5,085 research outputs found
Drift mobility of long-living excitons in coupled GaAs quantum wells
We observe high-mobility transport of indirect excitons in coupled GaAs
quantum wells. A voltage-tunable in-plane potential gradient is defined for
excitons by exploiting the quantum confined Stark effect in combination with a
lithographically designed resistive top gate. Excitonic photoluminescence
resolved in space, energy, and time provides insight into the in-plane drift
dynamics. Across several hundreds of microns an excitonic mobility of >10^5
cm2/eVs is observed for temperatures below 10 K. With increasing temperature
the excitonic mobility decreases due to exciton-phonon scattering.Comment: 3 pages, 3 figure
Magnetoresistance Induced by Rare Strong Scatterers in a High Mobility 2DEG
We observe a strong negative magnetoresistance at non-quantizing magnetic
fields in a high-mobility two-dimensional electron gas (2DEG). This strong
negative magnetoresistance consists of a narrow peak around zero magnetic field
and a huge magnetoresistance at larger fields. The peak shows parabolic
magnetic field dependence and is attributed to the interplay of smooth disorder
and rare strong scatterers. We identify the rare strong scatterers as
macroscopic defects in the material and determine their density from the peak
curvature.Comment: 5 pages, 4 figure
Novel metallic and insulating states at a bent quantum Hall junction
A non-planar geometry for the quantum Hall (QH) effect is studied, whereby
two quantum Hall (QH) systems are joined at a sharp right angle. When both
facets are at equal filling factor nu the junction hosts a channel with
non-quantized conductance, dependent on nu. The state is metallic at nu = 1/3,
with conductance along the junction increasing as the temperature T drops. At
nu = 1, 2 it is strongly insulating, and at nu = 3, 4 shows only weak T
dependence. Upon applying a dc voltage bias along the junction, the
differential conductance again shows three different behaviors. Hartree
calculations of the dispersion at the junction illustrate possible
explanations, and differences from planar QH structures are highlighted.Comment: 5 pages, 4 figures, text + figs revised for clarit
Tuning of structure inversion asymmetry by the -doping position in (001)-grown GaAs quantum wells
Structure and bulk inversion asymmetry in doped (001)-grown GaAs quantum
wells is investigated by applying the magnetic field induced photogalvanic
effect. We demonstrate that the structure inversion asymmetry (SIA) can be
tailored by variation of the delta-doping layer position. Symmetrically-doped
structures exhibit a substantial SIA due to impurity segregation during the
growth process. Tuning the SIA by the delta-doping position we grow samples
with almost equal degrees of structure and bulk inversion asymmetry.Comment: 4 pages 2 figure
Spin-sensitive Bleaching and Spin-Relaxation in QW's
Spin-sensitive saturation of absorption of infrared radiation has been
investigated in p-type GaAs QWs. It is shown that the absorption saturation of
circularly polarized radiation is mostly controlled by the spin relaxation time
of the holes. The saturation behavior has been investigated for different QW
widths and in dependence on the temperature with the result that the saturation
intensity substantially decreases with narrowing of QWs. Spin relaxation times
were experimentally obtained by making use of calculated (linear) absorption
coefficients for inter-subband transitions
Quantum Hall Effect in a Two-Dimensional Electron System Bent by 90 Degrees
Using a new MBE growth technique, we fabricate a two-dimensional electron
system which is bent around an atomically sharp 90 degree corner. In the
quantum Hall regime under tilted magnetic fields, we can measure equilibration
between both co- and counter-propagating edge channels of arbitrary filling
factor ratio. We present here 4-point magnetotransport characterization of the
corner junction with filling factor combinations which can all be explained
using the standard Landauer-Buttiker edge channel picture. The success of this
description confirms the realization of a new type of quantum Hall edge
geometry.Comment: 4 pages, figures included Typographical errors corrected, reference
adde
Dynamic masses for the close PG1159 binary SDSSJ212531.92-010745.9
SDSSJ212531.92-010745.9 is the first known PG1159 star in a close binary with
a late main sequence companion allowing a dynamical mass determination. The
system shows flux variations with a peak-to-peak amplitude of about 0.7 mag and
a period of about 6.96h. In August 2007, 13 spectra of SDSSJ212531.92-010745.9
covering the full orbital phase range were taken at the TWIN 3.5m telescope at
the Calar Alto Observatory (Alm\'{e}ria, Spain). These confirm the typical
PG1159 features seen in the SDSS discovery spectrum, together with the Balmer
series of hydrogen in emission (plus other emission lines), interpreted as
signature of the companion's irradiated side. A radial velocity curve was
obtained for both components. Using co-added radial-velocity-corrected spectra,
the spectral analysis of the PG1159 star is being refined.
