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

Static critical phenomena in ferromagnets with quenched disorder

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

The ideal magnetocaloric material would lay at the borderline of a first-order and a second-order phase transition. Hence, it is crucial to unambiguously determine the order of phase transitions for both applied magnetocaloric research as well as the characterization of other phase change materials. Although Ehrenfest provided a conceptually simple definition of the order of a phase transition, the known techniques for its determination based on magnetic measurements either provide erroneous results for specific cases or require extensive data analysis that depends on subjective appreciations of qualitative features of the data. Here we report a quantitative fingerprint of first-order thermomagnetic phase transitions: the exponent n from field dependence of magnetic entropy change presents a maximum of n > 2 only for first-order thermomagnetic phase transitions. This model-independent parameter allows evaluating the order of phase transition without any subjective interpretations, as we show for different types of materials and for the Bean–Rodbell model. Magnetocaloric materials often perform best when their magnetic transitions are at the boundary between first- and second-order behavior. Here the authors propose a simple criterion to determine the order of a transition, which may accelerate future magnetocaloric material searches.

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A quantitative criterion for determining the order of magnetic phase transitions using the magnetocaloric effect

This review describes the synthesis of 1,3- and 1,2-tetrachalcogenafulvalenes, the chemical transformation of substituents, reactions of the ring systems and the synthesis of polymers with 1,3-tetrathiafulvalene units in the backbone and in the side chain. For selected charge-transfer complexes of tetrachalcogenafulvalenes the relation between molecular structure and solid-state structure, respectively, and electric conductivity are discussed. Superconducting properties of some charge-transfer complexes with tetrachalcogenafulvalenes as donor are reported. Preparations of important starting compounds for the synthesis of tetrachalcogenafulvalenes (like 1,2- and 1,3-dithiole, 1,3-thiaselenole, and 1,3-diselenole derivatives) have been considered in this review. Selected properties (melting point, UV-VIS absorption data and oxidation potentials) of 1,2- and 1,3-tetrachalcogenafulvalenes are summarized in appropriate tables.

Synthesis, Reactions, and Selected Physico-Chemical Properties of 1,3-and 1,2-Tetrachalcogenafulvalenes

The critical specific heat of a glassy ferromagnet

Two phase transitions in TSeF-TCNQ?—The effects of doping with small concentrations of TTF-TCNQ

Effects of isostructural doping of the donor and acceptor stacks on the phase transitions and magnetic and electrical properties of tetrathiafulvalene-tetracyanoquinodimethane

Replacement of the sulphur atoms in the charge transfer salt tetrathiafulvalene (TTF)–2,5-diethyltetracyano-p-quinodimethane (DETCNQ) with selenium yields the isostructural and better conducting analogue tetraselenafulvalene (TSeF)–DETCNQ, in which the Peierls transition temperature is decreased.

‘Organic metals’. Isostructural tetrathia- and tetraselena-fulvalene salts of 2,5-diethyltetracyano-p-quinodimethane

Doping of methyltetracyano-p-quinodimethane into the ‘organic metal’ tetraselenafulvalene–tetracyano-p-quinodimethane obscures the phase transition at 28 K and increase the conductivity at 4 K by four orders of magnitude.

R. A. Craven

Papers

The critical specific heat of a glassy ferromagnet

Two phase transitions in TSeF-TCNQ?—The effects of doping with small concentrations of TTF-TCNQ

Effects of isostructural doping of the donor and acceptor stacks on the phase transitions and magnetic and electrical properties of tetrathiafulvalene-tetracyanoquinodimethane

‘Organic metals’. Isostructural tetrathia- and tetraselena-fulvalene salts of 2,5-diethyltetracyano-p-quinodimethane

‘Organic metals’: acceptor stack doping in the charge-transfer salt tetraselenafulvalene–tetracyano-p-quinodimethane (TSeF–TCNQ)