202 research outputs found
Positioning of self-assembled Ge islands on stripe-patterned Si (001) substrates
Self-assembled Ge islands were grown on stripe-patterned Si (001) substrates
by solid source molecular beam epitaxy. The surface morphology obtained by
atomic force microscopy (AFM) and cross-sectional transmission electron
microscopy images (TEM) shows that the Ge islands are preferentially grown at
the sidewalls of pure Si stripes along [-110] direction at 650o C or along the
trenches, whereas most of the Ge islands are formed on the top terrace when the
patterned stripes are covered by a strained GeSi buffer layer. Reducing the
growth temperature to 600oC results in a nucleation of Ge islands both on the
top terrace and at the sidewall of pure Si stripes. A qualitative analysis,
based on the growth kinetics, demonstrates that the step structure of the
stripes, the external strain field and the local critical wetting layer
thickness for the islands formation contribute to the preferential positioning
of Ge islands on the stripes.Comment: 10 pages, 7 figures, 1 table, the original paper is in print in J.
Appl. Phy
g-Factor Tuning and Manipulation of Spins by an Electric Current
We investigate the Zeeman splitting of two-dimensional electrons in an
asymmetric silicon quantum well, by electron-spin-resonance (ESR) experiments.
Applying a small dc current we observe a shift in the resonance field due to
the additional current-induced Bychkov-Rashba (BR) type of spin-orbit (SO)
field. This finding demonstrates SO coupling in the most straightforward way:
in the presence of a transverse electric field the drift velocity of the
carriers imposes an effective SO magnetic field. This effect allows selective
tuning of the g-factor by an applied dc current. In addition, we show that an
ac current may be used to induce spin resonance very efficiently.Comment: 4 pages, 4 figure
Valley splitting of Si/SiGe heterostructures in tilted magnetic fields
We have investigated the valley splitting of two-dimensional electrons in
high quality Si/SiGe heterostructures under tilted magnetic fields.
For all the samples in our study, the valley splitting at filling factor
() is significantly different before and after the
coincidence angle, at which energy levels cross at the Fermi level. On both
sides of the coincidence, a linear density dependence of on the
electron density was observed, while the slope of these two configurations
differs by more than a factor of two. We argue that screening of the Coulomb
interaction from the low-lying filled levels, which also explains the observed
spin-dependent resistivity, is responsible for the large difference of
before and after the coincidence.Comment: REVTEX 4 pages, 4 figure
Centrosymmetric PbTe/CdTe quantum dots coherently embedded by epitaxial precipitation
A concept for the fabrication of highly symmetric quantum dots that are
coherently embedded in a single crystalline matrix is demonstrated. In this
approach, the formation of the quantum dots is induced by a transformation of
an epitaxial 2D quantum well into an array of isolated precipitates with
dimensions of about 25 nm. The formation process is driven by the immiscibility
of the constituent materials resulting from their different lattice structures.
The investigated PbTe/CdTe heterosystem combines two different cubic lattices
with almost identical lattice constants. Therefore, the precipitated quantum
dots are almost strain free and near thermodynamic equilibrium they exhibit the
shape of small-rhombo-cubo-octahedrons. The PbTe/CdTe quantum dots, grown on
GaAs substrates, display intense room temperature luminescence at wavelength
around 3.2 micrometer, which makes them auspicious for applications in
mid-infrared photonic devices.Comment: 12 pages, 3 figure
Valley splitting in a Si/SiGe quantum point contact
We present the theory and measurement of valley splitting in a quantum point
contact (QPC) in a modulation doped Si/SiGe heterostructure. Our measurements
are performed on a submicron Schottky-gated device. An effective mass theory is
developed for a QPC formed in a quantum well, grown on a miscut substrate. Both
theory and experiments include a perpendicular magnetic field. Our results
indicate that both QPC and magnetic confinement can enhance the valley
splitting by reducing the spatial extent of the electronic wavefunction.
