In many materials with a highly anisotropic band structure, electron-phonon interactions lead to a novel type of ground state called the charge-density wave. The condensate is pinned to the underlying lattice by impurities and by boundary effects, but can, even for small electric fields, carry current in a fashion originally envisioned by Fr\"ohlich. This review discusses some of the underlying theories and the main experimental observations on this new collective transport phenomenon. The frequency- and electric-field-dependent conductivity, current oscillations, electric-field-dependent transport coefficients and elastic properties, together with nuclear-magnetic-resonance experiments, provide clear evidence for a translational motion of the condensate. Various theories, involving classical and quantum-mechanical concepts, are able to account for a broad variety of experimental findings, which were also made in the presence of combined dc and ac fields.

The dynamics of charge-density waves

This review attempts to present the most salient developments of research on organic conductors and superconductors during the past 10 years. A theoretical introduction treats instabilities of quasi-one-dimensional electron systems and associated precursor effects which are relevant to the experimental results on organic conductors. We then describe the characterization of quasi-one-dimensional organic conductors by their transport, optical and magnetic properties. Finally, two sections are devoted to the experimental investigation of the low temperature instabilities: lattice instability in TTF-TCNQ and related compounds, superconducting or antiferromagnetic instabilities in the (TMTSF) 2 X series. The importance of one-dimensional fluctuations is emphasized in both lattice and superconducting instabilities.

Organic conductors and superconductors

We review studies of the electromagnetic response of various classes of correlated electron materials including transition metal oxides, organic and molecular conductors, intermetallic compounds with $d$- and $f$-electrons as well as magnetic semiconductors. Optical inquiry into correlations in all these diverse systems is enabled by experimental access to the fundamental characteristics of an ensemble of electrons including their self-energy and kinetic energy. Steady-state spectroscopy carried out over a broad range of frequencies from microwaves to UV light and fast optics time-resolved techniques provide complimentary prospectives on correlations. Because the theoretical understanding of strong correlations is still evolving, the review is focused on the analysis of the universal trends that are emerging out of a large body of experimental data augmented where possible with insights from numerical studies.

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Electrodynamics of correlated electron materials

We introduce the development of conductive organic molecular assemblies including organic metals, superconductors, single component conductors, conductive films, and conductive liquids, particularl...

Development of Conductive Organic Molecular Assemblies: Organic Metals, Superconductors, and Exotic Functional Materials

Charge density wave conduction: A novel collective transport phenomenon in solids

Anomalous magnetoresistance in an organic conductor - (tmtsf)2pf6

Charge density wave noise in NbSE3

The spin susceptibility of organic metals is enhanced over the Pauli value at ambient temperature, where the mean free path is close to one lattice constant. The resistivity continues to follow the T 2 law in this regime. A theory is described which accounts for these unusual effects. The theory is somewhat analogous to polaron theory and makes use of the quadratic nature of the electron-phonon coupling. The electrons are localized by short-lived “phonon traps”, accounting for the transition to a Curie-like behaviour. Tunnelling between these nun-degenerate molecular sites gives rise to a T 2 law with a coefficient close to that of the low-temperature state, where the conductivity is metallic.

M. Weger

Papers

Anomalous magnetoresistance in an organic conductor - (tmtsf)2pf6

Charge density wave noise in NbSE3

Enhancement of susceptibility and the electrical resistivity of organic metals with a small mean free path

Nonlinear conductivity and narrow band noise in NbSe3

Phonon dispersion in systems with a planar Fermi surface