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Multiscale simulations of the electronic structure of III-nitride quantum wells with varied indium content: Connecting atomistic and continuum-based models
Carrier localization effects in III-N heterostructures are often studied in the frame of modified continuum-based models utilizing a single-band effective mass approximation. However, there exists no comparison between the results of a modified continuum model and atomistic calculations on the same underlying disordered energy landscape. We present a theoretical framework that establishes a connection between atomistic tight-binding theory and continuum-based electronic structure models, here a single-band effective mass approximation, and provide such a comparison for the electronic structure of (In,Ga)N quantum wells. In our approach, in principle, the effective masses are the only adjustable parameters since the confinement energy landscape is directly obtained from tight-binding theory. We find that the electronic structure calculated within effective mass approximation and the tight-binding model differ noticeably. However, at least in terms of energy eigenvalues, an improved agreement between the two methods can be achieved by adjusting the band offsets in the continuum model, enabling, therefore, a recipe for constructing a modified continuum model that gives a reasonable approximation of the tight-binding energies. Carrier localization characteristics for energetically low lying, strongly localized states differ, however, significantly from those obtained using the tight-binding model. For energetically higher lying, more delocalized states, good agreement may be achieved. Therefore, the atomistically motivated continuum-based single-band effective mass model established provides a good, computationally efficient alternative to fully atomistic investigations, at least at when targeting questions related to higher temperatures and carrier densities in (In,Ga)N systems
The Challenge of Wide-Field Transit Surveys: The Case of GSC 01944-02289
Wide-field searches for transiting extra-solar giant planets face the
difficult challenge of separating true transit events from the numerous false
positives caused by isolated or blended eclipsing binary systems. We describe
here the investigation of GSC 01944-02289, a very promising candidate for a
transiting brown dwarf detected by the Transatlantic Exoplanet Survey (TrES)
network. The photometry and radial velocity observations suggested that the
candidate was an object of substellar mass in orbit around an F star. However,
careful analysis of the spectral line shapes revealed a pattern of variations
consistent with the presence of another star whose motion produced the
asymmetries observed in the spectral lines of the brightest star. Detailed
simulations of blend models composed of an eclipsing binary plus a third star
diluting the eclipses were compared with the observed light curve and used to
derive the properties of the three components. Our photometric and
spectroscopic observations are fully consistent with a blend model of a
hierarchical triple system composed of an eclipsing binary with G0V and M3V
components in orbit around a slightly evolved F5 dwarf. We believe that this
investigation will be helpful to other groups pursuing wide-field transit
searches as this type of false detection could be more common than true
transiting planets, and difficult to identify.Comment: To appear in ApJ, v. 621, 2005 March 1
Fermi-Liquid Interactions in d-Wave Superconductor
This article develops a quantitative quasiparticle model of the
low-temperature properties of d-wave superconductors which incorporates both
Fermi-liquid effects and band-structure effects. The Fermi-liquid interaction
effects are found to be classifiable into strong and negligible renormalizaton
effects, for symmetric and antisymmetric combinations of the energies of
and quasiparticles, respectively. A particularly
important conclusion is that the leading clean-limit temperature-dependent
correction to the superfluid density is not renormalized by Fermi-liquid
interactions, but is subject to a Fermi velocity (or mass) renormalization
effect. This leads to difficulties in accounting for the penetration depth
measurements with physically acceptable parameters, and hence reopens the
question of the quantitative validity of the quasiparticle picture.Comment: 4 page
Orthorhombicity mixing of s- and d- gap components in without involving the chains
Momentum decoupling develops when forward scattering dominates the pairing
interaction and implies tendency for decorrelation between the physical
behavior in the various regions of the Fermi surface. In this regime it is
possible to obtain anisotropic s- or d-wave superconductivity even with
isotropic pairing scattering. We show that in the momentum decoupling regime
the distortion of the planes is enough to explain the experimental
reports for s- mixing in the dominantly d-wave gap of . In the
case of spin fluctuations mediated pairing instead, a large part of the
condensate must be located in the chains in order to understand the
experiments.Comment: LATEX file and 3 Postscript figure
Superconducting gap node spectroscopy using nonlinear electrodynamics
We present a method to determine the nodal structure of the energy gap of
unconventional superconductors such as high materials. We show how
nonlinear electrodynamics phenomena in the Meissner regime, arising from the
presence of lines on the Fermi surface where the superconducting energy gap is
very small or zero, can be used to perform ``node spectroscopy'', that is, as a
sensitive bulk probe to locate the angular position of those lines. In
calculating the nonlinear supercurrent response, we include the effects of
orthorhombic distortion and plane anisotropy. Analytic results presented
demonstrate a systematic way to experimentally distinguish order parameters of
different symmetries, including cases with mixed symmetry (for example,
and ). We consider, as suggested by various experiments, order parameters
with predominantly -wave character, and describe how to determine the
possible presence of other symmetries. The nonlinear magnetic moment displays a
distinct behavior if nodes in the gap are absent but regions with small,
finite, values of the energy gap exist.Comment: 18 pages, Revtex, 9 postscript figures. Submitted to Phys. Rev
Unveiling Relations in the Industry 4.0 Standards Landscape based on Knowledge Graph Embeddings
Industry~4.0 (I4.0) standards and standardization frameworks have been
proposed with the goal of \emph{empowering interoperability} in smart
factories. These standards enable the description and interaction of the main
components, systems, and processes inside of a smart factory. Due to the
growing number of frameworks and standards, there is an increasing need for
approaches that automatically analyze the landscape of I4.0 standards.
