Doping-induced variation of electron spin relaxation behavior in polypyrroles

Electron spin relaxation behaviors in polypyrrole (PPy) are examined using a pulsed electron spin resonance spectroscopy (ESR) technique. We have observed remarkable changes in T1−1 as well as T2−1 induced from the doping level variation for three types of PPy. T2−1 particularly exhibits a transition in the temperature dependence with the enhancement in doping levels from a motional narrowing effect into the Elliott mechanism by way of its intermediate state. Thus, the spin relaxation behaviors are interpreted in terms of the superstition of the above two mechanisms. Such a consideration enables us to estimate two parameters, the correlation rates τc−1 and the spin relaxation rate due to the Elliott mechanism Te−1. The former gives a measure of the hopping rate for a quasi-three-dimensional motion over several metallic regions, and the latter accounts for the spin relaxation process of the spins residing in the metallic crystalline region. Our treatment demonstrates that the spin relaxation behavior in PPy changes with the dopant levels in relation to several physical properties such as the ratio of the Pauli susceptibility to the total one, the metallic behavior in the conductivity, and the polaron band observed in the optical spectrum.