Tunable few-electron double quantum dots and Klein tunnelling in ultra-clean carbon nanotubes

Quantum dots defined in carbon nanotubes are a platform for both basic scientific studies and research into new device applications. In particular, they have unique properties that make them attractive for studying the coherent properties of single electron spins. To perform such experiments it is necessary to confine a single electron in a quantum dot with highly tunable barriers, but disorder has until now prevented tunable nanotube-based quantum-dot devices from reaching the single-electron regime. Here, we use local gate voltages applied to an ultra-clean suspended nanotube to confine a single electron in both a single quantum dot and, for the first time, in a tunable double quantum dot. This tunability is limited by a novel type of tunnelling that is analogous to that in the Klein paradox of relativistic quantum mechanics.Comment: 21 pages including supplementary informatio

Extraordinary carrier multiplication gated by a picosecond electric field pulse

The study of carrier multiplication has become an essential part of many-body physics and materials science as this multiplication directly affects nonlinear transport phenomena, and has a key role in designing efficient solar cells and electroluminescent emitters and highly sensitive photon detectors. Here we show that a 1-MVcm−1 electric field of a terahertz pulse, unlike a DC bias, can generate a substantial number of electron–hole pairs, forming excitons that emit near-infrared luminescence. The bright luminescence associated with carrier multiplication suggests that carriers coherently driven by a strong electric field can efficiently gain enough kinetic energy to induce a series of impact ionizations that can increase the number of carriers by about three orders of magnitude on the picosecond time scale

Electrically driven thermal light emission from individual single-walled carbon nanotubes

Light emission from nanostructures exhibits rich quantum effects and has broad applications. Single-walled carbon nanotubes (SWNTs) are one-dimensional (1D) metals or semiconductors, in which large number of electronic states in a narrow range of energies, known as van Hove singularities, can lead to strong spectral transitions. Photoluminescence and electroluminescence involving interband transitions and excitons have been observed in semiconducting SWNTs, but are not expected in metallic tubes due to non-radiative relaxations. Here, we show that in the negative differential conductance regime, a suspended quasi-metallic SWNT (QM-SWNT) emits light due to joule-heating, displaying strong peaks in the visible and infrared corresponding to interband transitions. This is a result of thermal light emission in 1D, in stark contrast with featureless blackbody-like emission observed in large bundles of SWNTs or multi-walled nanotubes. This allows for probing of the electronic temperature and non-equilibrium hot optical phonons in joule-heated QM-SWNTs