130 research outputs found
Sixième partie. — Etude par spectrométrie Mössbauer des alliages métalliques amorphes et des semi-conducteurs amorphes
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Enthalpies of formation of L12 intermetallics derived from heats of reordering
A new method is proposed for estimating the enthalpies of formation of L12 (fcc-ordered) intermetallics from the heat release measured during ordering of their disordered polymorphs. The method is applied to Cu3Au, Ni3Al, and Ni3Si. The resulting estimates of enthalpies of formation are close to values obtained by high temperature dissolution calorimetry. They also appear to be more precise than estimates based on Miedema's correlations provided that care is taken to account properly for the magnetic and lattice stability contributions to the formation enthalpies in the ordered and disordered states
Universal Cubic Eigenvalue Repulsion for Random Normal Matrices
Random matrix models consisting of normal matrices, defined by the sole
constraint , will be explored. It is shown that cubic
eigenvalue repulsion in the complex plane is universal with respect to the
probability distribution of matrices. The density of eigenvalues, all
correlation functions, and level spacing statistics are calculated. Normal
matrix models offer more probability distributions amenable to analytical
analysis than complex matrix models where only a model wth a Gaussian
distribution are solvable. The statistics of numerically generated eigenvalues
from gaussian distributed normal matrices are compared to the analytical
results obtained and agreement is seen.Comment: 15 pages, 2 eps figures. to appar in Physical Review
Etude par spectrométrie Mössbauer d'alliages Fe-Si-B amorphes
Amorphous Fe100-x-ySi yBx alloys have been studied by Môssbauer Spectroscopy. The fitting methods and some possible origins of the asymmetric broadenings of the peaks are discussed. A comparison of the hyperfine field distributions in alloys with and without silicon suggests that iron and silicon are located in similar sites (for the studied y values, y ≤ 9) while boron occupies interstitial sites with ≃ 9 iron atoms nearest neighbours. Crystallisation steps and mechanisms have been studied in Fe82Si3B15 and Fe76Si9B15 alloys. They differ from those found in Fe-B alloys because metastable borides are not observed.Des alliages amorphes Fe100-x- ySiyBx ont été étudiés par spectrométrie Môssbauer. Les méthodes de calcul et les origines possibles des dissymétries des spectres sont discutées. La comparaison des distributions de champ hyperfin des alliages avec et sans silicium suggère que le silicium et le fer occupent des sites semblables (pour les valeurs de y étudiées, y ≤ 9) alors que le bore occupe une position interstitielle avec ≃ 9 atomes de fer proches voisins. Les alliages Fe-B sont fortement stabilisés par le silicium. Les étapes et les mécanismes de cristallisation d'alliages Fe82Si3B 15 et Fe76Si9B15 ont été précisés. Ils diffèrent de ceux observés dans les alliages Fe-B car il ne se forme pas de borures métastables
Debye temperature of disordered bcc-Fe-Cr alloys
Debye temperature, TD, of Fe100-xCrx disordered alloys with 0<x<99.9 was
determined from the temperature dependence of the centre shift of 57Fe
Mossbauer spectra recorded in the temperature range of 80-300K. Its
compositional dependence shows an interesting non-monotonous behaviour. For
0<x<~45 as well as for ~75<x<~95 the Debye temperature is enhanced relative to
its value of a metallic iron, and at x=~3 there is a local maximum having a
relative height of ~12% compared to a pure iron. For ~45~95
the Debye temperature is smaller than the one for the metallic iron, with a
local minimum at x=~55 at which the relative decrease of TD amounts to ~12%.
The first maximum coincides quite well with that found for the spin-waves
stiffness coefficient, D0, while the pretty steep decrease observed for x>~95
which is indicative of a decoupling of the probe Fe atoms from the underlying
chromium matrix is likely related to the spin-density waves which constitute
the magnetic structure of chromium in that interval of composition. The
harmonic force constant calculated from the Debye temperature of the least
Fe-concentrated alloy (x>99.9) amounts to only 23% of the one characteristic of
a pure chromium.Comment: 15 pages, 7 figures, 26 reference
Topological correlations in soap froths
Correlation in two-dimensional soap froth is analysed with an effective
potential for the first time. Cells with equal number of sides repel (with
linear correlation) while cells with different number of sides attract (with
NON-bilinear) for nearest neighbours, which cannot be explained by the maximum
entropy argument. Also, the analysis indicates that froth is correlated up to
the third shell neighbours at least, contradicting the conventional ideas that
froth is not strongly correlated.Comment: 10 Pages LaTeX, 6 Postscript figure
Damage Spreading in the Ising Model
We present two new results regarding damage spreading in ferromagnetic Ising
models. First, we show that a damage spreading transition can occur in an Ising
chain that evolves in contact with a thermal reservoir. Damage heals at low
temperature and spreads for high T. The dynamic rules for the system's
evolution for which such a transition is observed are as legitimate as the
conventional rules (Glauber, Metropolis, heat bath). Our second result is that
such transitions are not always in the directed percolation universality class.Comment: 5 pages, RevTeX, revised and extended version, including 3 postscript
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The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. VII. Ophiuchus Observed with MIPS
We present maps of 14.4 deg^2 of the Ophiuchus dark clouds observed by the Spitzer Space Telescope Multiband Imaging Photometer for Spitzer (MIPS). These high-quality maps depict both numerous point sources and extended dust emission within the star-forming and non–star-forming portions of these clouds. Using PSF-fitting photometry, we detect 5779 sources at 24 μm and 81 sources at 70 μm at the 10 σ level of significance. Three hundred twenty-three candidate young stellar objects (YSOs) were identified according to their positions on the MIPS/2MASS K versus color-magnitude diagrams, as compared to 24 μm detections in the SWIRE extragalactic survey. We find that more than half of the YSO candidates, and almost all those with protostellar Class I spectral energy distributions, are confined to the known cluster and aggregates
Cost effectivenes of erlotinib versus chemotherapy for first-line treatment of non small cell lung cancer (NSCLC) in fit elderly patients participating in a prospective phase 2 study (GFPC 0504)
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