The system's lightcurve, obtained during three seasons of photometry with the
G\"ottingen 50cm and T\"ubingen 80cm telescopes, was fitted with both the
NIGHTFALL and PHOEBE binary simulation programs. An accurate mass determination
of the PG1159 component from the radial velocity measurements requires to first
derive the inclination, which requires light curve modelling and yields further
constraints on radii, effective temperature and separation of the system's
components. From the analysis of all data available so far, we present the
possible mass range for the PG1159 component of SDSSJ212531.92-010745.9.Comment: 8 pages, in "White dwarfs", proceedings of the 16th European White
Dwarf Workshop, eds. E. Garcia-Berro, M. Hernanz, J. Isern, S. Torres, to be
published in J. Phys.: Conf. Se
Forming and confining of dipolar excitons by quantizing magnetic fields
We show that a magnetic field perpendicular to an AlGaAs/GaAs coupled quantum
well efficiently traps dipolar excitons and leads to the stabilization of the
excitonic formation and confinement in the illumination area. Hereby, the
density of dipolar excitons is remarkably enhanced up to . By means of Landau level spectroscopy we study the density of excess
holes in the illuminated region. Depending on the excitation power and the
applied electric field, the hole density can be tuned over one order of
magnitude up to - a value comparable with typical
carrier densities in modulation-doped structures.Comment: 4.3 Pages, 4 Figure
Effect of initial spin polarization on spin dephasing and electron g factor in a high-mobility two-dimensional electron system
We have investigated the spin dynamics of a high-mobility two-dimensional
electron system (2DES) in a GaAs--AlGaAs single quantum well by
time-resolved Faraday rotation (TRFR) in dependence on the initial degree of
spin polarization, , of the 2DES. From to %, we observe
an increase of the spin dephasing time, , by an order of magnitude,
from about 20 ps to 200 ps, in good agreement with theoretical predictions by
Weng and Wu [Phys. Rev. B {\bf 68}, 075312 (2003)]. Furthermore, by applying an
external magnetic field in the Voigt configuration, also the electron
factor is found to decrease for increasing . Fully microscopic calculations,
by numerically solving the kinetic spin Bloch equations considering the
D'yakonov-Perel' and the Bir-Aronov-Pikus mechanisms, reproduce the most
salient features of the experiments, {\em i.e}., a dramatic decrease of spin
dephasing and a moderate decrease of the electron factor with increasing
. We show that both results are determined dominantly by the Hartree-Fock
contribution of the Coulomb interaction.Comment: 4 pages, 4 figures, to be published in PR
Engineering ultralong spin coherence in two-dimensional hole systems at low temperatures
For the realisation of scalable solid-state quantum-bit systems, spins in
semiconductor quantum dots are promising candidates. A key requirement for
quantum logic operations is a sufficiently long coherence time of the spin
system. Recently, hole spins in III-V-based quantum dots were discussed as
alternatives to electron spins, since the hole spin, in contrast to the
electron spin, is not affected by contact hyperfine interaction with the
nuclear spins. Here, we report a breakthrough in the spin coherence times of
hole ensembles, confined in so called natural quantum dots, in narrow
GaAs/AlGaAs quantum wells at temperatures below 500 mK. Consistently,
time-resolved Faraday rotation and resonant spin amplification techniques
deliver hole-spin coherence times, which approach in the low magnetic field
limit values above 70 ns. The optical initialisation of the hole spin
polarisation, as well as the interconnected electron and hole spin dynamics in
our samples are well reproduced using a rate equation model.Comment: 16 pages, 6 figure
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