474 research outputs found
Optimally Sparse Frames
Frames have established themselves as a means to derive redundant, yet stable
decompositions of a signal for analysis or transmission, while also promoting
sparse expansions. However, when the signal dimension is large, the computation
of the frame measurements of a signal typically requires a large number of
additions and multiplications, and this makes a frame decomposition intractable
in applications with limited computing budget. To address this problem, in this
paper, we focus on frames in finite-dimensional Hilbert spaces and introduce
sparsity for such frames as a new paradigm. In our terminology, a sparse frame
is a frame whose elements have a sparse representation in an orthonormal basis,
thereby enabling low-complexity frame decompositions. To introduce a precise
meaning of optimality, we take the sum of the numbers of vectors needed of this
orthonormal basis when expanding each frame vector as sparsity measure. We then
analyze the recently introduced algorithm Spectral Tetris for construction of
unit norm tight frames and prove that the tight frames generated by this
algorithm are in fact optimally sparse with respect to the standard unit vector
basis. Finally, we show that even the generalization of Spectral Tetris for the
construction of unit norm frames associated with a given frame operator
produces optimally sparse frames
Performance adaptive manufacturing processes
Part of:
Seliger, Günther (Ed.): Innovative solutions : proceedings / 11th Global Conference on Sustainable Manufacturing, Berlin, Germany, 23rd - 25th September, 2013. - Berlin: Universitätsverlag der TU Berlin, 2013. - ISBN 978-3-7983-2609-5 (online). - http://nbn-resolving.de/urn:nbn:de:kobv:83-opus4-40276. - pp. 296-301.Energy efficiency is of increasing importance towards sustainable manufacturing in the automotive industry, in particular due to growing environment regulations and rising electricity costs. Approaches within the manufacturing planning phase are insufficient to address dynamic influences during run-time (e.g., electricity tariffs or workload). Additionally, conventional production monitoring and control systems consider the ‘Overall Equipment Effectiveness‘ of manufacturing systems, but do not include related energy efficiency. This paper introduces a novel approach that combines these both aspects and provides more effectiveness based on socalled production variants. The latter are designed during the planning phase and used to adapt manufacturing behavior when facing dynamically changes during run-time. A simulation shows how dynamic adjustments of cycle times lead to a high reduction of energy costs while maintaining high throughputs
Upper critical field pecularities of superconducting YNi2B2C and LuNi2B2C
We present new upper critical field Hc2(T) data in a broad temperature region
from 0.3K to Tc for LuNi2B2C and YNi2B2C single crystals with well
characterized low impurity scattering rates. The absolute values for all T, in
particular Hc2(0), and the sizeable positive curvature (PC) of Hc2(T) at high
and intermediate T are explained quantitatively within an effective two-band
model. The failure of the isotropic single band approach is discussed in
detail. Supported by de Haas van Alphen data, the superconductivity reveals
direct insight into details of the electronic structure. The observed maximal
PC near Tc gives strong evidence for clean limit type II superconductors.Comment: 4 pages, 2 figures, Phys. Rev. Lett. accepte
Theory of de Haas-van Alphen Effect in Type-II Superconductors
Theory of quasiparticle spectra and the de Haas-van Alphen (dHvA) oscillation
in type-II superconductors are developed based on the Bogoliubov-de Gennes
equations for vortex-lattice states. As the pair potential grows through the
superconducting transition, each degenerate Landau level in the normal state
splits into quasiparticle bands in the magnetic Brillouin zone. This brings
Landau-level broadening, which in turn leads to the extra dHvA oscillation
damping in the vortex state. We perform extensive numerical calculations for
three-dimensional systems with various gap structures. It is thereby shown that
(i) this Landau-level broadening is directly connected with the average gap at
H=0 along each Fermi-surface orbit perpendicular to the field H; (ii) the extra
dHvA oscillation attenuation is caused by the broadening around each extremal
orbit. These results imply that the dHvA experiment can be a unique probe to
detect band- and/or angle-dependent gap amplitudes. We derive an analytic
expression for the extra damping based on the second-order perturbation with
respect to the pair potential for the Luttinger-Ward thermodynamic potential.
