Impurity-induced spin-orbit coupling and ESR line broadening in tetramethyltetraselenafulvalenium salts ((TMTSF)2X)

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Abstract
It is shown that replacement of TMTSF by TMTTF in the one-stack organic conductors (TMTSF)2-X (TMTSF and TMTTF are tetramethyltetraselenafulvalene and tetramethyltetrathiafulvalene, respectively, and X is a complex inorganic anion) promotes spin-orbit-coupled electronic transitions between donor conduction band states with a consequent increase in the electron spin resonance (ESR) linewidth ( Delta H). This results from the breaking of the inversion symmetry that relates any pair of donors in the pure salt, which symmetry forbids spin-orbit-coupled transitions between donor states. The predicted increase in Delta H, which is largely due to inter-stack electronic transitions, is in reasonable agreement with that observed when (TMTSF)2PF6 is doped with 3% (TMTTF)2PF6, but agrees with results on TMTTF-doped (TMTSF)2ClO4 only at considerably lower dopant concentrations.