Plenarvorträge - DPG-Tagungen

Halbleiterphysik Donnerstag
barrier. We observe definite peaks in the current and a modulations of α
as a function of the bias voltage.
HL 44.29 Do 16:30 Poster A
Electron Spin Decay Rates in single and coupled Quantum Dots
— •Johannes Voss and Daniela Pfannkuche — I. Institut für
Theoretische Physik, Universität Hamburg, Jungiusstr. 9, 20355 Hamburg,
Germany
Quantum systems with discrete spectra have become a field of both
experimental[1] and theoretical[2] research of particular interest for quantum
computing and electronics. Especially two level systems, e.g. the spin
of a single electron in a magnetic field, are suitable to represent qubits.
In this contribution we present numerical estimates for electron spin
decay rates in few electron quantum dots caused by hyperfine interaction
with the nuclear spins. For the calculations single quantum dots and laterally
coupled systems were considered. We investigate how the rates are
influenced by external magnetic fields and different forms of confining
potentials.
[1] R. Hanson et al., Phys. Rev. Lett. 91, 196802 (2003)
[2] A. Khaetskii, D. Loss, and L. Glazman, Phys. Rev. B 67, 195329
(2003)
HL 44.30 Do 16:30 Poster A
Strong magneto-resistance in double quantum dots due to
charge polarization — •Bernhard Wunsch 1 , David Jacob 2 , and
Daniela Pfannkuche 1 — 1 I. Institute of Theoretical Physics, University
of Hamburg, Jungiusstr. 9, D–20355 Hamburg, Germany —
2 Department of Applied Physics, University of Alicante, Cra San Vicente,
03690 San Vicente del Raspeig (Alicante), Spain
We report on magnetic field induced ground state crossings for vertically
coupled double quantum dots between states with same angular
momentum and spin but different isospin parity. In contrast to the
well known crossings between states of different angular momentum [1]
a crossing between states that differ only in parity can lead to a vertical
charge polarization.
We show that the polarization of the three electron ground state can be
detected in a transport experiment through the double quantum dot[2].
At the magnetic field, where originally the crossing occurred, we find a
strong suppression of the Coulomb peak associated with a transition between
two and three electrons in the double dot, due to the polarization
of the three electron ground state. Finally we want to point out, that for
a low electric field the mixing of the crossing states is very sensitive to
the magnetic field, which may prove to be useful for a realization of a
charge qubit in double quantum dots.
[1] H.Imamura and P. A.Maksysm and H.Aoki, Phys.Rev.B.59, 5817
(1999)
[2] S. Amaha and D. G. Austing and Y. Tokura and K. Muraki and
K. Ono and S.Tarucha, Solid State Commun. 119, 183 (2001)
HL 44.31 Do 16:30 Poster A
Thermoelectrical properties of single quantum dots — •Ralf
Scheibner, Hartmut Buhmann, and Laurens W. Molenkamp —
Experimentelle Physik III, Universität Würzburg
We analyze the thermoelectrical properties of lateral single quantum
dots in the coulomb blockade regime. For measuring the thermopower it
is necessary to apply a temperature gradient across the device. This is
achieved by an electron gas heating technique. The ability to tune the
temperature of the whole system and the coupling of the quantum dot
to the leads allows us to investigate the sequential, the cotunneling as
well as the Kondo regime. Temperature dependent measurements show
a crossover between the sequential and the cotunneling regime that we
compare with the calculations from [1]. In the Kondo regime the comparison
of conductance and thermopower measurements shows deviations
from Mott’s law suggesting a possible contribution of the electronic spin
entropy to the total entropy current that is the likely source of enhanced
thermopower in Co oxides [2].
[1] M.Turek and K.A. Matveev, ArXiv: cond-mat/ 0112445.
[2] Y. Wang, et.al., Nature, 423, 425, (2003).
