33 research outputs found
Increasing the {\nu} = 5 / 2 gap energy: an analysis of MBE growth parameters
The fractional quantized Hall state (FQHS) at the filling factor {\nu} = 5/2
is of special interest due to its possible application for quantum computing.
Here we report on the optimization of growth parameters that allowed us to
produce two-dimensional electron gases (2DEGs) with a 5/2 gap energy up to 135
mK. We concentrated on optimizing the MBE growth to provide high 5/2 gap
energies in "as-grown" samples, without the need to enhance the 2DEGs
properties by illumination or gating techniques. Our findings allow us to
analyse the impact of doping in narrow quantum wells with respect to
conventional DX-doping in AlxGa1-xAs. The impact of the setback distance
between doping layer and 2DEG was investigated as well. Additionally, we found
a considerable increase in gap energy by reducing the amount of background
impurities. To this end growth techniques like temperature reductions for
substrate and effusion cells and the reduction of the Al mole fraction in the
2DEG region were applied
The improved inverted AlGaAs/GaAs interface: its relevance for high-mobility quantum wells and hybrid systems
Two dimensional electron gases (2DEGs) realized at GaAs/AlGaAs single
interfaces by molecular-beam epitaxy (MBE) reach mobilities of about 15 million
cm^2/Vs if the AlGaAs alloy is grown after the GaAs. Surprisingly, the
mobilities may drop to a few millions for the identical but inverted
AlGaAs/GaAs interface, i.e. reversed layering. Here we report on a series of
inverted heterostructures with varying growth parameters including temperature,
doping, and composition. Minimizing the segregation of both dopants and
background impurities leads to mobilities of 13 million cm^2/Vs for inverted
structures. The dependence of the mobility on electron density tunes by a gate
or by illumination is found to be the identical if no doping layers exist
between the 2DEG and the respective gate. Otherwise, it differs significantly
compared to normal interface structures. Reducing the distance of the 2DEG to
the surface down to 50nm requires an additional doping layer between 2DEG and
surface in order to compensate for the surface-Schottky barrier. The
suitability of such shallow inverted structures for future
semiconductor-superconductor hybrid systems is discussed. Lastly, our
understanding of the improved inverted interface enables us to produce
optimized double-sided doped quantum wells exhibiting an electron mobility of
40 million cm^2/Vs at 1K.Comment: 19 pages, 9 figure
Controlled generation and detection of a thermal bias in Corbino devices under the quantum Hall regime
We present an experimental technique to generate and measure a temperature
bias in the quantum Hall effect of GaAs/AlGaAs Corbino samples. The bias is
generated by injecting an electrical current at a central resistive heater and
the resulting radial temperature drop is determine by local measurements of the
conductance between internal and external concentric rings. The experimental
results are in agreement with the predictions of numerical simulations of the
heat flow through the substrate. We also compare these results with previous
predictions based on the thermoelectric response of these devices.Comment: 6 pages, 5 figure
Quasiparticle Tunneling across an Exciton Condensate
The bulk properties of the bilayer quantum Hall state at total filling factor one have been intensively studied in experiment. Correlation induced phenomena such as Josephson-like tunneling and zero Hall resistance have been reported. In contrast, the edge of this bilayer state remains largely unexplored. Here, we address this edge physics by realizing quasiparticle tunneling across a quantum point contact. The tunneling manifests itself as a zero bias peak that grows with decreasing temperature. Its shape agrees quantitatively with the formula for weak quasiparticle tunneling frequently deployed in the fractional quantum Hall regime in single layer systems, consistent with theory. Interestingly, we extract a fractional charge of only a few percent of the free electron charge, which may be a signature of the theoretically predicted leakage between the chiral edge and the bulk mediated by gapless excitations
Quasiparticle Tunneling across an Exciton Condensate
The bulk properties of the bilayer quantum Hall state at total filling factor one have been intensively studied in experiment. Correlation induced phenomena such as Josephson-like tunneling and zero Hall resistance have been reported. In contrast, the edge of this bilayer state remains largely unexplored. Here, we address this edge physics by realizing quasiparticle tunneling across a quantum point contact. The tunneling manifests itself as a zero bias peak that grows with decreasing temperature. Its shape agrees quantitatively with the formula for weak quasiparticle tunneling frequently deployed in the fractional quantum Hall regime in single layer systems, consistent with theory. Interestingly, we extract a fractional charge of only a few percent of the free electron charge, which may be a signature of the theoretically predicted leakage between the chiral edge and the bulk mediated by gapless excitations
The genetic architecture of the human cerebral cortex
The cerebral cortex underlies our complex cognitive capabilities, yet little is known about the specific genetic loci that influence human cortical structure. To identify genetic variants that affect cortical structure, we conducted a genome-wide association meta-analysis of brain magnetic resonance imaging data from 51,665 individuals. We analyzed the surface area and average thickness of the whole cortex and 34 regions with known functional specializations. We identified 199 significant loci and found significant enrichment for loci influencing total surface area within regulatory elements that are active during prenatal cortical development, supporting the radial unit hypothesis. Loci that affect regional surface area cluster near genes in Wnt signaling pathways, which influence progenitor expansion and areal identity. Variation in cortical structure is genetically correlated with cognitive function, Parkinson's disease, insomnia, depression, neuroticism, and attention deficit hyperactivity disorder
Mapping an electron wave function by a local electron scattering probe
A technique is developed which allows for the detailed mapping of the electronic wave function in
two-dimensional electron gases with low-temperature mobilities up to 15 10 cm V s ´ 6 2 11 - - . Thin
(‘delta’) layers of aluminium are placed into the regions where the electrons reside. This causes
electron scattering which depends very locally on the amplitude of the electron wave function at the
position of the Al δ-layer. By changing the distance of this layer from the interface we map the shape of
the wave function perpendicular to the interface. Despite having a profound effect on the electron
mobiliy, the δ-layers do not cause a widening of the quantum Hall plateaus.peerReviewe
Gate induced quantum wires in GaAs/AlGaAs heterostructures by cleaved edge deposition
Electric conductors with dimensions reduced to the nanometer scale are the prerequisite of the quantum devices upon which the future advanced electronics is expected to be based. In the past, the fabrication of one-dimensional (1D) wires has been a particular challenge because they have to be defect-free over their whole length, which can be several tens µm. Excellent 1D wires have been produced by cleaving semiconductors (GaAs, AlGaAs) in ultra high vacuum and overgrowing the pristine edge surface by molecular beam epitaxy (MBE)1,2. Unfortunately, this cleaved edge overgrowth (CEO) technique did not find wide-spread use because it requires a series of elaborate steps that are difficult to accomplish. In this Letter, we present a greatly simplified variation of this technique where the cleaving takes place in ambient air and the MBE overgrowth is replaced by a standard deposition process. Wires produced by this cleaved edge deposition (CED) technique have properties that are as least as good as the traditional CEO ones. Due to its simplicity, the CED technique offers a generally accessible way to produce 1D devices.ISSN:2045-232
Spatial development of multiple-gap states in nonequilibrium superconductors
We have studied the gap instability in a superconductor under tunneling injection at high voltages by probing the spatial distribution of the phonon emission. A high sensitivity was achieved by using the fountain pressure of superfluid helium for detecting the phonons. Spatial structures were observed at gap depressions as small as 2%. From their spatial development we find that the quasiparticles diffuse into regions where their density is higher