Perturbed angular correlation study of Ta-181-doped PbTi1-xHfxO3 compounds

In this work, the hyperfine quadrupole interaction at Ta-doped PbTi1-xHfxO3 polycrystalline samples is studied for the first time. Powders with x=0.25, 0.50 and 0.75 were prepared and characterized by X-ray diffraction analysis. Perturbed Angular Correlation (PAC) analyses were done as a function of temperature, using low concentration Ta-181 nuclei as probes. In the ferroelectric and paraelectric phases of these compounds two sites were occupied by the probes. For each site the quadrupole frequency, asymmetry and relative distribution width parameters were obtained as a function of temperature above and below the Curie temperature (T-C). One of these sites was assigned to the regular Ti-Hf site, while the other one was assigned to some kind of defect. The behavior of the hyperfine parameters as a function of temperature was analyzed in terms of a recent published phase diagram and the presence of disorder below and above T-C. For the three compositions measured, the obtained hyperfine parameters present discontinuities which correspond to the ferroelectric-paraelectric phase transition. In both phases it was found broad frequency distributed interactions. The disorder in the electronic distribution would be responsible for the broad line width of the hyperfine interaction. (C) 2012 Elsevier B.V. All rights reserved.

Novel Gravity Distortion Evaluation on Large Antenna Array for Deep Space Communication

This paper presents a simple gravity evaluation model for large reflector antennas and the experimental example for a case study of one uplink array of 4x35-m antennas at X and Ka band. This model can be used to evaluate the gain reduction as a function of the maximum gravity distortion, and also to specify this at system designer level. The case study consists of one array of 35-m antennas for deep space missions. Main issues due to the gravity effect have been explored with Monte Carlo based simulation analysis.

On roughness measurement by angular speckle correlation

In this work, the influence of both characteristics of the lens and misalignment of the incident beams on roughness measurement is presented. To investigate how the focal length and diameter affect the degree of correlation between the speckle patterns, a set of experiments with different lenses is performed. On the other hand, the roughness when the beams separated by an amount are non-coincident at the same point on the sample is measured. To conclude the study, the uncertainty of the method is calculated.

Simulation-based evaluation of susceptibility distortion correction methods in diffusion MRI for connectivity analysis

Connectivity analysis on diffusion MRI data of the whole-brain suffers from distortions caused by the standard echo-planar imaging acquisition strategies. These images show characteristic geometrical deformations and signal destruction that are an important drawback limiting the success of tractography algorithms. Several retrospective correction techniques are readily available. In this work, we use a digital phantom designed for the evaluation of connectivity pipelines. We subject the phantom to a “theoretically correct” and plausible deformation that resembles the artifact under investigation. We correct data back, with three standard methodologies (namely fieldmap-based, reversed encoding-based, and registration- based). Finally, we rank the methods based on their geometrical accuracy, the dropout compensation, and their impact on the resulting connectivity matrices.

Analysis of pixel-mapping rounding on geometric distortion as a prediction for view synthesis distortion

We analyze the performance of the geometric distortion, incurred when coding depth maps in 3D Video, as an estimator of the distortion of synthesized views. Our analysis is motivated by the need of reducing the computational complexity required for the computation of synthesis distortion in 3D video encoders. We propose several geometric distortion models that capture (i) the geometric distortion caused by the depth coding error, and (ii) the pixel-mapping precision in view synthesis. Our analysis starts with the evaluation of the correlation of geometric distortion values obtained with these models and the actual distortion on synthesized views. Then, the different geometric distortion models are employed in the rate-distortion optimization cycle of depth map coding, in order to assess the results obtained by the correlation analysis. Results show that one of the geometric distortion models is performing consistently better than the other models in all tests. Therefore, it can be used as a reasonable estimator of the synthesis distortion in low complexity depth encoders.

Perceptual distortion modeling for side-by-side 3D video delivery

A frame-level distortion model based on perceptual features of the human visual system is proposed to improve the performance of unequal error protection strategies and provide better quality of experience to users in Side-by-Side 3D video delivery systems.