Plenarvorträge - DPG-Tagungen

Halbleiterphysik Donnerstag
HL 37 Quantenpunkte und -drähte: Transporteigenschaften
Zeit: Donnerstag 10:15–13:30 Raum: H13
HL 37.1 Do 10:15 H13
Spin Blockade in Capacitively Coupled Quantum Dots — •M.
C. Rogge, C. Fühner, U. F. Keyser, and R. J. Haug — Institut
für Festkörperphysik, Universität Hannover, D-30167 Hannover
We present transport measurements on a lateral double dot produced
by combining local anodic oxidation and electron beam lithography [1].
Our device is based on a GaAs/AlGaAs heterostructure containing a
two-dimensional electron system (2DES) 34 nm below the surface. We
use an atomic force microscope (AFM) to write the basic double dot
structure by local anodic oxidation (LAO) [2]. We complete our device
with a metallic top gate patterned with e-beam lithography to add the
function of controlled tunability of the interdot coupling. We investigate
our device in transport measurements in a 3He/4He dilution refrigerator
and demonstrate, that we can switch between capacitive and tunnel
coupling with top gate voltage. In the regime of capacitive coupling we
focus on the magnetic field dependence in the Coulomb blockade regime.
We observe oscillating peak positions and peak amplitudes for transport
over one dot. This is explained by the phenomenon of spin blockade [3],
which has not been observed in LAO-devices so far. We investigate this
effect and analyze the influence of capacitive interdot coupling in this
regime [4].
[1] M. C. Rogge et al., Appl. Phys. Lett. 83, 1163 (2003)
[2] U. F. Keyser et al., Appl. Phys. Lett. 76, 457 (2000)
[3] M. Ciorga et al., Phys. Rev. B 61, R16315 (2000)
[4] M. C. Rogge et al., cond-mat/0310469 (2003)
HL 37.2 Do 10:30 H13
Wave-function mapping of InAs quantum dots by scanning
tunneling spectroscopy — •Theophilos Maltezopoulos, Arne
Bolz, Christian Meyer, Christian Heyn, Wolfgang Hansen,
Markus Morgenstern, and Roland Wiesendanger — Institute of
Applied Physics, University of Hamburg, Jungiusstr. 11, D-20355 Hamburg,
Germany
Low temperature UHV scanning tunneling spectroscopy is used to investigate
the single-electron states and the corresponding squared wavefunctions
of individual and free-standing strain-induced InAs quantum
dots grown on GaAs(001) by molecular beam epitaxy. Several peaks are
found in dI/dV -curves above the dots, which belong to different singleelectron
states. Spatially resolved dI/dV -images at the peak positions
reveal (000)-, (100)-, (010)-, (200)-, and (300)-states, where the numbers
describe the number of nodes in [110]-, [110]-, and [001]-direction, respectively.
It is found, that the total number and energetic sequence of
states is different for different dots. Additionally, the (010)-state is often
missing, even when (200)- and (300)-states are present. This electronic
anisotropy is attributed to the shape asymmetry of the dots.
HL 37.3 Do 10:45 H13
Direct observation of tunneling escape from InAs/GaAs
quantum dots — •Erik Stock, Martin Geller, Roman Sellin,
and Dieter Bimberg — Technische Universität Berlin, Institut für
Festkörperphysik Hardenbergstr. 35, 10623 Berlin
A major challenge in the investigations of quantum dots (QDs) is to
distinguish between two competing charge carrier emission processes:
thermal activated and tunneling emission [1]. We report here the first
observation of the hole tunneling process via time resolved capacitance
transient spectroscopy (DLTS) measurements at low temperature, where
thermal emission is negligible. The escape time observed by us is independent
from the temperature but strongly depends on the applied reverse
bias, which determines the electric field in the QD layer. The measured
hole tunneling emission time constant is in the order of seconds (for electric
fields of about 100kV/cm), approximately three orders of magnitude
larger than for the ground state tunneling of electrons. The dependence of
the tunneling time constant on the electric field is compared to a model,
assuming a triangular barrier with a potential depth measured directly
by DLTS and photoluminescence excitation (PLE) measurements.
This work was funded by the Nanomat project of the European Commission
Growth Programme, contract number G5RD-CT-2001-00545, Intas
project 2001-774, and SFB 296 of DFG.
