Plenarvorträge - DPG-Tagungen

Halbleiterphysik Freitag
HL 47.6 Fr 12:15 H15
Realization of a new quantum wire structure fabricated by double
cleaved-edge overgrowth — •S. F. Roth, M. Grayson, M.
Bichler, D. Schuh, and G. Abstreiter — Walter Schottky Institut
TUM, 85748 Garching
We demonstrate a new quantum wire structure, in which the potential
over the whole length of the channel can be modulated with atomic
precision. Theory [1] shows, that if one exact potential barrier can be
introduced into the channel, the conductance obeys a power law in temperature.
With two barriers, effects associated with the Coulomb blockade
can be observed. Even a Mott-insulating state can be achieved by
inserting multiple barriers to modulate the potential periodically and
tuning the electron density with a gate. The geometry of our double
cleave wires is analogous to quantum wires grown by single cleaved-edge
overgrowth [2, 3], except that the channel is oriented along the substrate
growth direction. Therefore a variation in the substrate potential is directly
transferred to the adjacent quantum wire. The first step to realize
the more complicated structures mentioned above is to demonstrate a
simple quantum wire in this double cleave geometry. Preliminary results
show a conductance plateau at a value of about 2e 2 /h, which indicates
the existence of such a quantum wire. We will report on the behavior of
this feature with density, temperature and magnetic field.
[1] C. L. Kane, M. P. Fisher, Phys. Rev. B, 46, 15233 (1992)
[2] A. Yacoby, H. L. Stormer, N. S. Wingreen, L. N. Pfeiffer, K. W.
Baldwin and K. W. West, Phys. Rev. Lett., 77, 4612 (1996)
[3] M. Rother, Ph.D. thesis, Walter Schottky Institut TUM (1999)
HL 47.7 Fr 12:30 H15
Zur Streuung von Elektronen und Schallquanten in Quantendrähten
— •Frank Löcse — TU Chemnitz, Institut für Physik, 09107
Chemnitz
HL 48 Kohlenstoff/Diamant
Untersucht wird die Streuung von Elektronen und Schallquanten in
ballistischen und ungeordneten quasi-eindimensionalen Heterostrukturen
bei tiefen Temperaturen. Die zu diesem Zweck neu entwickelte semiklassische
Methode wird vorgestellt. Sowohl die analytischen als auch
die numerischen Ergebnisse zeigen, daß die Reduktion des Phasenraumes
eine Aufwertung der Streuprozesse im Hinblick auf den Energieumsatz
zur Folge hat. Die Streuung muß, im Gegensatz zur Elektronen-
Schallquanten Streuung in Bulk-Systemen, als inelastisch angesehen werden.
HL 47.8 Fr 12:45 H15
Theoretical prediction of a transport quantum logic gate —
•Matthias Sabathil and Peter Vogl — Walter Schottky Institut,
TU Muenchen
We present a systematic theoretical study of the realization of a quantum
logic gate based on a laterally structured high mobility 2DEG.
The realistic electronic structure has been calculated solving the 3D
Schroedinger-Poisson equation selfconsistently. For the prediction of the
transport properties we used the recently developed contact block reduction
(CBR) method that allows for the effiecient calculation of the
ballistic transmission for large 3D devices with multiple leads.
In ballistic quantum devices, a single Qubit may be realized by electrons
propagating within two adjacent quantum wires, representing the
0 and 1, respectively. Narrow windows in the barrier region between the
wires act as beam splitters, allowing the 0 and 1 states to interfer. Additional
gates are needed to control the phase shift within one of the
wires. Here, we propose a Mach-Zender type interferometer based on
an AlGaAs/GaAs high mobility 2DEG where the lateral confinement is
produced by top gates. We find that this device can operate up to a temperature
of several hundred mK. The calculations show a pronounced
switching behavior as a function of the applied gate voltage.
Zeit: Freitag 11:00–11:30 Raum: H17
HL 48.1 Fr 11:00 H17
Diamond based ion-sensitive field effect transitors — •Andreas
Härtl, Tobias Heimbeck, Jose Garrido, and Martin Stutzmann
— Walter Schottky Institut, Am Coulombwall 3, 85748 Garching
The surface termination of diamond by hydrogen induces a quasi twodimensional
hole accumulation directly at the surface with high carrier
concentrations (around 10 13 cm −2 ) and reasonable mobilities (50 - 100
cm 2 /V · s). This unique feature can be used for a novel design of ionsensitive
field effect transistor (ISFET) devices on diamond. The electrolyte
is in direct contact with the conductive channel. The species to
be detected interact with the surface of the diamond and change the resistance
of the hole channel. The sensitivity of these devices is expected
to be high due to the proximity of the electrolyte and the conductive
channel, in contrast to common silicon based devices, where an insulating
layer separates the electrolyte from the conductive channel. Other
advantages of diamond are its high degree of biocompatibility as well as
its large electrochemical potential window.
We have investigated ISFETs on H-terminated polycrystalline diamond.
With the surface completely hydrogenated, no pH-sensitivity was
found, whereas partially oxidized devices show pH-sensitivity with values
up to 70 mV/pH, exceeding the Nernst limit. Coulombic interaction between
surface adsorbates and charge carriers has tentatively been used to
HL 49 Spinabhängiger Transport III
explain this behaviour. With a suitable biofunctionalization of the surface,
the detection of enzymes, proteins and other biomolecules will be
possible.
HL 48.2 Fr 11:15 H17
Raman based structure assignment of Single-Walled Carbon
Nanotubes — •Hagen Telg 1 , Janina Maultzsch 1 , Stephanie
Reich 2 , Christian Thomsen 1 , and Frank Hennrich 3 — 1 Institut
für Festkörperphysik, Technische Universität Berlin, Germany —
2 Department of Engineering, University of Cambridge, United Kingdom
— 3 Institut für Physikalische Chemie, Universität Karlsruhe, Germany
Nanotube samples made by any method still contain all types of tubes
which differ in chiral angle and diameter. The radial breathing mode
(RBM), which strongly depends on the tube diameter, is frequently used
to determine the chiral angle of the tubes. We studied Raman spectra of
several samples varying in diameter distribution. We also distinguished
between isolated tubes in aqueous surfactant suspensions and bulk samples.
In order to obtain band gap information we took resonance profiles
for different energy regions in the visible. To assign the RBMs to a specific
nanotube type we compared our data with results from photoluminescence
measurements and theoretical predictions.
Zeit: Freitag 11:00–12:00 Raum: H13
HL 49.1 Fr 11:00 H13
Spin-Hall effect and anisotropic charge transport in semiconductors
— •John Schliemann, J. Carlos Egues, and Daniel Loss
— Department of Physics and Astronomy, University of Basel, Switzerland
In a two-dimensional electron gas as realized by a semiconductor quantum
well, the presence of spin-orbit coupling of both the Rashba and
Dresselhaus type leads to anisotropic dispersion relations and Fermi contours.
We study the effect of this anisotropy on the electrical conductivity
in the presence of fixed impurity scatterers. The conductivity also shows
in general an anisotropy which can be tuned by varying the Rashba coefficient.
This effect provides a method of detecting and investigating
spin-orbit coupling by measuring spin-unpolarized electrical currents in
the diffusive regime.
Moreover, we investigate the spin-Hall effect of both electrons and holes
in semiconductors using the Kubo formula in the correct zero-frequency
limit taking into account the finite momentum relaxation time of carriers
in real semiconductors. This approach allows to analyze the range of validity
of recent theoretical findings. In particular, the spin-Hall conductivity
vanishes for vanishing spin-orbit coupling if the correct zero-frequency