Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Dienstag
TT 17.3 Di 14:30 Poster A
Green’s functions method for modelling the interplay between
Coulomb blockade and Andreev reflection — •Ursula Schröter
— FB Physik, Universität Konstanz, Universitätsstraße 10, 78457 Konstanz
We want to investigate charge transport across a superconducting island
which is coupled to two superconducting reservoirs and whose potential
is adjustable via a gate. In single-electron-transistor structures
with weak coupling single electron tunneling is the dominant process
and Coulomb blockade occurs. For superconducting materials multiple
Andreev reflection (MAR) is also possible resulting in the transfer of
several elementary charges in one process. With transmissions for the
junctions no longer much smaller than one, as are found in mechanically
controllable break junctions [1], these higher order processes give subgap
contributions to the IV-characteristics. We regard two junctions in
series, at least one of which has a fairly high transmission. The question
is whether or not the MAR contribution to the current will be suppressed
because of Coulomb blockade. Based upon an algorithm for a single junction
[2] in a first ansatz we regard the two junctions as decoupled from
one another but for the charging state of the island. A second ansatz
shall include possible coherence over the entire structure. The two contacts
are treated as two ’perturbations’ by folding Green’s functions into
one another [3].
[1] E.Scheer et al., Nature 394, 154 (1998).
[2] J.C.Cuevas et al., Phys.Rev.B 54, 7366 (1996).
[3] U.Schröter, EPJ B 33, 297 (2003).
TT 17.4 Di 14:30 Poster A
Superconducting SETs in a Dissipative Environment: the JQP-
Cycle — •Konstanze Jähne and Frank Wilhelm — Lehrstuhl für
Theoretische Festkörperphysik, Sektion Physik der Universität München,
Theresienstr. 37, 80333 München
The properties of a mesoscopic device in the quantum limit are strongly
influenced by its electromagnetic environment, which destroys quantum
coherence and induces energy relaxation. We investigate the Josephsonquasiparticle
(JQP) process in a superconducting single-electron transistor
(SET), which is coupled to a controllable electromagnetic environment.
By using the P(E)-theory, we calculate transition rates for the
tunneling electrons and Cooper pairs and investigate the impact of the
environment on the transport through the device. This will be compared
with an experiment by A. Rimberg et al. The JQP process has an important
application in the read out of charge quantum bits.
TT 17.5 Di 14:30 Poster A
Preparation and Characterization of Magnetic Single Electron
Transistors — •Jürgen Schuler, Heribert Knoglinger, Josef
Höß, Christian Probst, Karl Neumaier, Achim Marx, and
Rudolf Gross — Walther-Meissner-Institut, Bayerische Akademie der
Wissenschaften, Walther-Meissner Straße 8, 85748 Garching
We have developed a process to fabricate ferromagnetic single electron
transistors (SET) based on the Al/Al2O3-tunnel junction technique and
the shadow evaporation technique. With this process we were able to fabricate
SETs with either the island or the leads made of nickel. The goal
of this work is to realize a device, which gives access to both controllable
single charging and magnetoresistive effects.
Apart from a short overview on the preparation and the measurement
setup developed in this work, we will show the characterization of the
devices in magnetic field up to 2 T in the normal conducting state. We
will compare several methods to determine the key parameters of the
device in the normal conducting state including numerical simulations in
the framework of the orthodox theory.
Based on our measurements in the superconducting regime at low magnetic
fields we will discuss the potential of these devices. Furthermore,
we discuss the abilities and the limits of the preparation techniques used
in this work.
TT 17.6 Di 14:30 Poster A
Coulomb blockade in benzol-dithiol: DFT calculations — •M.
Koentopp, F. Evers, and F. Weigend — Institut für Nanotechnologie,
Forschungszentrum Karlsruhe, 76021 Karlsruhe, Germany
For the development of future molecular-electronics devices an important
prerequisite is the ability to realize molecular switches, i.e. changing
the conductance properties of a molecule in a controlled fashion. A major
recent achievement has been the observation of Coulomb blockade in
single organic molecules by applying a gate voltage (S. Kubatkin et al.,
Nature 425, 698-701 (2003)).
