Muon spin relaxation as a probe of electron motion in conducting polymers

The use of implanted muons to probe the dynamics of electronic excitations in conducting polymers is reviewed. Early work on polyacetylene showed evidence for mobile solitons performing one-dimensional diffusion in the trans isomer and localized spins in the cis isomer. Subsequent muon studies on a range of conducting polymers have shown evidence for mobile polaronic excitations and microscopic transport properties for these polarons have been derived from the measurements. A theoretical framework was developed by Risch and Kehr to describe the intermittent hyperfine coupling between a static muon and an electron diffusing randomly through a chain of sites. This theory predicts a specific form for both the muon spin relaxation function and the field dependence of the relaxation rate. The experimental data are found to be described well by this model. Intrachain diffusion rates can be extracted from the data; in several cases an interchain diffusion rate can also be measured. The anisotropy of diffusion rates can be as high as 10⁴ at low temperatures, reducing typically to 10² or less at room temperature. The importance of molecular vibrational modes in controlling the electronic motion in the polymer has been shown.