Conduction-electron-spin resonance in organic conductors:αandβphases of di[bis(ethylenedithiolo)tetrathiafulvalene]triiodide [(BEDT-TTF)2I3]

The electron-spin resonance g value, linewidth, line shape, and spin susceptibility of single crystals of the α and β phases of di[bis(ethylenedithiolo)tetrathiafulvalene]triiodide [(BEDT-TTF${)}_2$ ${\mathrm{I}}_3$] are each found to show marked temperature and angular dependences in the temperature range from 5 to 300 K. In both phases, a Dysonian line-shape characteristic of conduction-electron-spin resonance is observed at room temperature when the microwave electric field is applied parallel to the quasi-two-dimensional conducting plane of the crystal. In the superconducting phase of β-(BEDT-TTF${)}_2$ ${\mathrm{I}}_3$ with appropriate crystal size, it is found that when the microwave electric field is applied perpendicular to the plane a Lorentzian line shape observed at room temperature is gradually converted to a Dysonian line shape with decreasing temperature. These phenomena are discussed in comparison with the transport parameters derived from the reflectance spectra and the conductivity measurements. The spin diffusion constants in the α and β phases are estimated to be about 0.2 and 0.3 ${\mathrm{cm}}^2$ ${\mathrm{s}}^{\mathrm{-}1}$, respectively. These constants are in agreement with the carrier diffusion constants evaluated independently by the analysis of the reflectance spectra. For the α phase, it is suggested from the temperature dependences of the g value and linewidth that there is a strong interaction between the triiodide and BEDT-TTF ions and the magnitude of the interaction changes suddenly with the metal-insulator transition.