Two-electron and four-electron periodicity in single-wall carbon nanotube quantum dots
Introduction
Carbon nanotubes are not only attractive for the building block of nanodevices [1], [2], but also suitable for an investigation of physics in low-dimensional systems [3], because of their extremely small diameter and quasi-one-dimensional structure. In this paper, we report experimental observations of two- and four-electron periodicity in Coulomb diamonds and Coulomb oscillations measured on single quantum dots formed in single-wall carbon nanotubes (SWNTs). The two-electron periodicity is arising from the spin degeneracy in a single quantum level, and is called the even–odd effect. Carbon nanotubes have an additional two-fold degeneracy called K–K′ subband degeneracy, in the unique band structure of the carbon nanotube. Combined with the spin degeneracy, the subband degeneracy results in the four-electron periodicity. Recently, these effects have been reported in various samples in different coupling regimes [4], [5], but the condition for the observation appears to be fully understood.
Section snippets
Experimental procedure
Single-wall carbon nanotubes (SWNTs), dispersed on a thermally oxidized Si wafer, were observed with an atomic force microscope (AFM), and SWNTs with their height smaller than 1 nm are selected to be contacted (inset in Fig. 1).The distance between the source–drain contacts was about 200 nm, and the height of the SWNT was about 0.8 nm. All the transport measurements, including the current (Id)–voltage (Vsd) curves for different gate voltages (Vg’s) and Coulomb oscillation measurements at the
Results and discussions
Fig. 1 shows Coulomb oscillations at some Vg range with Vsd changed from 1 mV(bottom) to 5 mV(top) at 1.5 K. As Vsd was increased, the current peak heights became larger and the widths became more broad. In addition, zero-dimensional (0D) shoulders were clearly observed in the current peaks. That is due to the stepwise increase of current channels when new quantum levels enter into the bias window set by Vsd. This fact confirms the transport in the quantum regime.
Fig. 2(a)shows Coulomb
Acknowledgements
We thank Dr. H. Maki, Dr. Y. Ishiwata, and Mr. D. Tsuya, all from RIKEN, for their experimental support.
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- 1
Also at: Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuda, Midori-ku, Yokohama 226-8503, Japan.
- 2
Also at: CREST, Japan Science and Technology (JST), Kawaguchi, Saitama 332-0012, Japan.