4,113 research outputs found
Calculation of Giant Magnetoresistance in Laterally Confined Multilayers
We have studied the Giant Magnetoresistance (GMR) for laterally confined
multilayers, e.g., layers of wires, using the classical Boltzmann equation in
the current-in-plane (CIP) geometry. For spin-independent specularity factors
at the sides of the wires we find that the GMR due to bulk and surface
scattering decreases with lateral confinement. The length scale at which this
occurs is of order the film thickness and the mean free paths. The precise
prefactor depends on the relative importance of surface and bulk scattering
anisotropies. For spin-dependent specularity factors at the sides of the wires
the GMR can increase in some cases with decreasing width. The origin of the
change in the GMR in both cases can be understood in terms of lateral
confinement changing the effective mean free paths within the layers.Comment: 18 pages, 7 figure
Boundary scattering of phonons: specularity of a randomly rough surface in the small perturbation limit
Scattering of normally incident longitudinal and transverse acoustic waves by
a randomly rough surface of an elastically isotropic solid is analyzed within
the small perturbation approach. In the limiting case of a large correlation
length compared with the acoustic wavelength, the specularity reduction is
given by , where is the RMS roughness and is the
acoustic wavevector, which is in agreement with the well-known Kirchhoff
approximation result often referred to as Ziman's equation [J. M. Ziman,
Electrons and Phonons (Clarendon Press, Oxford, 1960)]. In the opposite
limiting case of a small correlation length, the specularity reduction is found
to be proportional to , with the fourth power dependence on
frequency as in Rayleigh scattering. Numerical calculations for a Gaussian
autocorrelation function of surface roughness connect these limiting cases and
reveal a maximum of diffuse scattering at an intermediate value of . This
maximum becomes increasingly pronounced for the incident longitudinal wave as
the Poisson's ratio of the medium approaches 1/2 as a result of increased
scattering into transverse and Rayleigh surface waves. The results indicate
that thermal transport models using Ziman's formula are likely to overestimate
the heat flux dissipation due to boundary scattering, whereas modeling
interface roughness as atomic disorder is likely to underestimate scattering
Monte Carlo simulations for phonon transport in silicon nanomaterials
In nanostructures phonon transport behaviour is distinctly different to
transport in bulk materials such that materials with ultra low thermal
conductivities and enhanced thermoelectric performance can be realized. Low
thermal conductivities have been achieved in nanocrystalline materials that
include hierarchical sizes of inclusions and pores. Nanoporous structures
present a promising set of material properties and structures which allow for
ultra-low thermal conductivity, even below the amorphous limit. In this paper
we outline a semiclassical Monte Carlo code for the study of phonon transport
and present an investigation of the thermal conductivity in nanoporous and
nanocrystalline silicon. Different disordered geometry configurations are
incorporated to investigate the effects of pores and grain boundaries on the
phonon flux and the thermal conductivity, including the effects of boundary
roughness, pore position and pore diameter. At constant porosity, thermal
conductivity reduction is maximized by having a large number of smaller
diameter pores as compared to a small number of larger diameter pores.
Furthermore, we show that porosity has a greater impact on thermal conductivity
than the degree of boundary roughness. Our simulator is validated across
multiple simulation and experimental works for both pristine silicon channels
and nanoporous structures.Comment: 10 pages, 8 figure
A deep learning framework for quality assessment and restoration in video endoscopy
Endoscopy is a routine imaging technique used for both diagnosis and
minimally invasive surgical treatment. Artifacts such as motion blur, bubbles,
specular reflections, floating objects and pixel saturation impede the visual
interpretation and the automated analysis of endoscopy videos. Given the
widespread use of endoscopy in different clinical applications, we contend that
the robust and reliable identification of such artifacts and the automated
restoration of corrupted video frames is a fundamental medical imaging problem.
Existing state-of-the-art methods only deal with the detection and restoration
of selected artifacts. However, typically endoscopy videos contain numerous
artifacts which motivates to establish a comprehensive solution.
We propose a fully automatic framework that can: 1) detect and classify six
different primary artifacts, 2) provide a quality score for each frame and 3)
restore mildly corrupted frames. To detect different artifacts our framework
exploits fast multi-scale, single stage convolutional neural network detector.
We introduce a quality metric to assess frame quality and predict image
restoration success. Generative adversarial networks with carefully chosen
regularization are finally used to restore corrupted frames.
Our detector yields the highest mean average precision (mAP at 5% threshold)
of 49.0 and the lowest computational time of 88 ms allowing for accurate
real-time processing. Our restoration models for blind deblurring, saturation
correction and inpainting demonstrate significant improvements over previous
methods. On a set of 10 test videos we show that our approach preserves an
average of 68.7% which is 25% more frames than that retained from the raw
videos.Comment: 14 page
Phononic thermal conductivity in silicene: the role of vacancy defects and boundary scattering
We calculate the thermal conductivity of free-standing silicene using the
phonon Boltzmann transport equation within the relaxation time approximation.
In this calculation, we investigate the effects of sample size and different
scattering mechanisms such as phonon-phonon, phonon-boundary, phonon-isotope
and phonon-vacancy defect. Moreover, the role of different phonon modes is
examined. We show that, in contrast to graphene, the dominant contribution to
the thermal conductivity of silicene originates from the in-plane acoustic
branches, which is about 70\% at room temperature and this contribution becomes
larger by considering vacancy defects. Our results indicate that while the
thermal conductivity of silicene is significantly suppressed by the vacancy
defects, the effect of isotopes on the phononic transport is small. Our
calculations demonstrate that by removing only one of every 400 silicon atoms,
a substantial reduction of about 58\% in thermal conductivity is achieved.
Furthermore, we find that the phonon-boundary scattering is important in
defectless and small-size silicene samples, specially at low temperatures.Comment: 9 pages, 11 figure
- …