Plenarvorträge - DPG-Tagungen

Halbleiterphysik Dienstag
method of cleaved edge overgrowth [1]. For high barriers there are small
energy gaps (∆ ≪ ¯hωc) at the crossing points of the bent Landau bands
of the left and the right electron systems due to the weak coupling of the
counter propagating opposite edge channels. We present the solutions
of the Schrödinger equation for the whole system in the one electron
approximation which is especially interesting in the low barrier regime
(e.g. V0 = 3 ¯hωc) when the results strongly deviate from more crude approximations.
For low aluminium concentrations in the AlGaAs barrier
the dispersion relation can no longer be calculated as a superposition of
two symmetric Landau spectra. Instead a complete numerical calculation
must be performed in order to find the correct gaps and their positions.
Our theoretical results are compared to experimental data of CEO samples
which reveal energy gaps bigger and at different filling factors than
expected. The project is supported by the DFG Schwerpunktsprogramm
1092 Quanten-Hall-Systeme.
[1] W. Kang et al., Nature 403, 59 (2000)
HL 22.5 Di 16:15 H14
Spin structures in inhomogeneous fractional quantum Hall systems
— •Karel Vyborny and Daniela Pfannkuche — I. Institute
of Theoretical Physics, University of Hamburg, Jungiusstr. 9, D-20355
Hamburg, Germany
Huge longitudinal magnetoresistance effect in 2D electron systems was
shown to be a way to manipulate and measure spin of the host material
nuclei [1] and it can thus have important applications in the field of
quantum computers. Since there are two nearly degenerate ground states
of the electronic part of the system (polarized and singlet-state incompressible
liquids at filling factor 2/3) it is widely believed that the effect
is due to formation of domains which results into an enhanced resistance.
To our best knowledge there is however no direct evidence of existence
of such domains so far.
We study a finite size model of electrons in the lowest Landau level
which was formerly used to study the Fractional Quantum Hall Effect
(torus geometry, [2]). The system is now also subject to magnetic inhomogeneity
which makes one of the ground states more favorable in one
part and less favorable in another part of the system. We investigate how
the system answers to various types of inhomogeneities in spin quantities
and particle densities as well as in correlation functions.
[1] S. Kronmüller et al., Phys. Rev. Lett., 81, 2526 (1998), J. Smet
et al., Nature, 415, 281 (2002), S. Kraus et al., Phys. Rev. Lett, 89,
266801 (2002)
[2] D. Yoshioka, Phys. Rev. B, 29, 6833 (1984), T. Chakraborty,
Surf. Sci., 229, 16 (1990)
HL 22.6 Di 16:30 H14
On ground states of interacting Composite Fermions with spin
at half filling — •Matteo Merlo 1 , Nicodemo Magnoli 2 , Maura
Sassetti 1 , and Bernhard Kramer 3 — 1 Dipartimento di Fisica,
INFM-LAMIA, Università di Genova, Via Dodecaneso 33, I-16146 Genova,
Italy — 2 Dipartimento di Fisica, INFN, Università di Genova, Via
Dodecaneso 33, I-16146 Genova, Italy — 3 I. Institut für Theoretische
Physik, Universität Hamburg, Jungiusstraße 9, D-20355 Hamburg, Germany
The effects of interactions in a 2D electron system in a strong magnetic
field at the coincidence of two Landau levels with opposite spins and at
filling factors 1/2 are studied. Via the Chern-Simons gauge transformation,
the system is mapped to Composite Fermions. The fluctuations of
the gauge field induce an effective interaction between the Composite
Fermions which can be attractive in both the particle-particle [1,2] and
in the particle-hole channel. As a consequence, spin-zero ground states of
Composite Fermion particle-particle or particle-hole pairs can exist. The
competition between the two possible ground states is discussed and the
results are compared with experiments.
[1] E. Mariani, N. Magnoli, F. Napoli, M. Sassetti, and B. Kramer,
Phys. Rev. B 66, 241303 (2002) .
[2] B. Kramer, N. Magnoli, E. Mariani, M. Merlo, F. Napoli, and M.
Sassetti, in Quantum Phenomena in Mesoscopics Physics, Proceedings
of the International School of Physics Enrico Fermi, Varenna, 2002.
