Magnetic Properties of Conducting Organic Salts

Abstract

The physics of organic charge transfer salts is reviewed with emphasis on the highly conducting compounds based on TCNQ. Specific heat studies of a family of these salts show systematic variation of the Debye temperature, indicating that charge transfer in such systems is in some cases stabilized by the Madelung energy, but that polarization contributions are important. Requirements for achieving a metallic state in organic salts are discussed in terms of crystal structure, Coulomb interaction, and cation polarizability. Magnetic properties of (NMP)(TCNQ) are discussed in the general context of the metal‐insulator transition. Measurements of the transfer integral t, the on‐site Coulomb repulsion U, and the exchange interaction J, provide a numerical check on the relation J = 2t2/U, expected when the AF exchange arises from virtual charge transfer. In the low temperature AF regime, spin dynamics have been studied with NMR techniques. Results indicate that for spin ½ systems in 1‐d, spin wave excitations are to be treated as Fermions. The role of disorder is reviewed. The existence of metallic behavior in a symmetric cation salt, together with earlier studies, rules out cation disorder as the source of high conductivity.