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Use this search facility to find out more about the profile of our HPC-Europa2 visitors, the type of work they have been doing, and their project achievements.
In the context of material science the research studies in analytical transmission electron microscopy are mainly aimed to evaluate the local composition of nanostructures down to atomic scale. In this context scanning transmission electron microscopy (STEM) with a High angle annular dark field detector (HAADF) has proved high potentiality due to its unrivaled spatial resolution. However the quantitative application of this technique is limited by the need for accurate simulations necessary to create calibration curves for the experimental intensities. The most accurate simulation schemes are based on “Frozen Phonon” algorithms that demand large computational resources .
We have studied the possibility to investigate the composition in the InGaAsN quaternary alloy. This semiconductor system is particularly appealing to produce long wavelength optoelectronic devices lattice matched on GaAs .
 R F. Loane, et al. Acta Cryst. (1991) A47, 267-278  P.J. Klar Progress in Solid State Chemistry 31 (2003) 301–349
Parallel computing has been successfully applied to the algorithm of multislice for the simulation on STEM HAADF images. The simulation work was based on the parallel computing code STEM_CELL  (based on Kirkland’s multislice ) that has been modified for this project in order to produce a single file for each composition set containing the output for different experimental parameters namely specimen thickness and detector acceptance.
We have investigated the unit cell averaged intensity in the InGaAsN system for 26 sets of compositions of In and N for the comparison of simulated intensities with experiments.
The input for each set of compositions was a supercell generated by elastically relaxing randomly occupied cells of InGaAsN with the relevant In and N occupation. The elastic relaxation was obtained by using the well established valence force field method (see for details ).
The output was elaborated with the STEM_CELL graphical interface that permits to extract the relevant image from the complete output file and perform image analysis. The intensities for each set have been put on a sparse matrix. A first preliminary interpolation has been obtained by weighted average decreasing with distance to each point but more accurate algorithms are under developement.
A full post–processing of the simulations is still in progress but a few important results can be already described.
In facts we have found that the incorporation of In and N may have different effects on the unit cell averaged cell intensity. In facts the incorporation of In produces a classic chemical contrast given by the fact that the HAADF intensity is proportional to the square of the atomic number of the sample elements. The effect of Nitrogen on contrast is more related to the static disorder it introduces in the lattice.
In practice the difference between this two kinds of contribution can be appreciated by varying the HAADF detector angular acceptance and normalizing the intensity to those of the GaAs in the substrate. The simulation confirm this idea: the contribution to the normalized intensity deriving from In remains almost the same at any detector conditions. Conversely the Nitrogen incorporation may have opposite effect depending on the detector acceptance.
Therefore to a first approximation the N composition may be directly extracted from the difference of two normalized images with different detector regardless of In distribution. This result demonstrates that an accurate point to point comparison of two images can be used to produce a unique estimation for In and N separately.
This result has a large methodological and applicative relevance since a TEM based method for mapping in 2D both In and N in InGaAsN alloy was so far lacking. Such methodology will help the growth of InGaAsN for applications in optical fiber communication.
 E. Carlino, V. Grillo, P. Palazzari in Microscopy of Semiconducting Materials 2007, edited by A. G. Cullis and P. A. Midgley, IOP Conf. Proc. vol. 120, p. 177-180 and
 E.J. Kirkland, Advanced Computing in Electron Microscopy, Plenum, NewYork, 1998, p. 106.  V. Grillo et al. Physical Review B 77, 054103 (2008)
In the present work, a numerical study is conducted in order to investigate the unsteady aerodynamics of finite-span flapping rigid wings. The non-dimensional incompressible Navier-Stokes equations are solved in their velocity-pressure formulation using a second-order accurate in space and time finite-difference scheme. To tackle the problem of moving boundaries, the governing equations are solved on overlapping structured grids. The main goal is to investigate the wake topology and aerodynamic performance of low aspect-ratio flapping wings and their dependence on the flapping parameters (such as flapping frequency, flapping amplitude and Reynolds number). Specifically, the root-flapping motion characteristic of flying animals is studied. The numerical simulations are performed at a Reynolds number of Re = 1000 (a Reynolds number value often found in nature) and at different values of Strouhal number St.
In the present work, we have studied the root-flapping motion characteristic of flying animals, a subject virtually unexplored. It was found that, indeed, root-flapping motion produces wake structures similar to those of heaving or coupled heaving-and-pitching motions, but with the difference that the latter motions generate larger vortices and forces than root-flapping motion, presumably because the average velocity is higher across the span; aside from this, similar wake regimes occurs at similar St. In general, it was found that for values less than St < 0.25 we are in the drag production regime, for values approximately equal to St = 0.25 we produce little or no drag (or thrust), whereas for values higher that St > 0.25 we are in the thrust production regime.
The simulations also show that the wake of thrust producing, rigid finite-span wings is formed by vortex loops close to the wing surface, that slowly convert into vortex rings as they are convected downstream. It was also observed that the vortex rings are themselves inclined with respect to the free-stream; the angle of inclination of the vortex rings is found to be in the direction of their travel and in the streamwise direction for thrust producing configurations, whereas for drag producing configurations the angle of inclination is opposite to the direction of travel.
Additionally, the effect of aspec- ratio AR on the aerodynamic forces of finite-span wings was also assessed. It was observed that as we increase the wing AR, the aerodynamic forces also increase and this is chiefly attributed to the large area of high aspect ratio wings and to the decrease of three-dimensional effects in long wings. This observation leads us to think that the assumption of two-dimensionality has some validity for birds and insects, where the wings of many species tend to have relatively large aspect ratio.
All the qualitative and quantitative results found during this research support the hypothesis that: “flying and swimming animals cruise at a Strouhal number tuned for high power efficiency”
To develop models useful in many-body physics. While the model for solid HD is still under construction (a model including phonons, rotations, and their couplings is under way), here the question of polarized insulation in strongly correlated models was addressed via variational methods.
This project achieved the simple description of a metal-insulator transition (and metal-metal transitions away from half-filling) in which bound excitons play a key role. The metal-insulator transition in strongly correlated systems is in many ways still an open question. Variational wavefunctions (Baeriswyl and Baeriswyl-Gutzwiller projected wavefunctions) were studied via the well-known Gutzwiller ansatz applied in reciprocal space. The phase diagram was calculated resulting in a metal-insulator transition in which exciton binding plays the key role both in the metal-insulator transition at half-filling and the metal-metal transition away from half-filling. For the Baeriswyl-Gutzwiller projected wavefunction the transition occurs between a correlated metallic state of the Gutzwiller type and an insulator with bound excitons.