Consequently, the valley splitting can be much larger than the spin splitting
for small magnetic fields. We also observe different valley splittings for
different transverse modes in the QPC, supporting the notion that when steps
are present at the quantum well interface, the spatial extent of the
wavefunction plays a dominant role in determining the valley splitting.Comment: 23 pages, 14 figure
Multi-objective Optimization by Uncrowded Hypervolume Gradient Ascent
Evolutionary algorithms (EAs) are the preferred method for solving black-box
multi-objective optimization problems, but when gradients of the objective
functions are available, it is not straightforward to exploit these
efficiently. By contrast, gradient-based optimization is well-established for
single-objective optimization. A single-objective reformulation of the
multi-objective problem could therefore offer a solution. Of particular
interest to this end is the recently introduced uncrowded hypervolume (UHV)
indicator, which takes into account dominated solutions. In this work, we show
that the gradient of the UHV can often be computed, which allows for a direct
application of gradient ascent algorithms. We compare this new approach with
two EAs for UHV optimization as well as with one gradient-based algorithm for
optimizing the well-established hypervolume. On several bi-objective
benchmarks, we find that gradient-based algorithms outperform the tested EAs by
obtaining a better hypervolume with fewer evaluations whenever exact gradients
of the multiple objective functions are available and in case of small
evaluation budgets. For larger budgets, however, EAs perform similarly or
better. We further find that, when finite differences are used to approximate
the gradients of the multiple objectives, our new gradient-based algorithm is
still competitive with EAs in most considered benchmarks. Implementations are
available at https://github.com/scmaree/uncrowded-hypervolume.Comment: T.M.D. and S.C.M. contributed equally. The final authenticated
version is available in the conference proceedings of Parallel Problem
Solving from Nature - PPSN XVI. Changes in new version: removed statement
about Pareto compliance in abstract; added related work; corrected minor
mistake
Munigua, Spanien
Since more than 60 years the Madrid Department of the German Archaeological Institute has been conducting investigations at the Hispano-Roman municipium of Munigua. Current research focuses on a 3D-visualisation and reconstruction of the city. The project is accompanied by a thorough study of the function and use of the individual buildings in antiquity. During the campaigns of 2015 and 2016 the forum, the streets as well as the podium-temple of the city were analyzed. As a result, the voting process on the forum as well as the processional way through the city could be hypothetically reconstructed. A geophysical survey in the southern part of the city revealed the existence and structure of buildings
Emerging technologies revolutionise insect ecology and monitoring
Insects are the most diverse group of animals on Earth, but their small size and
high diversity have always made them challenging to study. Recent technologi-
cal advances have the potential to revolutionise insect ecology and monitoring.
We describe the state of the art of four technologies (computer vision, acoustic
monitoring, radar, and molecular methods), and assess their advantages, current
limitations, and future potential. We discuss how these technologies can adhere
to modern standards of data curation and transparency, their implications for
citizen science, and their potential for integration among different monitoring
programmes and technologies. We argue that they provide unprecedented
possibilities for insect ecology and monitoring, but it will be important to foster
international standards via collaborationpublishedVersio
A Novel Inhibitor of Human La Protein with Anti-HBV Activity Discovered by Structure-Based Virtual Screening and In Vitro Evaluation
Background: Over 350 million people worldwide are infected with hepatitis B virus (HBV), a major cause of liver failure and hepatocellular carcinoma. Current therapeutic agents are highly effective, but are also associated with development of viral resistance. Therefore, strategies for identifying other anti-HBV agents with specific, but distinctive mechanisms of action are needed. The human La (hLa) protein, which forms a stabilizing complex with HBV RNA ribonucleoprotein to promote HBV replication, is a promising target of molecular therapy. Aims: This study aimed to discover novel inhibitors of hLa that could inhibit HBV replication and expression. Methods: A multistage molecular docking approach was used to screen a Specs database and an in-house library against hLa binding sites. Sequential in vitro evaluations were performed to detect potential compounds with high scores in HepG2.2.15 cells. Results: Of the 26 potential compounds with high scores chosen for experimental verification, 12 had HBV DNA inhibition ratios of less than 50 % with P,0.05. Six had significant inhibition of HBV e antigen (HBeAg) levels, and 13 had significant inhibition of HBV surface antigen (HBsAg) levels by in vitro assays. Compounds HBSC-11, HBSC-15 and HBSC-34 (HBSC is system prefix for active compounds screened by the library) were selected for evaluation. HBSC-11 was found to have an obvious inhibitory effect on hLa transcription and expression
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