HL 44.32 Do 16:30 Poster A
Classical and quantum-mechanical behavior of the magnetoresistance
in open quantum dots — •Roland Brunner 1 ,
Ronald Meisels 1 , Friedemar Kuchar 1 , Mohamed Elhassan 2 ,
Jon Bird 2 , and Koji Ishibashi 3 — 1 Department of Physics, University
of Leoben, Austria — 2 Department of Elecctrical Engineering, ASU,
USA — 3 Semiconductor Laboratory, Saitama, Riken, Japan
We report on the investigation of ballistic magneto-transport in open
AlGaAs/GaAs dots and its response to microwaves in the frequency range
around 50 GHz. The results allow to distinguish between backscattering
peaks and Shubnikov-de-Haas oscillations in the transition range from
classical to quantum-mechanical behaviour. /par A particularly interesting
aspect of the physics of quantum dots regards the crossover from the
classical to the quantum-mechanical regime in open dots. In this contribution
we focus on the behaviour exhibited by open quantum dots in low
and medium magnetic fields. Open quantum dots are strongly coupled
to the 2DEG reservoirs via short constrictions. /par The experiments
were carried out in an array of three dots which were defined by contiguous
finger gate structures on top of a two dimensional electron system
in AlGaAs/GaAs using electron beam lithography. The connections to
neighbouring dots and to the 2DEG reservoirs are made by constrictions
(lithographic width 70 nm). The electronic dot diameter was determined
to be 185 nm. The experimental results are discussed with respect to
quasi-classical and quantum-mechanical calculations. Work supported by
FWF, project 15513, Austria.
HL 44.33 Do 16:30 Poster A
Fluorescence and Scattering Properties of Composites made
from Gold and Semicoductor Nanoparticles — •Melanie M.
de Souza, Eric Dulkeith, Gunar Raschke, Andrey L. Rogach,
Thomas A. Klar, and Jochen Feldmann — Photonics
and Optoelectronics Group, Department of Physics and CeNS, Ludwig-
Maximilians -Universität, Amalienstr. 54 D-80799 Munich, Germany
In the last decade there has been considerable interest in the field of
nanostructured materials. Intense research has centred on controlling the
surface properties of both metal and semiconductor nanoparticles (NPs)
to determine the surface/property relationships. Here we report on a
composite system comprising of stable colliods of both metal (Au) and
semiconductor (CdTe) NPs in solutions. In this composite system we observe
both efficient quenching of CdTe NP fluorescence and a change in
the scattering spectra of Au NPs. By creating well-defined spacer units
between nanoparticles (through the self-assembly of polyelectrolyte films
or the formation of a covalent bond) the distance-dependent fluorescence
of CdTe NPs is also investigated.
HL 44.34 Do 16:30 Poster A
Experimental indications for coherent coupling of two quantum
dots — •Gerhard Ortner 1 , M. Bayer 1 , A.V. Larionov 1 , I. Yugova
1 , G. Baldassarri H.v.H. 1 , P. Hawrylak 2 , S. Fafard 2 , and
Z. Wasilewski 2 — 1 Experimentelle Physik II, Otto-Hahn-Str. 4, 44221
Dortmund, Germany — 2 Institute of Microstructual Sciences, National
Research Council, Ottawa, K1AOR6, Canada
Semiconductor Quantum Dots (QDs) have attracted considerable interest
recently because of its potential for possible applications in quantum
information processing. For this purpose a quantum gate which entangles
the states of two quantum bits as the essential block of a quantum
processor is needed.
A pair of vertically aligned QDs has been suggested as a promising
candidate for an optically driven quantum gate. Recently the coherent
coupling of the exciton states in a single QD molecule due to tunneling
of carriers has been demonstrated in various experiments. These are:
anti-crossings in the fine structure of excitons in single QD molecules
and diamagnetic shifts in Faraday- and Voigt-configuration which depend
systematically on barrier width between the two dots. Similar systematic
dependences have been found for spin-splitting and biexciton properties
in single QD molecules.
HL 44.35 Do 16:30 Poster A
Tight-Binding Modellierung von Halbleiterquantenpunkten —
•Stefan Schulz und Gerd Czycholl — Institut für Theoretische
Physik, Universität Bremen
Es werden die elektronischen Eigenschaften von Halbleiterquantenpunkten
mit Hilfe eines empirischen scp 3 a Tight-Binding (ETB) Modells,
bei Berücksichtigung übernächster Nachbarkopplung, untersucht.
Dadurch ist eine mikroskopische Behandlung dieser niederdimensionalen