On the Kondo effect in carbon nanotubes at half halfing

TL;DR

This paper investigates the Kondo effect in carbon nanotube quantum dots, focusing on the half-filled shell (N=2), analyzing ground states, degeneracy, and exchange interactions through transport measurements.

Contribution

It provides new insights into the ground state degeneracy and exchange energy in CNT quantum dots, especially at half-filling, with experimental measurements of level mismatch and exchange interactions.

Findings

Kondo effect observed at half-filled shell N=2.

Exchange energy J found to be surprisingly small (<0.02).

Gaps observed at N=3 and over extended levels.

Abstract

In a single state of a quantum dot the Kondo effect arises due to the spin-degeneracy, which is present if the dot is occupied with one electron (N = 1). The eigenstates of a carbon nanotube quantum dot possess an additional orbital degeneracy leading to a four-fold shell pattern. This additional degeneracy increases the possibility for the Kondo effect to appear. We revisit the Kondo problem in metallic carbon nanotubes by linear and non-linear transport measurement in this regime, in which the four-fold pattern is present. We have analyzed the ground state of CNTs, which were grown by chemical vapor deposition, at filling N = 1, N = 2, and N = 3. Of particular interest is the half-filled shell, i.e. N = 2. In this case, the ground state is either a paired electron state or a state for which the singlet and triplet states are effectively degenerate, allowing in the latter case for the appearance of the Kondo effect. We deduce numbers for the effective missmatch d of the levels from perfect degeneracy and the exchange energy J. While d ~ 0.1 - 0.2 (in units of level spacing) is in agreement with previous work, the exchange term is found to be surprisingly small: J < 0.02. In addition we report on the observation of gaps, which in one case is seen at N = 3 and in another is present over an extended sequence of levels.