Microwave absorption at near-zero fields in conducting polymers

A low-field signal (LFS) of non-resonant microwave absorption is detected in nondegenerate ground state conducting polymers at relatively high temperatures and the intensity of this signal increases by three orders of magnitude on cooling to 4 K. Light n-doping of poly(p-phenylene) with sodium increases the polaron concentration and both the peak-to-peak intensity of the LFS and its onset temperature. However, in heavily n-doped or p-doped poly(p-phenylene), as well as in heat-treated ones, the LFS is strongly suppressed. Though the form of the LFS in conducting polymers is quite similar to the now well-known LFS observed in superconducting phases, its behavior as a function of magnetic field, temperature, and microwave power is different. Especially, the absence of both fine structure and hysteresis upon magnetization indicates a non-superconducting origin for the observed LFS in conducting polymers. This LFS might result from the increase of microwave absorption in low magnetic fields caused by a negative a.c. magnetoresistance due to a spin selective hopping process in a pair of two paramagnetic polarons, whose rate is reduced by field-dependent singlet-to-triplet transformation via hyperfine interaction. Spin flip on chain sites or during the hopping event provide alternate explanations for the magnetoresistance.