Synthetic Metals, 41-43 (1991) 3717-3727 371.7 MICROWAVE ABSORPTION AT NEAR-ZERO FIELDS IN CONDUCTING POLYMERS A.A. ZAKHIDOV, I.I. KHAIRULLIN, and V.Y. SOKOLOV Department of Thermophysics Uzbek SSR Academy of Sciences Katartal 28, Tashkent 700135, U.S.S.R. R.H. BAUGHMAN, Z. IQBAL, and M. MAXFIELD Allied-Signal Inc., Research and Technology Morristown, NJ 07962, U.S.A. B.L. RAMAKRISHNA Department of Chemistry and Center for Solid State Science Arizona State University, Tempe, AZ 85287, U.S.A. ABSTRACT 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 pammagnetic 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. INTRODUCTION The characteristic intense microwave absorption in low magnetic field (H) is well known in superconducting (SC) phases of the new high-T0 ceramic cuprates and has been extensively studied [1-9]. This type of absorption is usually observed near H = 0 in ESR spectra and is referred to here as the superconductor low field signal, SC-LFS. The SC-LFS is thought to be due to H-field microwave absorption at Josephson vortices in weakly coupled superconducting grains [7-10] and fluxon motion on granular surfaces and interfaces [11]. SC-LFS is therefore associated with the superconducting phase and appears only at the critical temperature, To. Because of the high sensitivity of the SC-LFS, it can be employed to find traces of new superconducting phases [12]. Using this method, we observed a small superconducting phase fraction in the Bi-Sr-Ca-Cu-O system with a Tc of 116 K [12], the volume content of which was so small (0.01%) that it could not be detected by resistivity measurements or SQUID magnetometry [13]. Very recently, a bulk superconductor at 116 K has been reported to exist in the Pb-doped Bi-Sr-Ca-Cu-O system [14]. Elsevier Sequoia/Printed in The Netherlands