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Physics of Charge Transport in Carbon Nanotubes
Carbon nanotubes (CNTs) are ideal model systems to study fundamental aspects of charge transport
in low-dimensions. On the one hand, a long CNT is a quantum wire. On the other hand, a CNT may
also become a zero-dimensional object under proper circumstances. This is because we work in
practice with CNTs of finite length determined by the attached electric contacts. A physicist
would call such a zero-dimensional object a quantum dot, while a chemist would refer to it
simply as a molecule. From a physical point of view, a zero-dimensional object is characterized
by a discrete energy spectrum just like any atom or molecule.
Using state-of-the-art electron-beam lithography, a single carbon nanotube can electrically be
configured with source and drain contacts and with additional gate electrodes. The latter allow
the tuning of the conduction properties. Using nanotubes of different diameter or length, we can
engineer some properties of these “artificial” molecules already during the fabrication process.
Even more so, we can tune important electrical properties just by changing potentials to gate
electrodes. This makes CNTs an interesting object to explore the fundamentals of electronics in
organic systems. This is the reason why research on CNTs has received a lot of attention within
the NCCR project Molecular Electronics.
It is only with CNTs that new correlation phenomena in zero dimensions could be studied very
recently. In other quantum dot systems, transparent superconducting and ferromagnetic contacts
could not be fabricated until today. We have studied in great detail, the so-called proximity
effect in a quantum dot for the first time. The power of CNTs for the realization of various
hybrid-systems has recently also led to new insight into spin-transport in low dimension. A
profound understanding of spin and charge correlations in reduced dimensions is required for
the future of on-chip quantum computing and quantum information processing and shall be further
addressed in carbon nanotubes serving as the ideal molecular model system of choice.
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Figure 1: Illustrates that hybrid devices are possible with CNTs. Here, X and Y
may be a normal metal, a superconductor and/or a ferromagnet. This offers a large
range of new possibilities. |
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Figure 2: Calculation of the differential conductance dI/dV of a set of equally
spaced quantum dot levels which are coupled to superconducting source and drain
electrodes. Dark blue corresponds to a low dI/dV, whereas white and all the other
colours correspond to large values of dI/dV. This intruiging conductance pattern
is due to coherent Andreev processes mediated between the reservoirs by a single
level in the quantum dot. Unlike multiple Andreev reflection (MAR) in weak-links,
the positions of the conductance peaks strongly depend on the gate voltage, i.e. on
the exact level-position with respect to the Fermi energies in the leads. We have
observed this striking effect for the first time making use of carbon nanotubes as
quantum dots. |
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Figure 3: a) Illustration of a second order Andreev process. This particular one
gives in general rise to a differential conductance peak at a source-drain voltage of
Vsd=Δ/e. (b) Actual measurement of the differential conductance
dI/dVsd as a function of gate-voltagE Vg and source-drain voltage Vsd on
carbon nanotube quantum dot coupled to superconducting source and drain contacts. Black
corresponds to high and white to low conductance. (c) A calculation of the
dI/dVsd using the parameters derived from the experiment. The calculation
corresponds to the stripe in (b) which is shaded.
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Multiple Andreev Reflections in a Carbon Nanotube Quantum Dot
M. R. Buitelaar, W. Belzig, T. Nussbaumer, B. Babic, C. Bruder, and C. Schönenberger
Phys. Rev. Lett. 91, 057005
Quantum dot coupled to a normal and a superconducting lead
M. Gräber, T. Nussbaumer, W. Belzig and C. Schönenberger
Nanotechnology 15, S479 (2004)
Electrical spin injection in multiwall carbon nanotubes with transparent ferromagnetic contacts
S. Sahoo, T. Kontos, C. Schönenberger, C. Sürgers
Applied Physics Letters, 112109 (2005)
Contact:
Christian Schönenberger |
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Institute of Physics
University of Basel
Switzerland
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