of planar molecules with the atomic orbitals overlapping along the stack. While in this direction the electrical conductivity can reach the value of conventional metals, in the perpendicular directions the conductivity is orders of magnitude lower, because the distance between the stacks is large and in addition they may be separated by counterions.

There are two prerequisite for a good electronic transport: the overlap of the orbitals and an electronic charge transfer between donor and acceptor molecules to generate partially filled bands. To achieve the latter requirement, most organic conductors are charge-transfer salts, consisting of donors and acceptors. They are arranged in alternating stacks, for example for TTF-CA, or in segregated stacks of anions and cations, for instance in (TMTSF)2PF6.

The breakthrough of organic conductors happened in the early 1970s with the synthesis of TTF-TCNQ which exhibits a room temperature conductivity of 103 (W cm)-1 and an anisotropy of more than a factor of 100. TTF-TCNQ is a charge-transfer compound with separate stacks of the cations TTF (charge donors) and anions TCNQ (electron acceptors). It has very good metallic properties down to a temperature of approximately 60 K where a metal-insulator transition occurs due to the development of a charge-density wave.

In order avoid the Fermi-surface instabilities that occur in one-dimension, the interaction between adjacent stacks has to be increase for example by going to the enlarged TMTTF and TMTSF molecule. These crystals do not undergo a charge-density wave transition, but magnetic interaction becomes important at low temperatures where magnetic order, spin-Peierls or spin-density wave transitions are observed. Some materials remain metallic down to lowest temperatures where they become superconducting.

 In the course of the last two decades, in particular the Bechgaard salts TMTSF, and its variant TMTTF where selenium is replaced by sulfur, turned out to be an excellent model for quasi-one-dimensional metals, superconductors, charge order, spin-density wave, spin chains, spin-Peierls , etc. depending on the degree of coupling along and perpendicular to the chains. The planar organic molecules are stacked along the a-direction with a distance of approximately 3.6 Å. In the b-direction the coupling between the chains is relatively small (but not negligible), and in the third direction the stacks are even separated by the inorganic anion, like PF6-, AsF6-, ClO4-, Br-, etc. Each organic molecule transfers half an electron to the counterions yielding a quarter-filled hole band. In general a small dimerization creates pairs of organic molecules; the conduction band gets splitted. In addition, spontaneous charge ordering (CO) can occur, leading to  two non-equivalent charged  cationic molcules in the crystal unit cell commonly observed in TMTTF salts.

The next step in the direction of increased interstack interaction is achieved for the BEDT-TTF molecules. The resulting salts are basically two-dimensional, with rather high conductivity within the BEDT-TTF layers and more or less insulating perpendicular to them. These systems have become model compounds for two-dimensional conductors, exhibiting Mott transitions, spin and charge order, spin frustration, etc.

The interest in organic materials is related to their high anisotropy, i.e., the electrical conductivity is metallic along one or two directions, while it is smaller by several orders of magnitude along the perpendicular directions. The anisotropy influences not only the charge transport in the normal state (free charge carriers along one direction, conductivity based on hopping in the perpendicular direction), but it also leads to a couple of unusual ground states, like charge and spin-density waves, and spin-Peierls state. For example, it is well-known that Landau's fundamental theory (stating that interacting particles can be described by the model of non-interacting particles after some suitable renormalization) breaks down in one dimension. One would like to know up to which level of anisotropy Landau's theory can be applied or can be modified.

The mechanism of superconductivity, in particular, the influence of phonons and of magnetic and charge fluctuations, in low-dimensional organic conductors is not yet completely explained. On the other hand, the symmetry of the superconducting state seems to be more complicated than for usual metals; there are strong arguments for d-wave symmetry of the order parameter, however, many aspects can be nicely explained by simple s-wave symmetry. There are some analogies to the class of the oxide-based high-temperature superconductors, like the two-dimensional layered structure, the influence of electronic correlations, and the close relation to magnetic phases. In addition charge degree of freedom seems to be important, and some organic conductors are candidates for charge-fluctuation driven superconductivity.

In contrast to inorganic crystals, organic systems have a much higher potential for molecular engineering, i.e., systematic and specific variations of the molecules which lead to the enhancement or suppression of particular phenomena.

Open Positions/Theses on Organic conductors

People
Links & Literature
  1. J.M. Williams et al., Science 252, 1501 (1991).
  2. D. Jerome, Science 252, 1509 (1991).
  3. J.M. Williams, Organic superconductors (Englewood Cliffs, NJ: Prentice Hall, 1992).
  4. S. Brown and G. Grüner, Charge and Spin Density Waves, Scientific American 270, No. 4, 28 (1994).
  5. M. Dressel, D. Schweitzer und H. Keller, Physikalische Blätter 50, 1145 (1994).
  6. J.-P. Farges,Organic conductors (New York: Dekker, 1994).
  7. T. Ishiguro, K. Yamaji, and G. Saito, Organic superconductors, 2nd edition (Berlin; Heidelberg: Springer, 1998).
  8. J. Wosnitza, Physikalische Blätter 56, Nr. 4, 41 (2000).
  9. N. Toyota, M. Lang and J. Müller, Low-Dimensional Molecular Materials (Berlin; Heidelberg: Springer 2007).
  10. A. Lebed, The Physics of Organic Superconductors and Conductors, (Berlin; Heidelberg: Springer 2008).
  11. M. Dressel, Quantum criticality in organic conductors? Fermi-liquid versus non-Fermi-liquid behavior, J. Phys.: Condens. Matter 23, 293201 (2011).