NMR evidence for the metallic nature of highly conducting polyaniline

Polyaniline doped with camphor sulphonic acid (PANI-CSA) has been shown to yield a material that, after casting from solution in meta-cresol, exhibits a temperature-independent magnetic susceptibility [Y. Cao, P. Smith, and A. J. Heeger, Synth. Met. 48, 91 (1992); N. S. Sariciftici, A. J. Heeger, and Y. Cao, Phys. Rev. B 49, 5988 (1994)]. We report recent ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ NMR experiments on uniformly ${}_{}{}^{13}{}_{}{}^{}\mathrm{enriched}$ PANI-CSA in which the ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ spin-lattice relaxation rates are shown to obey a modified Korringa relation for relaxation via the hyperfine coupling to conduction electrons. This observation of Korringa relaxation in polyaniline provides strong evidence for a metallic state in this material. An estimate is made of the Korringa enhancement factor that provides a measure of the degree of electron-electron correlations present. Two-dimensional spin-exchange experiments are also reported, which show that the ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ NMR signal results from a heterogeneity in the sample over at least a 30-Å distance scale. These results are discussed in terms of the spatial extent of the doping-induced defect.