[1] C.M.A. Kapteyn et al. Phys. Rev. B 60, (20), 14265 (1999)
HL 37.4 Do 11:00 H13
Spin effects in quantum dots — •Jens Könemann 1 , D. K.
Maude 2 , V. Avrutin 3 , A. Waag 3 , and R. J. Haug 1 — 1 Institut
für Festkörperphysik, Universität Hannover, Appelstrasse 2, D-30167
Hannover, Germany — 2 High Magnetic Field Laboratory, CNRS, 25
Avenue des Martyrs, BP 166, F-38042 Grenoble cedex 9, France —
3 Abteilung Halbleiterphysik, Universität Ulm, Albert-Einstein-Allee
45,D-89069 Ulm, Germany
We present spin-resolved measurements of the transport spectrum of
an individual localized state in a double-barrier resonant tunneling device
probed by single-electron tunneling [1]. We investigated the spin splitting
of ground and excited states and found a deviation to a linear increase
of spin splitting with magnetic field beyond 18 T. Astonishingly, we observed
at high magnetic fields above 14 T a flip of the resonance position
of the energetically higher spin component to a lower energy, whereas the
other spin-component remained unchanged. A hysteresis effect appeared
at the spin flip region as well. Such ”spin-flip”behaviour was observed
on different Fock-Darwin levels, where it occurred at different magnetic
fields. We relate this effect to the presence of magnetic impurities residual
inside the quantum well.
[1] J. Könemann et al., to appear in Physica E
HL 37.5 Do 11:15 H13
Coulomb effects in tunneling through a quantum dot stack — H.
Sprekeler, •G. Kießlich, A. Wacker, and E. Schöll — Institut
für Theoretische Physik, TU Berlin, Hardenbergstr. 36, 10623 Berlin
Resonant tunneling through two vertically coupled self-organized quantum
dots is studied by means of a Pauli master equation model. The
observation of double peaks in the current-voltage characteristic in a recent
experiment [1] as a signature of Coulomb interaction is analyzed in
terms of the tunnel coupling between the quantum dots and the coupling
to the contacts. Different regimes for the emitter chemical potential indicating
different peak scenarios in the tunneling current are discussed. By
comparison with a density matrix approach we show that the interplay
of coherent and incoherent effects in the stationary current can be fully
described by this approach.
[1] M. Borgstrom, T. Bryllert, T. Sass, B. Gustafson, L.-E. Wernersson,
W. Seifert, and L. Samuelson, Appl. Phys. Lett. 78, 3232 (2001)
[2] H. Sprekeler, G. Kießlich, A. Wacker, and E. Schöll, cond-mat/0309696
(unpublished)
HL 37.6 Do 11:30 H13
Fano and Kondo Resonances in Three-Leaded Quantum Dot
Systems — •Armin C. Welker, David Quirion, Juergen Weis
und Klaus v. Klitzing — Max-Planck-Institut fuer Festkoerperforschung,
D -70569 Stuttgart, Heisenbergstr. 1, Germany
The spectral function or density of states of a quantum dot coupled
to two leads can be investigated by weakly coupling a third lead to the
quantum dot. For doing so, the differential conductance to this third lead
is measured as a function of the bias voltage applied to this probing lead
in reference of the two main leads. The feasibility of this approach for
investigating the out-of-equilibrium splitting of the Kondo resonance has
been theoretically explored [1]. We present our preliminary experimental
results on a split-gate quantum dot indicating this splitting.
In addition, measurements on a quantum dot system are shown in the
regime of strong coupling to the two main leads where Fano-like resonances
occur. A peak in the differential conductance to the third lead is
observed whenever a Fano-like resonance is obtained in the conductance
between the two main leads. [1] E. Lebanon and A. Schiller, Phys. Rev.
B 65, 035308 (2002).
HL 37.7 Do 11:45 H13
Hole transport through an array of self-assembled Ge quantum
dots — •Kai-Martin Haendel 1 , U. Denker 2 , O. G. Schmidt 2 ,
and R. J. Haug 1 — 1 Institut für Festkörperphysik, Universität
Hannover, Appelstr. 2, 30167 Hannover — 2 Max-Planck-Institut für
Festkörperforschung, Heisenbergstr. 1, 70569 Stuttgart
In recent years, several features of quantum states in self-assembeld Ge
quantum dots have been demonstrated by various spectroscopic studies,
such as admittance, capacitance-voltage analysis, photoluminescence and
photoluminescence excitation spectroscopy.