Spin relaxation and diffusion in quasi-two-dimensional organic metals: The bis(ethylenedithiolo)tetrathiafulvalene compounds β-(BEDT-TTF)2X (X=I3 and IBr2)

Abstract

The electron-spin-resonance (ESR) linewidth and line shape for the quasi-two-dimensional organic metals β-(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.