Fano Lineshape and Phonon Softening in Single Isolated Metallic Carbon Nanotubes

Evolution of $G$-band modes of single metallic carbon nanotubes with the Fermi level shift is examined by simultaneous Raman and electron transport studies. Narrow Lorentzian line shape and upshifted frequencies are observed near the van Hove singularities. However, all $G$ modes soften and broaden at the band crossing point. The concurrent appearance of an asymmetric Fano line shape at this point indicates that phonon-continuum coupling is intrinsic to single metallic tubes. The apparent Lorentzian line shapes of as-synthesized metallic tubes are induced by ${\mathrm{O}}_2$ adsorption causing the Fermi level shift.