337 research outputs found
Multi-scale Modelling of Natural Composites Using Thermodynamics-Based Artificial Neural Networks and Dimensionality Reduction Techniques
Modelling natural composites, as the majority of real geomaterials,
requires facing their intrinsic multiscale nature. This allows to consider multiphysics
coupling occurring at the microscale, then reflected onto the macroscopic
behavior. Geotechnics is constantly requiring reliable constitutive models of natural
compolve large-scale engineering problems accurately and efficiently.
This need motivates the contribution. To capture in detail the macroscopic effects
of microssites to socopic processes, many authors have developed multi-scale numerical
schemes. A common drawback of such methods is the prohibitive computational
cost. Recently,Machine Learning based approaches have raised as promising alternatives
to traditional methods. Artificial Neural Networks – ANNs – have been
used to predict the constitutive behaviour of complex, heterogeneous materials,
with reduced calculation costs. However, a major weakness of ANN is the lack of
a rigorous framework based on principles of physics. This often implies a limited
capability to extrapolate values ranging outside the training set and the need of
large, high-quality datasets, on which performing the training. This work focuses
on the use of Thermodynamics-based Artificial Neural Networks – TANN – to
predict the constitutive behaviour of natural composites. Dimensionality reduction
techniques – DRTs – are used to embed information of microscopic processes
into a lower dimensional manifold. The obtained set of variables is used to characterize
the state of the material at the macroscopic scale. Entanglement of DRTs
with TANN allows to reproduce the complex nonlinear material response with
reduced computational costs and guarantying thermodynamic admissibility. To
demonstrate the method capabilities an application to a heterogeneous material
model is presented
Complex band structure and plasmon lattice Green's function of a periodic metal-nanoparticle chain
When the surface plasmon resonance in a metal-nanoparticle chain is excited
at one point, the response signal will generally decay down the chain due to
absorption and radiation losses. The decay length is a key parameter in such
plasmonic systems. By studying the plasmon lattice Green's function, we found
that the decay length is generally governed by two exponential decay constants
with phase factors corresponding to guided Bloch modes and one power-law decay
with a phase factor corresponding to that of free space photons. The results
show a high level of similarity between the absorptive and radiative decay
channels. By analyzing the poles (and the corresponding residues) of the
Green's function in a transformed complex reciprocal space, the dominant decay
channel of the real-space Green's function is understood.Comment: 19 pages, 3 figure
Mie resonances and bonding in photonic crystals
Isolated dielectric spheres support resonant electromagnetic modes which are
analogous to electronic orbitals and, like their electronic counterparts, can
form bonding or anti-bonding interactions between neighbouring spheres. By
irradiating the system with light at the bonding frequency an attractive
interaction is induced between the spheres. We suggest that by judicious
selection of bonding states we can drive a system towards a desired structure,
rather than rely on the structure dictated by gravitational or Van der Waals
forces, the latter deriving from the zero point energy population of a state.Comment: Minor changes in text, of explanatory nature. 6 pages, Latex, 6
figures, accepted by Europhysics Letter
Work in progress — Role of faculty in promoting lifelong learning: Initial findings
Calls for educational reform emphasize the
need for student-centered learning approaches that
foster lifelong learning. To be a lifelong learner includes
characteristics consistent with those of self-directed
learners, such as being curious, motivated, reflective,
analytical, persistent, flexible, and independent.
Instructor support of students’ self-directed learning
(SDL) development relies on understanding and
balancing these factors in the classroom. Engineering
educators play a critical role in influencing outcomes
related to SDL through their design of courses that
support students’ transitions from controlled to
autonomous learning behaviors. This study will examine
a variety of engineering courses and pedagogical
approaches. Each will be characterized using instructor
course information, recorded observations of instructorstudent
and student-student interactions, student and
instructor responses to surveys, and focus groups.
