Unified theory of segregated-stack organic charge-transfer solids: Magnetic properties

Abstract

A theoretical description of the complete family of quasi-one-dimensional segregated-stack charge-transfer solids within a single model has remained elusive mostly because of the rich variety of behavior within the family. In particular, materials with both strongly enhanced as well as almost unenhanced static magnetic susceptibilities are known. Furthermore, a large number of different interpretations of the static magnetic susceptibility data have led to an intense controversy over the role of Coulomb correlations in these narrow-band systems. By comparing structurally similar materials with different magnetic behavior we show that (a) these differences do not originate in differences in molecular properties or crystal structures and (b) none of the simple electron-electron or electron-phonon coupled models can explain the observed differences in susceptibility behavior. Within a previously proposed extended Hubbard model we then show that the susceptibility is expected to vary strongly and systematically as a function of the degree of charge transfer. Detailed comparisons of both the magnitude of the high-temperature susceptibilities as well as the temperatures at which the susceptibilities peak for a large number of materials are made with theoretical predictions to prove the validity of our model. In addition we discuss how other properties of the complete family can be explained and predicted within the present model, and show that the parameters of the model can be obtained from charge-transfer absorption data. Several of the newly synthesized materials have been suggested to have weak electron correlations based on weakly enhanced susceptibilities and the absence of the 4$k_F$ instability. We propose, however, that the above experimental features are consequences of the specific charge-transfer range within which these materials lie, and do not imply weak correlations.