TMTSF Salts: Quasi One-Dimensional Systems

Abstract

The first organic material which was discovered to show superconductivity was (TMTSF)₂PF₆. This salt is one of the so-called Bechgaard salts with the general formula (TMTSF)₂X, where X⁻ is a monovalent anion, first synthesized by Bechgaard et al. [3.1]. These compounds are charge transfer salts; the 2:1 salt formed by transferring one electron from two TMTSF molecules to one X. The crystal structures are isomorphous for various electron acceptors X. The structures remain the same even when the TMTSF molecule is replaced with a TMTTF molecule, which differs in that S atoms are substituted for the Se atoms of TMTSF (Fig. 1.1). This has allowed the investigation of the role of the constituent molecules and the crystal structure on the superconducting behavior. For example, the effect of intermolecular distances has been studied by modifying the molecular spacings under varying pressure and also by changing X. Although their superconducting transition temperatures are rather low (<3K), they have been model systems for the study of superconductivity in organic charge transfer salts at early stages and have provided a basis for understanding the much more complex and varied ET salts.