Elsevier

Chemical Physics

Volume 155, Issue 1, 1 August 1991, Pages 49-61
Chemical Physics

Material properties of low-dimensional charge-transfer salts. II. Mode-softening, Peierls transitions and van Hove singularities

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Abstract

The formation of soft modes in low-dimensional metallic systems with planar and nonplanar Fermi surfaces is studied in a tight-binding approximation. The electronic prerequisites allowing for instabilities at wave vectors (0, 0, 2kF) and (π/a, π/b, 2kF0) are studied in the zero-temperature limit. The optimum conditions to find the second type of mode-softening are realized in 2:1 donor-acceptor systems with an integral charge transfer and weaker intermolecular tight-binding interactions. The underlying electronic-structure properties (i.e. half-filled dispersions, smaller hopping integrals) shift logarithmic van Hove singularities in the electronic density of states to the Fermi level. Mode-softening and van Hove singularities enhance cooperatively the strength of the electron-phonon coupling. This coincidence may lead to low-temperature superconductivity in the strong coupling limit. The most important differences between one-dimensional electronic instabilities at wavevector (0, 0, 2kF) and suggested processes at (π/a, π/b, 2kF0) are quantified. The Peierls transition temperatures observed in (Y, Z-DCNQI)2Cu salts with di substituents Y, Z are reproduced by a simple interpolation scheme. The Peierls transitions in these compounds are explained by some type of “umbrella” model, i.e. a cooperative departure in the different Cu salts from a region where electronic instabilities are prevented by “three-dimensionality”.

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