Persistent Orbital Degeneracy in Carbon Nanotubes

TL;DR

This paper reports the observation of persistent orbital degeneracy in carbon nanotubes, revealing insights into electron reflection, shell filling, and low-energy excitations in quantum states.

Contribution

It demonstrates that electrons reflect without mode mixing at contacts, and uncovers new low-energy excitations in four-electron shells in nanotubes.

Findings

Electrons are reflected without mode mixing at contacts.

Two electrons in a degenerate shell form a triplet state at zero magnetic field.

Unexpected low-energy excitations occur at full four-electron shell filling.

Abstract

The quantum-mechanical orbitals in carbon nanotubes are doubly degenerate over a large number of states in the Coulomb blockade regime. We argue that this experimental observation indicates that electrons are reflected without mode mixing at the nanotube-metal contacts. Two electrons occupying a pair of degenerate orbitals (a ``shell'') are found to form a triplet state starting from zero magnetic field. Finally, we observe unexpected low-energy excitations at complete filling of a four-electron shell.