Franck-Condon blockade in suspended carbon nanotube quantum dots

Understanding the influence of vibrational motion of the atoms on electronic transitions in molecules constitutes a cornerstone of quantum physics, as epitomized by the Franck-Condon principle(1,2) of spectroscopy. Recent advances in building molecular-electronics devices(3) and nanoelectromechanical systems(4) open a new arena for studying the interaction between mechanical and electronic degrees of freedom in transport at the single-molecule level. The tunnelling of electrons through molecules or suspended quantum dots(5,6) has been shown to excite vibrational modes, or vibrons(6-9). Beyond this effect, theory predicts that strong electron-vibron coupling strongly suppresses the current flow at low biases, a collective behaviour known as Franck-Condon blockade(10,11). Here, we show measurements on quantum dots formed in suspended single-wall carbon nanotubes revealing a remarkably large electron-vibron coupling that, owing to the high quality and unprecedented tunability of our samples, allow a quantitative analysis of vibron-mediated electronic transport in the regime of strong electron-vibron coupling. This enables us to unambiguously demonstrate the Franck-Condon blockade in a suspended nanostructure. The large observed electron-vibron coupling could ultimately be a key ingredient for the detection of quantized mechanical motion(12,13). It also emphasizes the unique potential for nanoelectromechanical device applications based on suspended graphene sheets and carbon nanotubes.