The ªmetallicº state of conducting polymers continues to be a topic of interest and controversy. [1] Although disorder is generally recognized to play an important role in the physics of ªmetallicº polymers, the length scale of the disorder and the nature of the metal±insulator (M-I) transition are the central unresolved issues. [1±3] In particular, the question of whether disorder is present over a wide range of length scales or whether the properties are dominated by more macroscopic inhomogeneities has been a subject of considerable discussion. In the former case, [2] the M-I transition would be described by conventional localization physics (e.g., the Anderson transition), while in the latter case, the M-I transition would be better described in terms of percolation between metallic islands. [3] Recent progress in the processing of conducting polymers has significantly improved the quality of the materials with corresponding improvements in the electrical conductivity. An example is polypyrrole doped with PF 6 , PPy-PF 6 . [4] Transport studies [5] demonstrated that the improved material is more highly conducting and more homogeneous than that studied earlier. As is typical of conducting polymers, PPy-PF 6 is partially crystalline. The structural coherence length, x, is, however, only »20±50 , less than any length used to characterize the electronic properties near the M-I transition, i.e., less than the inelastic scattering length (L in » 300 ) in the metallic regime, and less than the localization length (L c » 200±300 ) in the insulating regime. [2,5] The corresponding transport data in the critical regime and the crossover from metal to insulator have been successfully analyzed in terms of conventional disorderinduced localization. [5] In spite of the evidence for the disorder-induced M-I transition as inferred from the transport [5] and optical measurements, [6] the metallic state of PPy-PF 6 remains a subject of controversy. Kohlman et al. [7] reported infrared (IR) reflectance measurements, R(o), which they analyzed in terms of the frequency-(o-) dependent optical constants. They reported a zero-crossing in the dielectric function, e 1 (o), at o » 250 cm ±1 (well below the p-electron plasma frequency at 1.2 eV). At frequencies below the zero-crossing, they reported e 1 (o) becoming increasingly negative. This low-frequency zero-crossing is not consistent with a disordered metal near the M-I transition; Kohlman et al. attributed the zero-crossing to the plasma resonance of a low density of ªdelocalized carriersº with a long scattering time (t » 10 ±11 s). They concluded that metallic PPy-PF 6 is inhomogeneous, consisting of a composite of metallic islands (crystalline regions) embedded in an amorphous matrix and interpreted the M-I transition in terms of percolation between the metallic islands. The inference of a small fraction of carriers with long relaxation time was used to predict ultra-high conductivity polymers in which all the carriers were delocalized with similarly long scattering times. [7]...
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