Tunneling in suspended carbon nanotubes assisted by longitudinal phonons

TL;DR

This paper investigates phonon-assisted tunneling in suspended carbon nanotube quantum dots, revealing strong electron-phonon coupling and harmonic phonon modes through experimental measurements and a Franck-Condon model.

Contribution

It demonstrates the role of longitudinal phonons in tunneling processes and introduces a Franck-Condon-based model to describe phonon-assisted tunneling in nanotubes.

Findings

Observation of equally spaced steps in I-V characteristics.

Identification of longitudinal phonon modes as the excitation spectrum.

Evidence of strong electron-phonon coupling with a factor of order unity.

Abstract

Current-voltage characteristics of suspended single-wall carbon nanotube quantum dots show a series of steps equally spaced in voltage. The energy scale of this harmonic, low-energy excitation spectrum is consistent with that of the longitudinal low-k phonon mode (stretching mode) in the nanotube. Agreement is found with a Franck-Condon-based model in which the phonon-assisted tunneling process is modeled as a coupling of electronic levels to underdamped quantum harmonic oscillators. Comparison with this model indicates a rather strong electron-phonon coupling factor of order unity.