ESR study of the temperature dependence of metallic complexes of alkali-metal-doped polyacetylene

In this work, we have followed the evolution of the electron-spin-resonance (ESR) linewidth ($\ensuremath{\Delta}{H}_{\mathrm{p}.\mathrm{p}.}$) of highly-alkali-metal-doped polyacetylene, as a function of the nature of the dopant and the temperature in the range 4-300 K. The room-temperature linewidth increases with increasing atomic number ($Z$) of the dopant, suggesting a significant contribution to the width from the spin-orbit coupling of the unpaired electrons on the dopant site. This linewidth behavior versus $Z$ follows a ${Z}^{\ensuremath{\alpha}}$ law, with $\ensuremath{\alpha}\ensuremath{\simeq}2.3\ifmmode\pm\else\textpm\fi{}0.7$. In the case of Li-, K-, and Rb-doped films, the linewidth decreases with decreasing temperature down to 100 K for Li-doped films and \ensuremath{\sim}50 K for K- and Rb-doped ones where it starts to increase. This temperature behavior can be analyzed in terms of gradual transition from a high-temperature metallic state where interchain electron hopping plays a dominant role, to a low-temperature semiconducting state where electrons are mainly confined along the chains.