24 research outputs found
Many-body aspects of positron annihilation in the electron gas
We investigate positron annihilation in electron liquid as a case study for
many-body theory, in particular the optimized Fermi Hypernetted Chain (FHNC-EL)
method. We examine several approximation schemes and show that one has to go up
to the most sophisticated implementation of the theory available at the moment
in order to get annihilation rates that agree reasonably well with experimental
data. Even though there is basically just one number to look at, the
electron-positron pair distribution function at zero distance, it is exactly
this number that dictates how the full pair distribution behaves: In most
cases, it falls off monotonously towards unity as the distance increases. Cases
where the electron-positron pair distribution exhibits a dip are precursors to
the formation of bound electron--positron pairs. The formation of
electron-positron pairs is indicated by a divergence of the FHNC-EL equations,
from this we can estimate the density regime where positrons must be localized.
This occurs in our calculations in the range 9.4 <= r_s <=10, where r_s is the
dimensionless density parameter of the electron liquid.Comment: To appear in Phys. Rev. B (2003
Effects of a calcium treatment on origin and composition of nonmetallic inclusions in the production of aluminiumkilled steel for long products
Multiple view object cosegmentation using appearance and stereo cues
Abstract. We present an automatic approach to segment an object in calibrated images acquired from multiple viewpoints. Our system starts with a new piecewise planar layer-based stereo algorithm that estimates a dense depth map that consists of a set of 3D planar surfaces. The algorithm is formulated using an energy minimization framework that combines stereo and appearance cues, where for each surface, an appearance model is learnt using an unsupervised approach. By treating the planar surfaces as structural elements of the scene and reasoning about their visibility in multiple views, we segment the object in each image independently. Finally, these segmentations are refined by probabilistically fusing information across multiple views. We demonstrate that our approach can segment challenging objects with complex shapes and topologies, which may have thin structures and non-Lambertian surfaces. It can also handle scenarios where the object and background color distributions overlap significantly. Key words: object cosegmentation, multiview segmentation, multiview stereo
New evaluation of the angular correlation of positron annihilation spectra for noble and d-transition metals
Electron momentum density in yttrium
A simultaneous analysis of high-resolution directional Compton profiles and
two-dimensional angular correlation of positron annihilation experimental data
has been performed by studying both a directional anisotropy of measured
spectra and reconstructed densities. The results were compared with theoretical
fully-relativistic augmented plane-wave calculations with and without including
correlation effects. Estimated symmetry selection rules have allowed us to
establish some values of Fermi momenta. Both experiments show exactly the same
shape of the anisotropy of the momentum densities, in agreement with the band
structure results. In the positron annihilation data electron-positron
correlations are not seen while in both experiments electron-electron
correlations are observed