518 research outputs found
Ab initio many-body calculation of excitons in solid Ne and Ar
Absorption spectra, exciton energy levels and wave functions for solid Ne and
Ar have been calculated from first principles using many-body techniques.
Electronic band structures of Ne and Ar were calculated using the GW
approximation. Exciton states were calculated by diagonalizing an exciton
Hamiltonian derived from the particle-hole Green function, whose equation of
motion is the Bethe-Salpeter equation. Singlet and triplet exciton series up to
n=5 for Ne and n=3 for Ar were obtained. Binding energies and
longitudinal-transverse splittings of n=1 excitons are in excellent agreement
with experiment. Plots of correlated electron-hole wave functions show that the
electron-hole complex is delocalised over roughly 7 a.u. in solid Ar.Comment: 6 page
Magnetic Miniband Structure and Quantum Oscillations in Lateral Semiconductor Superlattices
We present fully quantum-mechanical magnetotransport calculations for
short-period lateral superlattices with one-dimensional electrostatic
modulation. A non-perturbative treatment of both magnetic field and modulation
potential proves to be necessary to reproduce novel quantum oscillations in the
magnetoresistance found in recent experiments in the resistance component
parallel to the modulation potential. In addition, we predict oscillations of
opposite phase in the component perpendicular to the modulation not yet
observed experimentally. We show that the new oscillations originate from the
magnetic miniband structure in the regime of overlapping minibands.Comment: 6 pages with 4 figure
Schwangerschaftsdepression und deren Behandlung
Zusammenfassung: Etwa 11% aller schwangeren Frauen leiden unter einer behandlungsbedürftigen Depression, welche unbehandelt mit Risiken wie Frühgeburt oder niederem Geburtsgewicht verbunden ist. Da manche Symptome der Depression oft der Schwangerschaft zugeschrieben werden, ist die Diagnose einer Schwangerschaftsdepression nicht immer leicht. Eine weitere Herausforderung ist die Wahl der geeigneten Therapiemaßnahme. Als Behandlungsmöglichkeiten stehen neben Psycho- und Pharmakotherapie auch die Elektrokrampftherapie (EKT) sowie Lichttherapie zur Auswahl. Aktuelle Berichte über Auswirkungen von Antidepressiva auf die Entwicklung des Ungeborenen und postpartale Anpassungsschwierigkeiten führen zu großen Verunsicherungen. Die Entscheidung, welche Therapiemaßnahme ergriffen wird, kann nur mit der werdenden Mutter zusammen getroffen werden; ein sorgfältiges Abwägen der Vor- und Nachteile der Behandlung ist dafür Voraussetzun
Theoretical study of interacting hole gas in p-doped bulk III-V semiconductors
We study the homogeneous interacting hole gas in -doped bulk III-V
semiconductors. The structure of the valence band is modelled by Luttinger's
Hamiltonian in the spherical approximation, giving rise to heavy and light hole
dispersion branches, and the Coulomb repulsion is taken into account via a
self-consistent Hartree-Fock treatment. As a nontrivial feature of the model,
the self-consistent solutions of the Hartree-Fock equations can be found in an
almost purely analytical fashion, which is not the case for other types of
effective spin-orbit coupling terms. In particular, the Coulomb interaction
renormalizes the Fermi wave numbers for heavy and light holes. As a
consequence, the ground state energy found in the self-consistent Hartree-Fock
approach and the result from lowest-order perturbation theory do not agree. We
discuss the consequences of our observations for ferromagnetic semiconductors,
and for the possible observation of the spin-Hall effect in bulk -doped
semiconductors. Finally, we also investigate elementary properties of the
dielectric function in such systems.Comment: 9 pages, 5 figures, title slightly changed in the course of editorial
process, a few references added, version to appear in Phys. Rev.
Retrospective analysis of a nonforecasted rain-on-snow flood in the Alps – a matter of model limitations or unpredictable nature?
A rain-on-snow flood occurred in the Bernese Alps, Switzerland, on 10 October
2011, and caused significant damage. As the flood peak was unpredicted by the
flood forecast system, questions were raised concerning the causes and the
predictability of the event. Here, we aimed to reconstruct the anatomy of
this rain-on-snow flood in the Lötschen Valley (160 km<sup>2</sup>) by
analyzing meteorological data from the synoptic to the local scale and by
reproducing the flood peak with the hydrological model WaSiM-ETH (Water Flow
and Balance Simulation Model). This in order to gain process understanding
and to evaluate the predictability.
