459 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
Kondo effect in a few-electron quantum ring
A small quantum ring with less than 10 electrons was studied by transport
spectroscopy. For strong coupling to the leads a Kondo effect is observed and
used to characterize the spin structure of the system in a wide range of
magnetic fields. At small magnetic fields Aharonov-Bohm oscillations influenced
by Coulomb interaction appear. They exhibit phase jumps by at the
Coulomb-blockade resonances. Inside Coulomb-blockade valleys the Aharonov-Bohm
oscillations can also be studied due to the finite conductance caused by the
Kondo effect. Astonishingly, the maxima of the oscillations show linear shifts
with magnetic field and gate voltage.Comment: 4 pages, 4 figure
Combined atomic force microscope and electron-beam lithography used for the fabrication of variable-coupling quantum dots
We have combined direct nanofabrication by local anodic oxidation with
conventional electron-beam lithography to produce a parallel double quantum dot
based on a GaAs/AlGaAs heterostructure. The combination of both nanolithography
methods allows to fabricate robust in-plane gates and Cr/Au top gate electrodes
on the same device for optimal controllability. This is illustrated by the
tunability of the interdot coupling in our device. We describe our fabrication
and alignment scheme in detail and demonstrate the tunability in
low-temperature transport measurements.Comment: 4 pages, 3 figure
Highly efficient single photon emission from single quantum dots within a two-dimensional photonic bandgap
We report highly efficient single photon generation from InGaAs
self-assembled quantum dots emitting within a two-dimensional photonic bandgap.
A strongly suppressed multiphoton probability is obtained for single quantum
dots in bulk GaAs and those emitting into the photonic bandgap. In the latter
case, photoluminescence saturation spectroscopy is employed to measure a ~17
times enhancement of the average photon extraction efficiency, when compared to
quantum dots in bulk GaAs. For quantum dots in the photonic crystal we measure
directly an external quantum efficiency up to 26%, much higher than for quantum
dots on the same sample without a tailored photonic environment. The results
show that highly efficient quantum dot single photon sources can be realized,
without the need for complex nanopositioning techniques
Efficient Spatial Redistribution of Quantum Dot Spontaneous Emission from 2D Photonic Crystals
We investigate the modification of the spontaneous emission dynamics and
external quantum efficiency for self-assembled InGaAs quantum dots coupled to
extended and localised photonic states in GaAs 2D-photonic crystals. The
2D-photonic bandgap is shown to give rise to a 5-10 times enhancement of the
external quantum efficiency whilst the spontaneous emission rate is
simultaneously reduced by a comparable factor. Our findings are quantitatively
explained by a modal redistribution of spontaneous emission due to the modified
local density of photonic states. The results suggest that quantum dots
embedded within 2D-photonic crystals are suitable for practical single photon
sources with high external efficiency
Aharonov-Bohm oscillations of a tunable quantum ring
With an atomic force microscope a ring geometry with self-aligned in-plane
gates was directly written into a GaAs/AlGaAs-heterostructure. Transport
measurements in the open regime show only one transmitting mode and
Aharonov-Bohm oscillations with more than 50% modulation are observed in the
conductance. The tuning via in-plane gates allows to study the Aharonov-Bohm
effect in the whole range from the open ring to the Coulomb-blockade regime.Comment: 3 pages, 3 figure
Activated Transport in the individual Layers that form the =1 Exciton Condensate
We observe the total filling factor =1 quantum Hall state in a
bilayer two-dimensional electron system with virtually no tunnelling. We find
thermally activated transport in the balanced system with a monotonic increase
of the activation energy with decreasing below 1.65. In the
imbalanced system we find activated transport in each of the layers separately,
yet the activation energies show a striking asymmetry around the balance point.
This implies that the gap to charge-excitations in the {\em individual} layers
is substantially different for positive and negative imbalance.Comment: 4 pages. 4 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
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