Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Donnerstag
TT 27 Nanoelektronik III: Molekulare Elektronik
Zeit: Donnerstag 09:30–12:45 Raum: H19
Hauptvortrag TT 27.1 Do 09:30 H19
Molecular wires in electromagnetic fields — •Sigmund Kohler 1 ,
Jörg Lehmann 1 , Sébastien Camalet 1,2 , and Peter Hänggi 1
— 1 Institut für Physik, Universität Augsburg, 86135 Augsburg —
2 Laboratoire de Physique, ENS Lyon, Frankreich
Electromagnetic ac fields can alter significantly the transport properties
of mesoscopic systems like molecular wires. Resonant excitations
of electrons can e.g. enhance drastically the currents through molecules.
The opposite phenomenon also exists: a proper off-resonant driving field
reduces the coherent transport across the molecule resulting in a strong
current suppression. Moreover, near such current suppressions, we find
characteristic minima and maxima of the shot noise [1]. This effect allows
to manipulate current fluctuations by external fields. Molecular wires in
laser fields may also be used to study the so-called ratchet effect: in asymmetric
molecules, an ac field induces a dc current even in the absence of
any bias voltage [2].
The external field is modelled by a periodic time-dependence of the
Hamiltonian. This requires a generalization of established transport theories
like, e.g., the Landauer formula. Such a generalization, which is
based on the Floquet theorem, will be presented and the main differences
to the static situation will be discussed.
[1] S. Camalet et al., Phys. Rev. Lett. 90, 210602 (2003).
[2] J. Lehmann et al., Phys. Rev. Lett. 88, 228305 (2002).
TT 27.2 Do 10:00 H19
Electronic transport through single molecules — •H. v.
Löhneysen 1,2 , F. Hennrich 3 , M.M. Kappes 3 , R. Krupke 3 ,
M. Mayor 3 , J. Reichert 3 und H.B. Weber 3 — 1 Physikalisches
Institut, Universität Karlsruhe, 76128 Karlsruhe — 2 Forschungszentrum
Karlsruhe, Institut für Festkörperphysik, 76344 Karlsruhe —
3 Forschungszentrum Karlsruhe, Institut für Nanotechnologie, 76344
Karlsruhe
Electronic transport measurements through single π-conjugated molecules
can be realized using mechanically controlled break junctions to
couple thiol end groups of the molecules to two gold electrodes. We have
investigated transport through π-conjugated molecules which differ
by their spatial symmetry and π-conjugated connectivity. The current
voltage characteristics (IV s) of the metal-molecule-metal system reflect
the spatial symmetry and topology of the molecules with respect to the
direction of current flow indicating that transport occurs indeed through
single molecules [1]. Fluctuations in the IV s are a manifestation of the
variation of level spacings of the system, which depend crucially on the
bonding between thiol end groups and Au electrodes.
For the future electronics, carbon nanotubes are the prime candidates.
Recent progress in the controlled deposition of nanotubes between
electrodes and separation of metallic and semiconducting nanotubes [2]
is reported.
[1] J. Reichert et al., Phys. Rev. Lett. 88, 176804 (2002)
[2] R. Krupke et al., Science 301, 344 (2003)
TT 27.3 Do 10:15 H19
Electronic Transport through C60 — •Tobias Böhler, Jochen
Gebing, and Elke Scheer — FB Physik - Universität Konstanz
We present an experiment to measure the differential conductance of a
single or few C60 molecules embedded in between the single atom tips of a
mechanically controllable break junction (MCB). The C60 is evaporated
onto an opened break junction under high vacuum conditions. Then I-
V-curves are taken at room temperature. The tip electrodes of the MCB
are fabricated of aluminium or gold.
The I-V-curves are linear for voltages |U| ≤ 0.2V and show slowly
increasing conductance for higher voltages with time dependent fluctuations.
After the evaporation of C60 the fluctuations are suppressed, but
the nonlinearities of the I-V-curves remain unaffected. Thus, it appears
that the C60 molecules stabilize the gold contact across which a maximum
voltage of 1V can be applied.
