Spin relaxation and diffusion in quasi-two-dimensional organic metals: The bis(ethylenedithiolo)tetrathiafulvalene compounds beta -(BEDT-TTF)2X (X=I

Abstract

The electron-spin-resonance (ESR) linewidth and line shape for the quasi-two-dimensional organic metals \ensuremath{\beta}-(BEDT-TTF${)}_{2}$X [BEDT-TTF is bis(ethylenedithiolo)tetrathiafulvalene, ${\mathrm{C}}_{10}$${\mathrm{H}}_{8}$${\mathrm{S}}_{8}$, and X is ${\mathrm{I}}_{3}$ or ${\mathrm{IBr}}_{2}$] are examined over the range from 4 to 300 K. The linewidth of both salts exhibits nearly linear temperature dependence consistent with metallic behavior. The spin-relaxation time ${T}_{2}$ derived from the linewidth of the ${\mathrm{IBr}}_{2}$ salt is always longer than that of the ${\mathrm{I}}_{3}$ salt, whereas the spin-diffusion times ${T}_{D}$ are very close to each other. It is found that in both salts, ${T}_{D}$ is comparable to ${T}_{2}$ below 20 K, while ${T}_{D}$ is much larger than ${T}_{2}$ above 100 K. From the analysis of the angular dependence of the g value and linewidth, it is found that the principal directions of the g tensor are slightly different from those of the linewidth tensor. This suggests that the ESR linewidth of these organic metals would not be simply dominated by Elliott's spin-orbit coupling mechanism.