Pumping of vibrational excitations in a Coulomb blockaded suspended carbon nanotube

TL;DR

This paper reports on the observation of vibrational excitations in a suspended carbon nanotube quantum dot within the Coulomb blockade regime, revealing non-equilibrium mechanical mode occupation driven by electronic transport.

Contribution

It demonstrates gate-dependent vibrational excitations in a Coulomb blockaded nanotube and models the interplay between electronic and vibrational states causing mode pumping.

Findings

Vibrational excitations occur in Coulomb blockade regime.

Non-equilibrium occupation of mechanical modes is driven by cotunnel-assisted tunneling.

Vibrational mode pumping is linked to electronic-vibrational interplay.

Abstract

Low-temperature transport spectroscopy measurements on a suspended few-hole carbon nanotube quantum dot are presented, showing a gate-dependent harmonic excitation spectrum which, strikingly, occurs in the Coulomb blockade regime. The quantized excitation energy corresponds to the scale expected for longitudinal vibrations of the nanotube. The electronic transport processes are identified as cotunnel-assisted sequential tunneling, resulting from non-equilibrium occupation of the mechanical mode. They appear only above a high-bias threshold at the scale of electronic nanotube excitations. We discuss models for the pumping process that explain the enhancement of the non-equilibrium occupation and show that it is connected to a subtle interplay between electronic and vibrational degrees of freedom.