Finally, the students’ capacity for SDL will be measured
using the Motivated Strategies for Learning
Questionnaire. This approach should provide for rich,
contextualized descriptions of what instructors and
learners do, how instructors and students relate to each
other, and how students view their classrooms. This
work-in-progress paper will describe our initial work in
this multiyear study
Scattering of elastic waves by periodic arrays of spherical bodies
We develop a formalism for the calculation of the frequency band structure of
a phononic crystal consisting of non-overlapping elastic spheres, characterized
by Lam\'e coefficients which may be complex and frequency dependent, arranged
periodically in a host medium with different mass density and Lam\'e
coefficients. We view the crystal as a sequence of planes of spheres, parallel
to and having the two dimensional periodicity of a given crystallographic
plane, and obtain the complex band structure of the infinite crystal associated
with this plane. The method allows one to calculate, also, the transmission,
reflection, and absorption coefficients for an elastic wave (longitudinal or
transverse) incident, at any angle, on a slab of the crystal of finite
thickness. We demonstrate the efficiency of the method by applying it to a
specific example.Comment: 19 pages, 5 figures, Phys. Rev. B (in press
A simple formula for the L-gap width of a face-centered-cubic photonic crystal
The width of the first Bragg's scattering peak in the (111)
direction of a face-centered-cubic lattice of air spheres can be well
approximated by a simple formula which only involves the volume averaged
and over the lattice unit cell, being the
(position dependent) dielectric constant of the medium, and the effective
dielectric constant in the long-wavelength limit approximated
by Maxwell-Garnett's formula. Apparently, our formula describes the asymptotic
behaviour of the absolute gap width for high dielectric contrast
exactly. The standard deviation steadily decreases well below
1% as increases. For example for the sphere filling
fraction and . On the interval , our
formula still approximates the absolute gap width (the relative
gap width ) with a reasonable precision, namely with a standard
deviation 3% (4.2%) for low filling fractions up to 6.5% (8%) for the
close-packed case. Differences between the case of air spheres in a dielectric
and dielectric spheres in air are briefly discussed.Comment: 13 pages, 4 figs., RevTex, two references added. For more info see
http://www.amolf.nl/external/wwwlab/atoms/theory/index.htm
Onset of magnetism in B2 transition metals aluminides
Ab initio calculation results for the electronic structure of disordered bcc
Fe(x)Al(1-x) (0.4<x<0.75), Co(x)Al(1-x) and Ni(x)Al(1-x) (x=0.4; 0.5; 0.6)
alloys near the 1:1 stoichiometry, as well as of the ordered B2 (FeAl, CoAl,
NiAl) phases with point defects are presented. The calculations were performed
using the coherent potential approximation within the Korringa-Kohn-Rostoker
method (KKR-CPA) for the disordered case and the tight-binding linear
muffin-tin orbital (TB-LMTO) method for the intermetallic compounds. We studied
in particular the onset of magnetism in Fe-Al and Co-Al systems as a function
of the defect structure. We found the appearance of large local magnetic
moments associated with the transition metal (TM) antisite defect in FeAl and
CoAl compounds, in agreement with the experimental findings. Moreover, we found
that any vacancies on both sublattices enhance the magnetic moments via
reducing the charge transfer to a TM atom. Disordered Fe-Al alloys are
ferromagnetically ordered for the whole range of composition studied, whereas
Co-Al becomes magnetic only for Co concentration >0.5.Comment: 11 pages with 9 embedded postscript figures, to be published in
Phys.Rev.
Primary angiosarcoma of the lung and pleura
A 46 year old male smoker was admitted for severe continuing hemoptysis. Chest-X-rays and chest computed tomography revealed nodular infiltrates and bilateral hemothorax. Fiberoptic bronchoscopy resulted to non-diagnostic cytological and microbiological findings. Open lung and pleural biopsies after right thoracotomy revealed epithelioid angiosarcoma and further staging assessment showed secondary brain and liver foci. The patient received several courses of chemotherapy but he died one month later. The clinical, radiological, pathological, histochemical and therapeutic aspects of the disease are discussed. Angiosarcoma, a rare tumour with poor prognosis should be taken into consideration in the differential diagnosis of hemoptysis
Screened Coulomb interactions in metallic alloys: I. Universal screening in the atomic sphere approximation
We have used the locally self-consistent Green's function (LSGF) method in
supercell calculations to establish the distribution of the net charges
assigned to the atomic spheres of the alloy components in metallic alloys with
different compositions and degrees of order. This allows us to determine the
Madelung potential energy of a random alloy in the single-site mean field
approximation which makes the conventional single-site density-functional-
theory coherent potential approximation (SS-DFT-CPA) method practically
identical to the supercell LSGF method with a single-site local interaction
zone that yields an exact solution of the DFT problem. We demonstrate that the
basic mechanism which governs the charge distribution is the screening of the
net charges of the alloy components that makes the direct Coulomb interactions
short-ranged. In the atomic sphere approximation, this screening appears to be
almost independent of the alloy composition, lattice spacing, and crystal
structure. A formalism which allows a consistent treatment of the screened
Coulomb interactions within the single-site mean-filed approximation is
outlined. We also derive the contribution of the screened Coulomb interactions
to the S2 formalism and the generalized perturbation method.Comment: 28 pages, 8 figure
Resonant Visible Light Modulation with Graphene
Fast modulation and switching of light at visible and near-infrared (vis-NIR)
frequencies is of utmost importance for optical signal processing and sensing
technologies. No fundamental limit appears to prevent us from designing
wavelength-sized devices capable of controlling the light phase and intensity
at gigaherts (and even terahertz) speeds in those spectral ranges. However,
this problem remains largely unsolved, despite recent advances in the use of
quantum wells and phase-change materials for that purpose. Here, we explore an
alternative solution based upon the remarkable electro-optical properties of
graphene. In particular, we predict unity-order changes in the transmission and
absorption of vis-NIR light produced upon electrical doping of graphene sheets
coupled to realistically engineered optical cavities. The light intensity is
enhanced at the graphene plane, and so is its absorption, which can be switched
and modulated via Pauli blocking through varying the level of doping.
Specifically, we explore dielectric planar cavities operating under either
tunneling or Fabry-Perot resonant transmission conditions, as well as Mie modes
in silicon nanospheres and lattice resonances in metal particle arrays. Our
simulations reveal absolute variations in transmission exceeding 90% as well as
an extinction ratio >15 dB with small insertion losses using feasible material
parameters, thus supporting the application of graphene in fast electro-optics
at vis-NIR frequencies.Comment: 17 pages, 13 figures, 54 reference
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