191 research outputs found
Quasiparticle band structure based on a generalized Kohn-Sham scheme
We present a comparative full-potential study of generalized Kohn-Sham
schemes (gKS) with explicit focus on their suitability as starting point for
the solution of the quasiparticle equation. We compare quasiparticle
band structures calculated upon LDA, sX, HSE03, PBE0, and HF functionals for
exchange and correlation (XC) for Si, InN and ZnO. Furthermore, the HSE03
functional is studied and compared to the GGA for 15 non-metallic materials for
its use as a starting point in the calculation of quasiparticle excitation
energies. For this case, also the effects of selfconsistency in the
self-energy are analysed. It is shown that the use of a gKS scheme as a
starting point for a perturbative QP correction can improve upon the
deficiencies found for LDA or GGA staring points for compounds with shallow
bands. For these solids, the order of the valence and conduction bands is often
inverted using local or semi-local approximations for XC, which makes
perturbative calculations unreliable. The use of a gKS starting point
allows for the calculation of fairly accurate band gaps even in these difficult
cases, and generally single-shot calculations following calculations
using the HSE03 functional are very close to experiment
Dielectric tensor of monoclinic GaO single crystals in the spectral range eV
The dielectric tensor of -GaO was determined by generalized
spectroscopic ellipsometry in a wide spectral range from to
as well as by calculation including quasiparticle bands and
excitonic effects. The dielectric tensors obtained by both methods are in
excellent agreement with each other and the observed transitions in the
dielectric function are assigned to the corresponding valence bands. It is
shown that the off-diagonal element of the dielectric tensor reaches values up
to and cannot be neglected. Even in the
transparent spectral range where it is quite small () it causes a rotation of the dielectric axes around the symmetry axis of up
to
Verfahren und Protokolle für energiebewusste, gemeinsame Ressourcenverwendung mit mobilen Geräten
Voraussetzung für die gemeinsame Nutzung von Betriebsmitteln mobiler Geräte sind Verfahren, die es erlauben, den Energiebedarf für die Bereitstellung von Ressourcen abzuschätzen, unter mehreren Ressourcenanbietern einen geeigneten auszuwählen und diesen für den erbrachten Energieaufwand zu entschädigen. Diese Arbeit stellt hierfür entwickelte Verfahren vor, die insbesondere das geringe Energiebudget mobiler Geräte berücksichtigen
GW band structure of InAs and GaAs in the wurtzite phase
We report the first quasiparticle calculations of the newly observed wurtzite
polymorph of InAs and GaAs. The calculations are performed in the GW
approximation using plane waves and pseudopotentials. For comparison we also
report the study of the zinc-blende phase within the same approximations. In
the InAs compound the In 4d electrons play a very important role: whether they
are frozen in the core or not, leads either to a correct or a wrong band
ordering (negative gap) within the Local Density Appproximation (LDA). We have
calculated the GW band structure in both cases. In the first approach, we have
estimated the correction to the pd repulsion calculated within the LDA and
included this effect in the calculation of the GW corrections to the LDA
spectrum. In the second case, we circumvent the negative gap problem by first
using the screened exchange approximation and then calculating the GW
corrections starting from the so obtained eigenvalues and eigenfunctions. This
approach leads to a more realistic band-structure and was also used for GaAs.
For both InAs and GaAs in the wurtzite phase we predict an increase of the
quasiparticle gap with respect to the zinc-blende polytype.Comment: 9 pages, 6 figures, 3 table
Retrospective Measurement of Students’ Extracurricular Activities with a Self-administered Calendar Instrument
With the expansion of all-day schooling in Germany, students’ extracurricular activities are being brought into greater focus in educational and social sciences. However, the diverse range of activities and individual biographies makes it difficult to gather data on the variety and periods of extracurricular activities in classroom-based surveys. This paper introduces a tailored calendar instrument that was applied by the Study on the Development of All-Day Schools (StEG) to retrospectively survey the activities of senior students since the fifth grade. Unlike other calendar applications, the calendar was not filled in by trained staff but self-administered by the students in a group setting. We discuss methodological issues regarding this procedure by examining the current state of research and by sharing experiences of tests of the instrument prior to the survey. By further analysing the survey data, we find no indication that the calendar task induces higher non-response as a result of overburdening the respondents. Calendar elements with an open-ended format resulted in heterogeneous reports, which were nonetheless mostly suitable for further analysis. According to our findings, the number of reported activities does not vary for students with longer intervals of retrospection. From our results, we conclude that a calendar instrument can be successfully applied in classroom-based surveys but should be implemented with a step-by-step procedure under a supervisor’s guidance
Electronic and Optical Properties of Alkaline Earth Metal Fluoride Crystals with the Inclusion of Many-Body Effects: A Comparative Study on Rutile MgF2 and Cubic SrF2
We conducted a systematic investigation using state-of-the-art techniques on the electronic and optical properties of two crystals of alkaline earth metal fluorides, namely rutile MgF2
and cubic SrF2
. For these two crystals of different symmetry, we present density functional theory (DFT), many-body perturbation theory (MBPT), and Bethe–Salpeter equation (BSE) calculations. We calculated a variety of properties, namely ground-state energies, band-energy gaps, and optical absorption spectra with the inclusion of excitonic effects. The quantities were obtained with a high degree of convergence regarding all bulk electronic and optical properties. Bulk rutile MgF2
has distinguished ground-state and excited-state properties with respect to the other cubic fluoride SrF2
and the other members of the alkaline earth metal fluoride family. The nature of the fundamental gaps and estimates of the self-energy and excitonic effects for the two compounds are presented and discussed in detail. Our results are in good accordance with the measurements and other theoretical–computational data. A comparison is made between the excitation and optical properties of bulk rutile MgF2
, cubic SrF2
, and the corresponding clusters, for which calculations have recently been published, confirming strong excitonic effects in finite-sized systems
Overcoming a Communication Barrier on the Way Towards a Global Sensor Network
In a global sensor network different sensor platforms will be deployed. A
grave obstacle on the way of building sensor networks out of different sensor nodes are incompatible implementations of network protocol stacks used with different sensor node platforms. We describe our efforts to overcome this obstacle in a heterogeneous sensor network consisting out of MICAz Motes and Sun SPOTs, both using an IEEE 802.15.4 radio chip. We explain the major differences in the respective network stacks and our approach to bridge them. A network stack that bridges the gap between different platforms allows for more flexible and robust networks
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