HL 22.7 Di 16:45 H14
Two-dimensional High Mobility Holes on (110) GaAs — •Frank
Fischer, Matthew Grayson, Dieter Schuh, Max Bichler, and
Gerhard Abstreiter — Walter Schottky Institut, Technische Universität
München, Garching, Germany
High mobility two-dimensional hole gases (2DHG) in GaAs have
previously been realised by using Si as dopant on the (311)A surface
[1]. On the (110) GaAs surface Si can act as both, donor and acceptor
[2]. But up to now only high-mobility two-dimensional electron gases
have been achieved on that orientation. We report on the first growth
of high mobility two-dimensional modulation doped hole gases on the
(110) GaAs orientation. We were able to achieve a 1.2K mobility of
µ = 1.7 × 10 5 cm 2 /Vs at a density of n = 2.3 × 10 11 /cm 2 .
We will report studies of anisotropic conduction in this system as
previously performed on the (311) surface [3].
[1] M. Henini, P. J. Rodgers, P. A. Crump, B. L. Gallagher, G. Hill, J.
Cryst. Growth 150(1-4), 441 (1995)
[2] E. S. Tok, J. H. Neave, M. J. Ashwin, B. A. Joyce, T. S. Jones, J.
Appl. Phys. 83(8), 4160 (1998)
[3] S. J. Papadakis, E. P. De Poortere, M. Shayegan, R. Winkler, Phys.
Rev. Lett. 84 (24), 5592 (2000)
HL 22.8 Di 17:00 H14
A non-perturbative solution for Bloch electrons in constant
magnetic fields — •Alex Trellakis and Peter Vogl — Walter
Schottky Institut, Technische Universität München, 85748 Garching
A general theoretical approach for the non-perturbative Bloch solution
of Schrödinger’s equation in the presence of a periodic crystal potential
and a constant magnetic field is presented. Using a singular gauge transformation
based on a two-dimensional lattice of magnetic flux lines, an
equivalent quantum system with a now periodic vector potential is obtained.
For rational values of the magnetic field the periods of the vector
potential and the crystal potential become commensurable, and the system
forms a magnetic superlattice for which then Bloch’s theorem applies.
Extensions of the approach to particles with spin and many-body
systems and connections to the theory of magnetic translation groups are
discussed.
HL 22.9 Di 17:15 H14
Spin-polarisierte Randzustände von Quanten-Hall Systemen
auf Siliziumbasis — •Carsten A. G. Kentsch und Dieter P.
Kern — Institut für Angewandte Physik, Auf der Morgenstelle 10, 72076
Tübingen
Spin-polarisierte Elektronen finden sich in den Randzuständen
zweidimensionaler Elektronengase (2DEG) im starken Magnetfeld. Sie
können dazu dienen, mittels Transportmessungen Streuung zwischen
Spinzuständen zu untersuchen. 2DEG-Systeme in Silizium erfahren
in letzter Zeit wieder Beachtung, da das Hauptisotop von Silizium
keinen Kernspin hat und somit die Spin-Streuung von Elektronen
mit dem Grundmaterial viel geringer ausfällt als z.B. bei GaAs.
Eine längere Lebensdauer spin-polarisierter Elektronen ist die Folge.
Diese können zur Detektion von Kerndrehimpuls-Zustände von gezielt
eingebrachten Phosphoratomen dienen, die sich als Quanten-Bits in
Quanten-Computern eignen.
Es wurden Hallbar-Srukturen auf Basis von MOSFET-Transistoren
mit Poly-Silizium Topgate hergestellt und bei 8 Tesla und 1,5 Kelvin charakterisiert.
Landau-Niveaus wurden bis zum n=1 Niveau bei Shubnikovde
Haas Oszillationen beobachtet. Die Randzustände sollen durch Split-
Gate Anordnungen im Topgate manipuliert werden.
HL 22.10 Di 17:30 H14
Equilibration between spin split edge channels studied by timeresolved
measurements — •Gennadiy Sukhodub, Frank Hohls,
and Rolf Haug — Institut für Festkörper Physik, Universität Hannover,
Appelstraße 2, D-30167 Hannover, Germany
The concept of edge channels (EC) utilizes a two-dimensional electron
system in the regime of integer quantum Hall effect as an object for study
of the interactions between one-dimensional (1D) channels. Particularly,
the scattering between these 1D states can be investigated in a sample
which allows for selective population of different EC. In our system a
short voltage pulse is applied to one of the two EC at filling factor ν = 2.
After interaction over a distance of 20µm with a not excited state these
two EC are separated and a time-resolved current in each channel is detected
independently. Selective population and detection is realized by
well-established cross gates technique. Dependent on the polarity of the
applied pulse two scattering regimes with significantly different equilibration
lengths are possible. We studied the influence of the pulse amplitude
on the equilibration in the case of weak scattering. We obtain characteristic
time scales for the adiabatic and the equilibrated regime of the current
transport. There is a strong indication of the involvement of the higher