Torsional-oscillation-induced spin-orbit coupling and ESR line broadening in tetramethyltetraselenafulvalenium salts [(TMTSF)2X]

Spin-orbit-coupled electronic transitions between donor conduction-band states of the organic conductors (TMTSF${)}_2$X (TMTSF is tetramethyltetraselenafulvalene and X is a complex inorganic anion), which cause the electron-spin-resonance (ESR) line broadening in these compounds, are shown to be activated by torsional oscillations of the donor molecules about their long in-plane axes. These oscillations break the symmetry restriction forbidding spin-orbit transitions within or between donor stacks comprised of the same, identically oriented, centrosymmetric molecules. Both intra- stack and interstack transitions occur, the latter between donor stacks displaced one unit along the crystallographic b axis, and contribute to the ESR linewidth. The application of this model to the tetramethyltetrathiafulvalene analogues of these compounds, i.e., (TMTTF${)}_2$X, is also briefly discussed.