Here, we report theoretical calculations on the Coulomb blockade for a
paradigmatic system, namely the molecule 1,4 benzol-dithiol. We use the
nonequilibrium Green’s function formalism in combination with density
functional calculations with the program package TURBOMOLE. The
number of atoms in the simulated electrodes is large enough to ensure
that Fermi level mismatch does not occur. The gate is realized by point
charges distributed homogenously on a square grid in a plane parallel to
the C-atoms. Charging effects are included in a manner that in principle
is exact. Screening of the gate by the electrodes (charge neutrality) is
thereby taken fully into account.
We observe the expected shift of the transmission resonances with increasing
gate voltage. These shifting levels lead to spikes in the transmission
when they cross the Fermi energy.
TT 17.7 Di 14:30 Poster A
Computersimulations of electronic charge transport in
nanowires — •Markus Dreher and Peter Nielaba — Lehrstuhl
für Theoretische Physik, Fachbereich Physik, Universität Konstanz,
D-78457 Konstanz, collaboration with Jan Heurich and Carlos Cuevas
(Universität Karlsruhe)
With a molecular dynamic simulation we calculated the structure of
gold nanocontacts which were pulled apart [1]. In order to prevent heating
up the system during the stretching process we established a Nosé-Hoover
thermostat. The gold atoms are interacting via an effective-mediumtheory-potential
incorporating many body effects [2]. The formation of
single atom chains was observed.
In cooperation with Heurich and Cuevas, who developed -by using a
tight-binding-model and Green function techniques- a program for the
current through a nanocontact, conductivity curves (in different channels)
have been determined and are in good agreement with the experimental
data [1].
The conductance is not totally determined by the atoms of the constriction,
but also by atom rearrangements in the vicinity of the constriction.
[1] M. Dreher, Diplomarbeit, Konstanz, 2002
[2] K. Jacobsen et al., A semi-empirical effective medium theory for metals
and alloys, Surface Science, 366, 394 (1996)
TT 17.8 Di 14:30 Poster A
Transport properties of single-atom contacts of zinc — •P.
Konrad 1 , P. Brenner 2 , A. Mayer-Gindner 2 , H. v. Löhneysen 2 ,
and E. Scheer 1 — 1 Physics Department, University of Konstanz —
2 Institute of Physics, Physics Department, University of Karlsruhe
We report low-temperature measurements of the electrical transport
properties of single-atom zinc contacts fabricated by different variations
of the mechanically-controllable breakjunction technique, including the
notched-wire method [1], the lithographic method [2], and using whiskers.
From investigations of the statistical behavior in the normalconducting
state as well as from the analysis of individual contacts in the superconducting
state we find indications for the presence of preferred atomic
configurations. In particular there exist contacts with a single, well transmitted
transport channel.
In lithographic breakjunctions made of thin films we find nonlinearities
in the current-voltage characteristics that we attribute to Coulomb
blockade due to the granular structure of the film. These latter findings
are in qualitative agreement with a recent prediction by Avishai et al.
[3].
[1] C. Muller, J. M. van Ruitenbeek and J. L. de Jongh, Physica C191,
485 (1992)
[2] J. M. van Ruitenbeek et al., Rev. Sci. Instrum. 67, 108 (1996)
[3] Y. Avishai, A. Golub and A. D. Zaikin, Europhys. Lett. 54, 640 (2001)
TT 17.9 Di 14:30 Poster A
Wechselwirkungseffekte in kleinen Quantenpunkten —
•Stephan Weiß, Boris Reusch und Reinhold Egger — Institut
für Theoretische Physik IV, Heinrich-Heine-Universität Düsseldorf,
40225 Düsseldorf
Wir untersuchen Wechelwirkungseffekte in kleinen Quantenpunkten
(ne ≤ 10) mit Hilfe der Pfad-Integral-Monte-Carlo-Methode (PIMC).
Das Verhalten der Elektronen im Quantenpunkt wird wesentlich
durch Kopplungsparameter, z.B. den Brücknerparameters rs, sowie
die Spin-Bahn-Wechselwirkung αr, bestimmt. Bei festem Gesamtspin
des Systems, ist es möglich den Übergang von Fermi-Flüssigkeit zur
Wigner-Kristallisation zu verstehen [1][2]. Spin-Bahn-Effekte werden
durch Berücksichtigung des Rashba-Kopplungsterms beschrieben [3].