This formula reproduces all our numerical results excellently, and is used to
estimate band-specific gap amplitudes from available data on NbSe_2, Nb_3Sn,
and YNi_2B_2C. The obtained value for YNi_2B_2C is fairly different from the
one through a specific-heat measurement, indicating presence of gap anisotropy
in this material. C programs to solve the two-dimensional Bogoliubov-de Gennes
equations are available at http://phys.sci.hokudai.ac.jp/~kita/index-e.html .Comment: 16 pages, 11 figure
Comparison of coherent and weakly incoherent transport models for the interlayer magnetoresistance of layered Fermi liquids
The interlayer magnetoresistance of layered metals in a tilted magnetic field
is calculated for two distinct models for the interlayer transport. The first
model involves coherent interlayer transport and makes use of results of
semi-classical or Bloch-Boltzmann transport theory. The second model involves
weakly incoherent interlayer transport where the electron is scattered many
times within a layer before tunneling into the next layer. The results are
relevant to the interpretation of experiments on angular-dependent
magnetoresistance oscillations (AMRO) in quasi-one- and quasi-two-dimensional
metals. We find that the dependence of the magnetoresistance on the direction
of the magnetic field is identical for both models except when the field is
almost parallel to the layers. An important implication of this result is that
a three-dimensional Fermi surface is not necessary for the observation of the
Yamaji and Danner oscillations seen in quasi-two- and quasi-one-dimensional
metals, respectively. A universal expression is given for the dependence of the
resistance at AMRO maxima and minima on the magnetic field and scattering time
(and thus the temperature). We point out three distinctive features of coherent
interlayer transport: (i) a beat frequency in the magnetic oscillations of
quasi-two-dimensional systems, (ii) a peak in the angular-dependent
magnetoresistance when the field is sufficiently large and parallel to the
layers, and (iii) a crossover from a linear to a quadratic field dependence for
the magnetoresistance when the field is parallel to the layers. Properties (i)
and (ii) are compared with published experimental data for a range of
quasi-two-dimensional organic metals and for Sr2RuO4.Comment: 21 pages, RevTeX + epsf, 4 figures. Published version. Subsection
added. References update
Franchising as a Strategy for Combining Small and Large Group Advantages (Logics) in Social Entrepreneurship:A Hayekian Perspective
This article develops a Hayekian perspective on social franchising that distinguishes between the end-connected logic of the small group and the rule-connected logic of the big group. Our key claim is that mission-driven social entrepreneurs often draw on the small-group logic when starting their social ventures and then face difficulties when the process of scaling shifts their operations toward a big-group logic. In this situation, social franchising offers a strategy to replicate the small group despite systemwide scaling, to mobilize decentrally accessible social capital, and to reduce agency costs through mechanisms of self-selection and self-monitoring. By employing a Hayekian perspective, we are thus able to offer an explanation as to why social franchising is a suitable scaling strategy for some social entrepreneurship organizations and not for others. We illustrate our work using the Ashoka Fellow Wellcome
Time-dependent density functional study of the electronic spectra of oligoacenes in the charge states -1, 0, +1, and +2
We present a systematic theoretical study of the five smallest oligoacenes
(naphthalene, anthracene, tetracene, pentacene, and hexacene) in their
anionic,neutral, cationic, and dicationic charge states. We used density
functional theory (DFT) to obtain the ground-state optimised geometries, and
time-dependent DFT (TD-DFT) to evaluate the electronic absorption spectra.
Total-energy differences enabled us to evaluate the electron affinities and
first and second ionisation energies, the quasiparticle correction to the
HOMO-LUMO energy gap and an estimate of the excitonic effects in the neutral
molecules. Electronic absorption spectra have been computed by combining two
different implementations of TD-DFT: the frequency-space method to study
general trends as a function of charge-state and molecular size for the
lowest-lying in-plane long-polarised and short-polarised
electronic transitions, and the real-time propagation scheme to obtain the
whole photo-absorption cross-section up to the far-UV. Doubly-ionised PAHs are
found to display strong electronic transitions of character
in the near-IR, visible, and near-UV spectral ranges, like their singly-charged
counterparts. While, as expected, the broad plasmon-like structure with its
maximum at about 17-18 eV is relatively insensitive to the charge-state of the
molecule, a systematic decrease with increasing positive charge of the
absorption cross-section between about 6 and about 12 eV is observed for each
member of the class.Comment: 38 pages, 11 figures, 7 tables, accepted for publication in Chemical
Physic
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