370 research outputs found
Anomalous magnetoresistance peak in (110) GaAs two-dimensional holes: Evidence for Landau-level spin-index anticrossings
We measure an anomalous magnetoresistance peak within the lowest Landau level
(nu = 1) minimum of a two-dimensional hole system on (110) GaAs.
Self-consistent calculations of the valence band mixing show that the two
lowest spin-index Landau levels anticross in a perpendicular magnetic field B
consistent with where the experimental peak is measured, Bp. The temperature
dependence of the anomalous peak height is interpreted as an activated behavior
across this anticrossing gap. Calculations of the spin polarization in the
lowest Landau levels predict a rapid switch from about -3/2 to +3/2 spin at the
anticrossing. The peak position Bp is shown to be affected by the confinement
electrostatics, and the utility of a tunable anticrossing position for
spintronics applications is discussed.Comment: 4 pages, 4 figure
Magnetic field dependent transmission phase of a double dot system in a quantum ring
The Aharonov-Bohm effect is measured in a four-terminal open ring geometry
based on a Ga[Al]As heterostructure. Two quantum dots are embedded in the
structure, one in each of the two interfering paths. The number of electrons in
the two dots can be controlled independently. The transmission phase is
measured as electrons are added to or taken away from the individual quantum
dots. Although the measured phase shifts are in qualitative agreement with
theoretical predictions, the phase evolution exhibits unexpected dependence on
the magnetic field. For example, phase lapses are found only in certain ranges
of magnetic field.Comment: 5 pages, 4 figure
Optically Probing Spin and Charge Interactions in an Tunable Artificial Molecule
We optically probe and electrically control a single artificial molecule
containing a well defined number of electrons. Charge and spin dependent
inter-dot quantum couplings are probed optically by adding a single
electron-hole pair and detecting the emission from negatively charged exciton
states. Coulomb and Pauli blockade effects are directly observed and
hybridization and electrostatic charging energies are independently measured.
The inter-dot quantum coupling is confirmed to be mediated predominantly by
electron tunneling. Our results are in excellent accord with calculations that
provide a complete picture of negative excitons and few electron states in
quantum dot molecules.Comment: shortened version: 6 pages, 3 figures, 1 table, to appear in Phys.
Rev. Let
New Anisotropic Behavior of Quantum Hall Resistance in (110) GaAs Heterostructures at mK Temperatures and Fractional Filling Factors
Transport experiments in high mobility (110) GaAs heterostructures have been
performed at very low temperatures 8 mK. At higher Landau-Levels we observe a
transport anisotropy that bears some similarity with what is already seen at
half-odd-integer filling on (001) oriented substrates. In addition we report
the first observation of transport anisotropies within the lowest Landau-Level.
This remarkable new anisotropy is independent of the current direction and
depends on the polarity of the magnetic field.Comment: 3 Pages, 4 figures, Latex, uses elsart.cls and physart.cls, to be
published in Physica E Added reference, made contact configuration more clea
Direct observation of acoustic phonon mediated relaxation between coupled exciton states in a single quantum dot molecule
We probe acoustic phonon mediated relaxation between tunnel coupled exciton
states in an individual quantum dot molecule in which the inter-dot quantum
coupling and energy separation between exciton states is continuously tuned
using static electric field. Time resolved and temperature dependent optical
spectroscopy are used to probe inter-level relaxation around the point of
maximum coupling. The radiative lifetimes of the coupled excitonic states can
be tuned from ~2 ns to ~10 ns as the spatially direct and indirect character of
the wavefunction is varied by detuning from resonance. Acoustic phonon mediated
inter-level relaxation is shown to proceed over timescales comparable to the
direct exciton radiative lifetime, indicative of a relaxation bottleneck for
level spacings in the range $\Delta E\$ ~3-6 meV.Comment: 6 pages, 4 figures, submitted for publicatio
Direct Observation of Controlled Coupling in an Individual Quantum Dot Molecule
We report the direct observation of quantum coupling in individual quantum
dot molecules and its manipulation using static electric fields. A pronounced
anti-crossing of different excitonic transitions is observed as the electric
field is tuned. Comparison of our experimental results with theory shows that
the observed anti-crossing occurs between excitons with predominant spatially
\emph{direct} and \emph{indirect} character. The electron component of the
exciton wavefunction is shown to have molecular character at the anti-crossing
and the quantum coupling strength is deduced optically. In addition, we
determine the dependence of the coupling strength on the inter-dot separation
and identify a field driven transition of the nature of the molecular ground
state.Comment: 11 pages, 4 figures submitted to Physical Review Letter
Nanometer-scale sharpness in corner-overgrown heterostructures
A corner-overgrown GaAs/AlGaAs heterostructure is investigated with
transmission and scanning transmission electron microscopy, demonstrating
self-limiting growth of an extremely sharp corner profile of 3.5 nm width. In
the AlGaAs layers we observe self-ordered diagonal stripes, precipitating
exactly at the corner, which are regions of increased Al content measured by an
XEDS analysis. A quantitative model for self-limited growth is adapted to the
present case of faceted MBE growth, and the corner sharpness is discussed in
relation to quantum confined structures. We note that MBE corner overgrowth
maintains nm-sharpness even after microns of growth, allowing the realization
of corner-shaped nanostructures.Comment: 4 pages, 3 figure
Electrical control of the exciton-biexciton splitting in a single self-assembled InGaAs quantum dots
We report on single InGaAs quantum dots embedded in a lateral electric field
device. By applying a voltage we tune the neutral exciton transition into
resonance with the biexciton using the quantum confined Stark effect. The
results are compared to theoretical calculations of the relative energies of
exciton and biexciton. Cascaded decay from the manifold of single
exciton-biexciton states has been predicted to be a new concept to generate
entangled photon pairs on demand without the need to suppress the fine
structures splitting of the neutral exciton
Asymmetric optical nuclear spin pumping in a single uncharged quantum dot
A highly asymmetric dynamic nuclear spin pumping is observed in a single self
assembled InGaAs quantum dot subject to resonant optical pumping of the neutral
exciton transition leading to a large maximum polarization of 54%. This dynamic
nuclear polarization is found to be much stronger following pumping of the
higher energy Zeeman state. Time-resolved measurements allow us to directly
monitor the buildup of the nuclear spin polarization in real time and to
quantitatively study the dynamics of the process. A strong dependence of the
observed dynamic nuclear polarization on the applied magnetic field is found,
with resonances in the pumping efficiency being observed for particular
magnetic fields. We develop a model that fully accounts for the observed
behaviour, where the pumping of the nuclear spin system is due to
hyperfine-mediated spin flip transitions between the states of the neutral
exciton manifold.Comment: published version; 4+ pages, 3 figures (eps
Highly Non-linear Excitonic Zeeman Spin-Splitting in Composition-Engineered Artificial Atoms
Non-linear Zeeman splitting of neutral excitons is observed in composition
engineered In(x)Ga(1-x)As self-assembled quantum dots and its microscopic
origin is explained. Eight-band k.p simulations, performed using realistic dot
parameters extracted from cross-sectional scanning tunneling microscopy, reveal
that a quadratic contribution to the Zeeman energy originates from a spin
dependent mixing of heavy and light hole orbital states in the dot. The dilute
In-composition (x<0.35) and large lateral size (40-50 nm) of the quantum dots
investigated is shown to strongly enhance the non-linear excitonic Zeeman gap,
providing a blueprint to enhance such magnetic non-linearities via growth
engineering
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