Another method is the fabrication of a MCB made of C60 without
metallic electrodes by evaporating the molecules onto a lithographic
mask. The conductance of the junction is much smaller (≈ 150MΩ) with
similar nonlinear I-V-curves for larger voltages.
TT 27.4 Do 10:30 H19
Electron tunneling through a single Co-complex — •Maarten
Wegewijs 1 , Christian Romeike 1 , Wolfgang Wenzel 2 und Herbert
Schoeller 1 — 1 Institut für Theoretische Physik - Lehrstuhl A ,
RWTH Aachen , 52056 Aachen , Germany — 2 Forschungszentrum Karlsruhe
, Institut für Nanotechnologie , 76021 Karlsruhe , Germany
We theoretically consider electron tunneling through a transition-metal
complex coupled to metallic electrodes. A single Co 2+/3+ ion is fixed between
a left and right terpyridine ligand, each of which is in turn coupled
to a linker molecule with a functionalized end group to connect to one of
the electrodes. It was found experimentally [1] that by varying the type
of linker molecules, a strong Kondo effect in the current appears at low
temperatures in Coulomb blocked transport regimes due to a stronger
coupling to the electrodes.
We have investigated the conjecture that the tunneling is dominated
by single atom-like d-orbitals of the ion by electronic structure calculations.
We show that microscopically the ligands of this particular complex
function as a tunnel barrier due to the different character (symmetry)
of the highest ligand (π) and two Co 2+/3+ -ion orbitals (σ). The role of
the linker molecules connecting the ligands to the electrodes is therefore
crucial. We discuss the simplest phenomenological tunneling model
consistent with our findings (molecular states, addition energy, tunneling
matrix elements).
[1] J. Park et al, Nature 417, 722 (2002)
TT 27.5 Do 10:45 H19
Shot noise in tunneling transport through molecules and quantum
dots — •Axel Thielmann 1 , Matthias H. Hettler 1 , Jürgen
König 2 , and Gerd Schön 1,3 — 1 Forschungszentrum Karlsruhe, Institut
für Nanotechnologie, 76021 Karlsruhe, Germany — 2 Institut für Theoretische
Physik III, Ruhr-Universität Bochum, 44780 Bochum, Germany
— 3 Institut für Theoretische Festkörperphysik , Universtität Karlsruhe,
76128 Karlsruhe, Germany
We consider charge transport through single molecules coupled weakly
to metal electrodes via tunneling barriers. The molecule is represented
by the Anderson impurity model which is the simplest model that includes
Coulomb interactions U. We calculate the current and current
noise within a first-order perturbation expansion in the coupling strength
using a diagrammatic technique. Analytical results for current, noise and
the Fano factor, F,(depending on the ratio of the couplings to the left
and right electrodes) are presented and discussed for all transport regimes
involving a single interacting (molecular) level (Phys. Rev. B 68, 115105
(2003)). Furthermore an extended model with two levels will be considered,
where the effects of asymmetric coupling and photon-relaxation can
be studied. Negative differential conductance (NDC) behavior leading to
super-poissonian noise (F > 1) can be observed in the case of asymmetric
coupling to the leads and is distroyed by strong relaxation rates. In
addition to the numerical evaluation analytical expressions for several
transport regimes will be presented.
11:00 Pause
TT 27.6 Do 11:15 H19
Vibrational Effects in the Conductance through a Molecular
Bridge — •Michael Hartung, Klaus Richter, and Gianaurelio
Cuniberti — Institute for Theoretical Physics, University of Regensburg,
93040 Regensburg , Germany
We study the conductance of a single molecule sandwiched between
two electrodes and taking into account the center of mass motion of the
molecule.
The starting point is a tight-binding model Hamiltonian, which includes
a linear coupling between the electronic degrees of freedom and
the bosonic motion. The conductance is calculated within the scattering
matrix formalism. The coupling to the electronic degrees of freedom
shows the interesting effect such as enhancing the off resonant conductance.
TT 27.7 Do 11:30 H19
Role of vibrational modes in the transport through single
molecules — •Juan Carlos Cuevas, Fabian Pauly, and Gerd
Schön — Institut für Theoretische Festkörperphysik, Universität Karlsruhe,
D-76128 Karlsruhe