Strong electromechanical coupling in ultra-short carbon nanotube quantum dots

We study electromechanical coupling in suspended single-wall carbon nanotubes using low-temperature electron transport. Using a feedback-controlled electromigration method $[1]$, we create gate-tuneable single quantum dots whose lengths range from tens of nm down to $\approx$ 3 nm. We observe current suppression of low bias stretching vibron sidebands due to the Franck-Condon blockade, and extract the electron-vibron coupling strength, $g$, both in the electron and hole doped regimes in the same devices. We observe strong $g$ and are exploring its dependence on mechanical strain in the tube. Due to a positive feedback mechanism between tunneling electrons and bending mode vibrations of the nanotubes, we observe bending mode frequencies up to the 100 GHz range $[2]$. The bending mode frequency is found to be tuneable by a factor of two by applying electrostatic strain. \\ $[1]$ J.O. Island \textit{et al}. Appl. Phys. Lett. \textbf{99}, 243106 (2011) \\ $[2]$ J.O. Island \textit{et al}. Nano. Lett. \textbf{12}, 4564 (2012)