Evolution of SU(4) Transport Regimes in Carbon Nanotube Quantum Dots

TL;DR

This paper investigates how conductance regimes in carbon nanotube quantum dots evolve with contact transparency, revealing Kondo effects, mixed valence behavior, and a small energy scale influencing temperature-dependent conductance.

Contribution

It provides a comprehensive experimental study of conductance evolution in nanotube quantum dots and introduces a simple, parameter-free formula to describe the mixed valence regime.

Findings

Kondo behavior observed for 1-3 electrons in open contacts

Identification of a mixed valence regime with four-electron periodicity

Discovery of a small energy scale affecting conductance temperature dependence

Abstract

We study the evolution of conductance regimes in carbon nanotubes with doubly degenerate orbitals (``shells'') by controlling the contact transparency within the same sample. For sufficiently open contacts, Kondo behavior is observed for 1, 2, and 3 electrons in the topmost shell. As the contacts are opened more, the sample enters the ``mixed valence'' regime, where different charge states are strongly hybridized by electron tunneling. Here, the conductance as a function of gate voltage shows pronounced modulations with a period of four electrons, and all single-electron features are washed away at low temperature. We successfully describe this behavior by a simple formula with no fitting parameters. Finally, we find a surprisingly small energy scale that controls the temperature evolution of conductance and the tunneling density of states in the mixed valence regime.