Coupling of Vibrational Modes to the Electron-Hole Continuum in Doped Single-Wall Carbon Nanotubes

Abstract

Electron-phonon coupling is at the heart of many physical phenomena like superconductivity, charge transport and the Raman effect (Ferrari2007). Previously, it has been shown that a coupling of vibrational modes to the low-energy electronic continuum absorption in graphene and carbon nanotubes results in transparency windows in the mid-infrared spectral region (Lapointe2012, Lapointe2017). The reduced absorption at the vibrational energies can be attributed to so-called Fano (anti-)resonances (Fano1961).Here, we present a systematic study of the infrared continuum absorption and such antiresonances in purely semiconducting, monochiral carbon nanotube (CNT) thin-films. We observe strong antiresonances for the Raman active D- and G-phonons, which acquire oscillator strength via coupling to the bright Drude-like intraband continuum in doped CNTs. Both the spectrum of the continuum absorption and the amplitude of the Fano resonances critically depend on the doping level. With increasing carrier concentration, we find a redshift of the Drude peak accompanied with a non-monotonic doping level dependence of the antiresonance intensity. Our observations show that carrier localization and delocalization as well as disorder induced by defects play a critical role for both charge transport and electron-phonon coupling as evidenced by the Fano resonances. We suggest that our findings also have implications for other low-dimensional systems with a low-energy electronic continuum absorption.C. Ferrari, Solid State Commun. 2007, 143, 47-57.Lapointe et al., Phys. Rev. Lett. 2012, 109, 097402.Lapointe et al., J. Phys. Chem. C 2017, 121, 9053-9062.Fano, Phys. Rev. 1961, 124, 1866-1878.