Quantum transport in AuCl 3 doped polyacetylene studied by ESR $ A. Bartl a,* , L. Dunsch a , Y.W. Park b , E.S. Choi b , D.S. Suh b a Institut fu Èr Festko Èrper- und Werkstofforschung, Helmoltzstr. 20, D-01069 Dresden, Germany b Department of Physics, Seoul National University, Seoul 151-742, South Korea Abstract Starting with a review of general principles of transport processes in polyacetylene (PA) which can be studied by electron spin resonance (ESR) new results on highly doped PA are presented. The AuCl 3 doped PA samples stored under vacuum at room temperature in ESR sample tubes reach a steady state in the temperature dependent intensity and linewidth behavior after three to four month storage. The ESR spectra show two lines with an equal g-factor. In particular, the linewidth of the small line demonstrates a remarkable property. The temperature dependence of the linewidth shows a maximum of DB  0:30 mT at 30 K, and it is constant at DB  0:17 mT above this temperature in the whole temperature region between 70 and 300 K. The ESR data are discussed in a two spin model of delocalized states at the PA chain and ®xed spins near the dopants. Broadening and motional narrowing in¯uence the ESR linewidth. The interpretation points to the metallic behavior of the highly doped PA at low temperatures. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Polyacetylene; Electron paramagnetic resonance spectroscopy; Doping; Metallic behavior 1. Introduction Heavily doped polyacetylene (PA) is known to have unique metallic properties, but the conduction mechanism is different from ordinary metals. Dynamical change of electronic states occurs upon doping, which is fundamen- tally due to the existence of dopants interacting with con- jugated polymer chains [1]. From the ®rst discovery of the insulator-metal transition by doping metallic PA has been widely investigated experimentally and theoretically [2]. However, it is dif®cult to understand the intrinsic properties of the metallic PA due to the complex morphology of the doped material. Its metallic properties, such as the tempera- ture independent magnetic susceptibility, the high electrical conductivity and the linear temperature dependent thermo- electric power, were widely studied [3±5]. One of the remarkable features in polyacetylene is the spin-charge inversion of conjugation defects: neutral defects carry a spin and charged defects are spinless [6]. The magnetic moment of the neutral defect (neutral soliton) can be seen in the ESR signal and in the static susceptibility as a paramagnetic contribution originated in unpaired electrons. In the metallic state when there is a ®nite density of states at the Fermi level and the Peierls transition is suppressed, the delocalized electrons will lead to a Pauli paramagnetism which contributes to the static suscept- ibility too. In the last years there has been a considerable progress in understanding the unusual properties of this material. Evi- dence for the motion of the solitons can be obtained from the ESR spectra. The linewidth of the ESR spectrum is generally de®ned as the distance DHpp between both peaks of the derivative ESR spectrum. Many different effects can con- tribute to the ®nite linewidth:  unresolved hyperfine splitting combined with motional narrowing;  interactions between unpaired electrons of different nature: conduction, fixed and mobile electrons;  exchange narrowing. In this way, the ESR is able to study energy and mass transport processes in PA by using neutral solitons or other paramagnetic species as probes. During the thermal isomeri- zation while cis-PA is transformed to trans-PA, these paramagnetic defects are created. The properties of neutral solitons were studied by ESR of undoped PA focused on the temperature dependence of the ESR linewidth. By these studies two narrowing mechanisms were found: a tempera- ture dependent and a temperature independent mechanism. In the last years a consistent picture of the neutral soliton dynamics was installed and by using different magnetic Synthetic Metals 117 (2001) 21±25 $ Dedicated to the Nobel Laureates Alan Heeger, Alan Mac Diarmid and Hideki Shirakawa. * Corresponding author. 0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0379-6779(00)00533-6