Standardization frameworks classify standards according to their functions into
layers and dimensions. However, similar standards can be classified differently
across the frameworks, producing, thus, interoperability conflicts among them.
Semantic-based approaches that rely on ontologies and knowledge graphs, have
been proposed to represent standards, known relations among them, as well as
their classification according to existing frameworks. Albeit informative, the
structured modeling of the I4.0 landscape only provides the foundations for
detecting interoperability issues. Thus, graph-based analytical methods able to
exploit knowledge encoded by these approaches, are required to uncover
alignments among standards. We study the relatedness among standards and
frameworks based on community analysis to discover knowledge that helps to cope
with interoperability conflicts between standards. We use knowledge graph
embeddings to automatically create these communities exploiting the meaning of
the existing relationships. In particular, we focus on the identification of
similar standards, i.e., communities of standards, and analyze their properties
to detect unknown relations. We empirically evaluate our approach on a
knowledge graph of I4.0 standards using the Trans family of embedding
models for knowledge graph entities. Our results are promising and suggest that
relations among standards can be detected accurately.Comment: 15 pages, 7 figures, DEXA2020 Conferenc
Frame Theory for Signal Processing in Psychoacoustics
This review chapter aims to strengthen the link between frame theory and
signal processing tasks in psychoacoustics. On the one side, the basic concepts
of frame theory are presented and some proofs are provided to explain those
concepts in some detail. The goal is to reveal to hearing scientists how this
mathematical theory could be relevant for their research. In particular, we
focus on frame theory in a filter bank approach, which is probably the most
relevant view-point for audio signal processing. On the other side, basic
psychoacoustic concepts are presented to stimulate mathematicians to apply
their knowledge in this field
Disorder and chain superconductivity in YBa_2Cu_3O_{7-\delta}
The effects of chain disorder on superconductivity in YBa_2Cu_3O_{7-\delta}
are discussed within the context of a proximity model. Chain disorder causes
both pair-breaking and localization. The hybridization of chain and plane
wavefunctions reduces the importance of localization, so that the transport
anisotropy remains large in the presence of a finite fraction of
oxygen vacancies. Penetration depth and specific heat measurements probe the
pair-breaking effects of chain disorder, and are discussed in detail at the
level of the self-consistent T-matrix approximation. Quantitative agreement
with these experiments is found when chain disorder is present.Comment: 4 pages, 2 figures, submitted to PRB rapid communication
Transit Photometry as an Exoplanet Discovery Method
Photometry with the transit method has arguably been the most successful
exoplanet discovery method to date. A short overview about the rise of that
method to its present status is given. The method's strength is the rich set of
parameters that can be obtained from transiting planets, in particular in
combination with radial velocity observations; the basic principles of these
parameters are given. The method has however also drawbacks, which are the low
probability that transits appear in randomly oriented planet systems, and the
presence of astrophysical phenomena that may mimic transits and give rise to
false detection positives. In the second part we outline the main factors that
determine the design of transit surveys, such as the size of the survey sample,
the temporal coverage, the detection precision, the sample brightness and the
methods to extract transit events from observed light curves. Lastly, an
overview over past, current and future transit surveys is given. For these
surveys we indicate their basic instrument configuration and their planet
catch, including the ranges of planet sizes and stellar magnitudes that were
encountered. Current and future transit detection experiments concentrate
primarily on bright or special targets, and we expect that the transit method
remains a principal driver of exoplanet science, through new discoveries to be
made and through the development of new generations of instruments.Comment: Review chapte
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