Plenarvorträge - DPG-Tagungen

Halbleiterphysik Freitag
limit is performed.
References: J. Schliemann, J. C. Egues, and D. Loss, Phys. Rev. Lett.
90, 146801 (2003).
J. Schliemann and D. Loss, Phys. Rev. B 68, 165311 (2003).
J. Schliemann and D. Loss, cond-mat/0310108.
HL 49.2 Fr 11:15 H13
Two-dimensional hole precession in an all-semiconductor spin
field effect transistor — •Marco G. Pala 1,2 , Michele Governale
1,3 , Jürgen König 1,4 , Ulrich Zülicke 1,5 , and Giuseppe Iannaccone
2 — 1 Institut für Theoretische Festkörperphysik, Universität
Karlsruhe, Germany — 2 Dipartimento di Ingegneria dell’Informazione,
Università degli Studi di Pisa, Italy — 3 NEST-INFM & Scuola Normale
Superiore, Pisa, Italy — 4 Institut für Theoretische Physik III, Ruhr-
Universität Bochum, Germany — 5 Institute of Fundamental Sciences,
Massey University, New Zealand
We present a theoretical study of a spin field-effect transistor
[1] realized in a quantum well formed in a p–
doped ferromagnetic-semiconductor- nonmagnetic-semiconductorferromagnetic-semiconductor
hybrid structure. Based on an envelopefunction
approach for the hole bands, we derive the full theory of coherent
transport through the device, which includes both heavy- and light-hole
subbands, proper modeling of the mode matching at the interfaces,
integration over injection angles, Rashba spin precession [2], interference
effects due to multiple reflection, and gate-voltage dependence. [1]
S. Datta and B. Das, Appl. Phys. Lett. 56, 665, (1990). [2] E. I. Rashba,
Fiz. Tverd. Tela (Leningrad) 2, 1224 (1960) [Sov. Phys. Solid State 2,
1109 (1960)].
HL 49.3 Fr 11:30 H13
Magnetic moment induced by spin-polarized currents in a
semiconductor heterostructure — •Frank Lehmann, Charles
Gould, Christian Rüster, Peter Grabs, Georg Schmidt, and
Laurens W. Molenkamp — Unversität Würzburg, Physikalisches
Institut (EP 3), Am Hubland, D-97974 Würzburg, Germany
In recent years, several experiments have successfully demonstrated the
electrical injection of spin-polarized currents into semiconductors. These
experiments are sensitive to the polarization of the current but not to the
magnetic moment induced by the spin-polarized carriers. We have now
successfully performed SQUID experiments that provide direct evidence
of a current induced magnetization in an electrical spin injection device.
The magnetic moment of a ZnBeMnSe/GaAs heterostructure was measured
using a SQUID magnetometer and lock-in technique at various
magnetic fields, currents, and temperatures. When the background magnetic
moment that results from Biot-Savart‘s law is eliminated, we observe
a clear change in the magnetic moment at the field and current
configurations for which spin injecton is expected. The field and temperature
dependence show that the effect is related to the magnetization
of the DMS. We interpret our results in terms of (i) the band bending
that occurs in the DMS at higher eletric fields and that reduces the spinpolarization
in the magnetic material and (ii) spin accumulation in the
GaAs.
HL 49.4 Fr 11:45 H13
Positive cross correlations in a three-terminal quantum dot with
ferromagnetic contacts — •Wolfgang Belzig, Audrey Cottet,
and Christoph Bruder — Department für Physik, Universität Basel,
Klingelbergstr. 82, 4056 Basel, Schweiz
We study current fluctuations in a three-terminal quantum dot with
ferromagnetic leads in the sequential tunneling regime. Dynamical spin
blockade on the dot leads to positive zero-frequency cross-correlations of
the currents in the output leads. We include the influence of spin-flip scattering
and identify favorable conditions for the experimental observation
of this effect with respect to polarization of the contacts and tunneling
rates.