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The atmospheric drivers of this rain-on-snow flood were (i) sustained
snowfall followed by (ii) the passage of an atmospheric river bringing warm
and moist air towards the Alps. As a result, intensive rainfall (average of
100 mm day<sup>-1</sup>) was accompanied by a temperature increase that
shifted the 0° line from 1500 to 3200 m a.s.l. (meters above
sea level) in 24 h with a maximum increase of 9 K in
9 h. The south-facing slope of the valley received significantly more
precipitation than the north-facing slope, leading to flooding only in
tributaries along the south-facing slope. We hypothesized that the reason for
this very local rainfall distribution was a cavity circulation combined with
a seeder-feeder-cloud system enhancing local rainfall and snowmelt along the
south-facing slope.
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By applying and considerably recalibrating the standard hydrological model
setup, we proved that both latent and sensible heat fluxes were needed to
reconstruct the snow cover dynamic, and that locally high-precipitation sums
(160 mm in 12 h) were required to produce the estimated flood
peak. However, to reproduce the rapid runoff responses during the event, we
conceptually represent likely lateral flow dynamics within the snow cover
causing the model to react "oversensitively" to meltwater.
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Driving the optimized model with COSMO (Consortium for Small-scale
Modeling)-2 forecast data, we still failed to simulate the flood because
COSMO-2 forecast data underestimated both the local precipitation peak and
the temperature increase. Thus we conclude that this rain-on-snow flood was,
in general, predictable, but requires a special hydrological model setup and
extensive and locally precise meteorological input data. Although, this data
quality may not be achieved with forecast data, an additional model with a
specific rain-on-snow configuration can provide useful information when
rain-on-snow events are likely to occur
Ground state energies of quantum dots in high magnetic fields: A new approach
We present a new method for calculating ground state properties of quantum
dots in high magnetic fields. It takes into account the equilibrium positions
of electrons in a Wigner cluster to minimize the interaction energy in the high
field limit. Assuming perfect spin alignment the many-body trial function is a
single Slater determinant of overlapping oscillator functions from the lowest
Landau level centered at and near the classical equilibrium positions. We
obtain an analytic expression for the ground state energy and present numerical
results for up to N=40.Comment: 4 pages, including 2 figures, contribution to the Proceedings of
EP2DS-14, submitted to Physica
Magnetic ground state and spin fluctuations in MnGe chiral magnet as studied by Muon Spin Rotation
We have studied by muon spin resonance ({\mu}SR) the helical ground state and
fluctuating chiral phase recently observed in the MnGe chiral magnet. At low
temperature, the muon polarization shows double period oscillations at short
time scales. Their analysis, akin to that recently developed for MnSi [A. Amato
et al., Phys. Rev. B 89, 184425 (2014)], provides an estimation of the field
distribution induced by the Mn helical order at the muon site. The refined muon
position agrees nicely with ab initio calculations. With increasing
temperature, an inhomogeneous fluctuating chiral phase sets in, characterized
by two well separated frequency ranges which coexist in the sample. Rapid and
slow fluctuations, respectively associated with short range and long range
ordered helices, coexist in a large temperature range below T = 170 K. We
discuss the results with respect to MnSi, taking the short helical period,
metastable quenched state and peculiar band structure of MnGe into account.Comment: 13 pages, 11 figure
Commensurability effects in Andreev antidot billiards
An Andreev billiard was realized in an array of niobium filled antidots in a
high-mobility InAs/AlGaSb heterostructure. Below the critical temperature T_C
of the Nb dots we observe a strong reduction of the resistance around B=0 and a
suppression of the commensurability peaks, which are usually found in antidot
lattices. Both effects can be explained in a classical Kubo approach by
considering the trajectories of charge carriers in the semiconductor, when
Andreev reflection at the semiconductor-superconductor interface is included.
For perfect Andreev reflection, we expect a complete suppression of the
commensurability features, even though motion at finite B is chaotic.Comment: 4 pages, 4 figure
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