588 research outputs found
Model fitting and Bayesian inference via power expectation propagation
We study a message passing approach to power expectation propagation for Bayesian model fitting and inference. Power expectation propagation is a class of variational approximations based on the notion of α-divergence that extends two notable approximations, namely mean field variational Bayes and expectation propagation. An illustration on a simple model allows to grasp benefits and complexities of this methodology and sets the basis for applications on more complex models
Shape-independent scaling of excitonic confinement in realistic quantum wires
The scaling of exciton binding energy in semiconductor quantum wires is
investigated theoretically through a non-variational, fully three-dimensional
approach for a wide set of realistic state-of-the-art structures. We find that
in the strong confinement limit the same potential-to-kinetic energy ratio
holds for quite different wire cross-sections and compositions. As a
consequence, a universal (shape- and composition-independent) parameter can be
identified that governs the scaling of the binding energy with size. Previous
indications that the shape of the wire cross-section may have important effects
on exciton binding are discussed in the light of the present results.Comment: To appear in Phys. Rev. Lett. (12 pages + 2 figures in postscript
Polaron effects in electron channels on a helium film
Using the Feynman path-integral formalism we study the polaron effects in
quantum wires above a liquid helium film. The electron interacts with
two-dimensional (2D) surface phonons, i.e. ripplons, and is confined in one
dimension (1D) by an harmonic potential. The obtained results are valid for
arbitrary temperature (), electron-phonon coupling strength (), and
lateral confinement (). Analytical and numerical results are
obtained for limiting cases of , , and . We found the
surprising result that reducing the electron motion from 2D to quasi-1D makes
the self-trapping transition more continuous.Comment: 6 pages, 7 figures, submitted to Phys. Rev.
Mean parameter model for the Pekar-Fr\"{o}hlich polaron in a multilayered heterostructure
The polaron energy and the effective mass are calculated for an electron
confined in a finite quantum well constructed of
layers. To simplify the study we suggest a model in which parameters of a
medium are averaged over the ground-state wave function. The rectangular and
the Rosen-Morse potential are used as examples.
To describe the confined electron properties explicitly to the second order
of perturbations in powers of the electron-phonon coupling constant we use the
exact energy-dependent Green function for the Rosen-Morse confining potential.
In the case of the rectangular potential, the sum over all intermediate virtual
states is calculated. The comparison is made with the often used leading term
approximation when only the ground-state is taken into account as a virtual
state. It is shown that the results are quite different, so the incorporation
of all virtual states and especially those of the continuous spectrum is
essential.
Our model reproduces the correct three-dimensional asymptotics at both small
and large widths. We obtained a rather monotonous behavior of the polaron
energy as a function of the confining potential width and found a peak of the
effective mass. The comparison is made with theoretical results by other
authors. We found that our model gives practically the same (or very close)
results as the explicit calculations for potential widths .Comment: 12 pages, LaTeX, including 5 PS-figures, subm. to Phys. Rev. B, new
data are discusse
Triple-negative breast cancer: Present challenges and new perspectives
Contains fulltext :
83751.pdf (publisher's version ) (Open Access)21 p
Designing touch-enabled electronic flight bags in SAR helicopter operations
In order to benefit from potential reduced operational costs and crew workload airlines are increasingly interested in touchscreen-based Electronic Flight Bags (EFB). This paper focuses on the specific domain of Search and Rescue (SAR) Helicopters. A first set of results aiming to explore and understand potential benefits and challenges of an EFB in a SAR environment will be presented. A review of related work, operational observations and interviews with pilots were conducted to understand and specify the use context. Digital Human Modelling (DHM) software was used to determine physical constraints of an EFB in this type of flight deck. A scenario was developed which will be used in future to define features, content and functionality that a SAR pilot may wish to see in an EFB. Developed initial interface design guidelines are presented
Breast imaging technology: Application of magnetic resonance imaging to angiogenesis in breast cancer
Magnetic resonance imaging (MRI) techniques enable vascular function to be mapped with high spatial resolution. Current methods for imaging in breast cancer are described, and a review of recent studies that compared dynamic contrast-enhanced MRI with histopathological indicators of tumour vascular status is provided. These studies show correlation between in vivo dynamic contrast measurements and in vitro histopathology. Dynamic contrast enhanced MRI is also being applied to assessment of the response of breast tumours to treatment
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