HL 50 Quantenpunkte und -drähte: Optische Eigenschaften III
Zeit: Freitag 11:00–13:30 Raum: H14
HL 50.1 Fr 11:00 H14
Optical Detection of Single-Electron Spin Decoherence in
a Quantum Dot — •Oliver Gywat 1 , Hans-Andreas Engel
1 , Daniel Loss 1 , R.J. Epstein 2 , F. Mendoza 2 , and D.D.
Awschalom 2 — 1 Department of Physics and Astronomy, University
of Basel, Switzerland — 2 Center for Spintronics and Quantum
Computation, University of California, Santa Barbara, USA
We propose a method based on optically detected magnetic resonance
(ODMR) to measure the decoherence time T2 of a single electron spin in
a semiconductor quantum dot. The electron spin resonance (ESR) of a
single excess electron on a quantum dot is probed by circularly polarized
laser excitation. Due to Pauli blocking, optical excitation is only possible
for one of the electron spin states. The photoluminescence is modulated
due to the ESR which enables the measurement of electron spin decoherence.
We study different possible schemes for such an ODMR setup.
(cond-mat/0307669)
HL 50.2 Fr 11:15 H14
Optical properties of localized excitons in InGaN quantum
structures — •Til Bartel 1 , Matthias Dworzak 1 , Martin
Strassburg 2 , Axel Hoffmann 1 , Andre Strittmatter 1 , and
Dieter Bimberg 1 — 1 Institut für Festkörperphysik, Technische
Universität Berlin, Hardenbergstr. 36, 10623 Berlin — 2 Georgia State
University, Dep. of Physics and Astronomy, Atlanta, GA-30303
Although InGaN structures already find application in optoelectronics,
its optical transitions are still poorly understood and are subject to
current investigation. Indium fluctuations in InGaN quantum-wells are
centers of localization for excitons and show quantum dot-like behavior
at low temperatures.
Wurzite InGaN/GaN single quantum wells were grown by metal organic
chemical vapor deposition on Si(111) and their luminescence was
investigated. High resolution µ-photoluminescence (µ-PL) spectra of
masked samples showed lines as narrow as the resolution limit. This is
interpreted as an indication for δ-shaped density of states. Excitation
density dependence allows the identification of excitons and biexcitones
with positive and negative binding energies. Time resolved PL experiments
show decay rates of less than 1 ns and investigation of temperature
dependence of the photoluminescence reveal typical S-shape behavior.
These results demonstrate the quantum dot-like character of the indium
fluctuations.
HL 50.3 Fr 11:30 H14
Selective optical charging of self-assembled InGaAs quantum
dots — •Miro Kroutvar, Yann Ducommun, Jon J Finley, Max
Bichler, and Gerhard Abstreiter — Walter Schottky Institut,
Technische Universit”at M”unchen, Am Coulombwall 3, 85748 Garching,
Germany
Charge and spin excitations in individual quantum dots (QDs) have
been proposed as QBITS for implementation of quantum logic. One of
the main challenges for these applications is the selective creation of
electrons in such systems and control of their spin degree of freedom.
We present a QD charging device which enables to optically generate
single charges and potentially spins in sub-ensembles of self-assembled
InGaAs QDs. The device consists of a single layer of InGaAs QDs embedded
within the intrinsic region of a GaAs Schottky photodiode. The
charge storage mechanism relies on selective exciton ionization following
resonant optical generation. Our results demonstrate unambiguously
selective charging with extremely long (¿25 µs) charge storage lifetimes
at low temperatures (10K). Analysis of the energy and temperature dependence
of the charge storage signal permits investigation of the role of
exciton-phonon coupling during the QD resonant absorption process. In
addition, thermally-activated redistribution of resonantly stored charge
among the QD ensemble is identified to be the cause of a time-dependent
loss of spectral selectivity at elevated temperatures. Our storage device
enables to probe directly resonant carrier excitation processes and potentially
electron spin dynamics in micro-ensembles of semiconductor QDs.