Electron-hole symmetry in a semiconducting carbon nanotube quantum dot

TL;DR

This study demonstrates near perfect electron-hole symmetry in a semiconducting carbon nanotube quantum dot, revealing insights into its energy spectrum and electron-electron interactions, and showing the nanotube can be free of charged impurities.

Contribution

It provides the first direct measurement of electron-hole symmetry in a semiconducting nanotube, highlighting the absence of charged impurities and strong electron-electron interactions.

Findings

Electron-hole symmetry observed in nanotube spectrum.

Small Zeeman spin splitting detected.

Strong electron-electron interactions indicated.

Abstract

Optical and electronic phenomena in solids arise from the behaviour of electrons and holes (unoccupied states in a filled electron sea). Electron-hole symmetry can often be invoked as a simplifying description, which states that electrons with energy above the Fermi sea behave the same as holes below the Fermi energy. In semiconductors, however, electron-hole symmetry is generally absent since the energy band structure of the conduction band differs from the valence band. Here we report on measurements of the discrete, quantized-energy spectrum of electrons and holes in a semiconducting carbon nanotube. Through a gate, an individual nanotube is filled controllably with a precise number of either electrons or holes, starting from one. The discrete excitation spectrum for a nanotube with N holes is strikingly similar to the corresponding spectrum for N electrons. This observation of near perfect electron-hole symmetry demonstrates for the first time that a semiconducting nanotube can be free of charged impurities, even in the limit of few-electrons or holes. We furthermore find an anomalously small Zeeman spin splitting and an excitation spectrum indicating strong electron-electron interactions.