Electron-spin dynamics of polarons in lightly doped polypyrroles

This paper examines the spin dynamics of polarons in lightly doped polypyrrole (PPy), on the basis of the temperature dependence of spin-relaxation rates determined by a pulsed ESR technique. Several experiments using pulse equipment indicate that observed spins are averaged out over several levels and that one spin species is detected, while the susceptibility suggests the coexistence of Pauli and Curie types of spins. We consider that the discrepancy originates from a rapid motion of the spins distributing over possible levels. The spin-relaxation rates scarcely changed for a variety of dopants, but we observed remarkable decreases resulting from the deuteration of PPy in $T_1^{-1}$ as well as $T_2^{-1}.$ Two models, the quasi-one-dimensional (Q1D) diffusion model and the random diffusion model, indicate that observed spins diffuse quasi-three-dimensionally. We point out that the Q1D diffusion model is not necessarily desirable for such a less anisotropic motion. In contrast, the random diffusion model supposing an exponential type of correlation function can successfully interpret the decreases resulting from the deuteration in $T_1^{-1}$ and $T_2^{-1}$ as attributed to the difference in local fields (hyperfine field), independently of the temperature. We emphasize that the spin-correlation rate deduced from the latter model gives an estimate of the hopping rate of polarons in lightly doped PPy.