789 research outputs found
Ab initio study on the magneto-structural properties of MnAs
The magnetic and structural properties of MnAs are studied with ab initio
methods, and by mapping total energies onto a Heisenberg model. The stability
of the different phases is found to depend mainly on the volume and on the
amount of magnetic order, confirming previous experimental findings and
phenomenological models. It is generally found that for large lattice constants
the ferromagnetic state is favored, whereas for small lattice constants
different antiferromagnetic states can be stabilized. In the ferromagnetic
state the structure with minimal energy is always hexagonal, whereas it becomes
orthorhombically distorted if there is an antiferromagnetic component in the
hexagonal plane. For the paramagnetic state the stable cell is found to be
orthorhombic up to a critical lattice constant of about 3.7 Angstrom, above
which it remains hexagonal. This leads to the second order structural phase
transition between paramagnetic states at about 400 K, where the lattice
parameter increases above this critical value with rising temperature due to
the thermal expansion. For the paramagnetic state an analytic approximation for
the magnitude of the orthorhombic distortion as a function of the lattice
constant is given. Within the mean field approximation the dependence of the
Curie temperature on the volume and on the orthorhombic distortion is
calculated. For orthorhombically distorted cells the Curie temperature is much
smaller than for hexagonal cells. This is mainly due to the fact that some of
the exchange coupling constants in the hexagonal plane become negative for
distorted cells. With these results a description of the susceptibility as
function of temperature is given
Statistical compiler tuning
Modern compilers implement a number of optimization switches and they must be configured carefully in order to obtain the best performance. However, there exist few strategies to configure these compiler switches or flags. This is caused by the fact that the performance of a code is both dependent on the target architecture and the application. Additionally, the effect of the compiler optimizations is highly dependent on other compiler optimizations which are employed, causing the actual effect to be masked and not predictable. In this thesis, we propose to use statistical analysis to determine the effectiveness of the compiler optimizations. This enables us to construct systematic methodologies for determining settings of compiler optimizations automatically. The proposed methodologies are all independent from the implementation of compilers and implementation of applications, therefore, it is easy to apply our methodologies to any combination of compilers and applications. This versatility makes our results unique compared to other approaches. Additionally, with our methodologies, users can choose their optimization objective, for example, execution time or code size, etc. From the results shown in this thesis, we can concluded that the statistical tuning of compiler optimization is both possible and useful.UBL - phd migration 201
Potencialidade do Uso de Composto Produzido a Partir de Lodo de Esgoto Urbano e Poda Verde de Árvore.
bitstream/CNPDIA/10451/1/CiT25_2004.pd
Método de Aproveitamento Biossólido Proveniente de Lodo de Esgoto Residencial Através de Processo de Compostagem Seguido de Biodigestão Anaeróbia.
bitstream/CNPDIA/10461/1/DOC13_2004.pd
Finite-Size and surface effects in maghemite nanoparticles: Monte Carlo simulations
Finite-size and surface effects in fine particle systems are investigated by
Monte Carlo simulation of a model of a -FeO (maghemite) single
particle. Periodic boundary conditions have been used to simulate the bulk
properties and the results compared with those for a spherical shaped particle
with free boundaries to evidence the role played by the surface on the
anomalous magnetic properties displayed by these systems at low temperatures.
Several outcomes of the model are in qualitative agreement with the
experimental findings. A reduction of the magnetic ordering temperature,
spontaneous magnetization, and coercive field is observed as the particle size
is decreased. Moreover, the hysteresis loops become elongated with high values
of the differential susceptibility, resembling those from frustrated or
disordered systems. These facts are consequence of the formation of a surface
layer with higher degree of magnetic disorder than the core, which, for small
sizes, dominates the magnetization processes of the particle. However, in
contradiction with the assumptions of some authors, our model does not predict
the freezing of the surface layer into a spin-glass-like state. The results
indicate that magnetic disorder at the surface simply facilitates the thermal
demagnetization of the particle at zero field, while the magnetization is
increased at moderate fields, since surface disorder diminishes ferrimagnetic
correlations within the particle. The change in shape of the hysteresis loops
with the particle size demonstrates that the reversal mode is strongly
influenced by the reduced atomic coordination and disorder at the surface.Comment: Twocolumn RevTex format. 19 pages, 15 Figures included. Submitted to
Phys. Rev.
Carnosine scavenging of glucolipotoxic free radicals enhances insulin secretion and glucose uptake
The worldwide prevalence of diabetes has risen to 8.5% among adults, which represents a staggering rise in prevalence from 4.7% in 1980. Whilst some treatments work by increasing insulin secretion, over time their effectiveness decreases. We aim to increase insulin secretion by developing strategies that work through mechanisms independent of current treatment options. Isolated CD1 mouse islets, INS-1 pancreatic β-cells, or C2C12 mouse myotubes were incubated in standard tissue culture media, or media supplemented with 28 mM glucose, 200 μM palmitic acid, and 200 μM oleic acid as a cellular model of diabetic glucolipotoxicity. Intracellular reactive species content was assayed using 2′,7′-dichlorofluorescein diacetate dye, inducible nitric oxide synthase levels determined by Western blot, 3-nitrotyrosine and 4-hydrpxnonenal both assayed by ELISA, insulin secretion quantified using ELISA or radioimmunoassay, and glucose uptake determined through 2-deoxy glucose 6 phosphate luminescence. Our data indicate that carnosine, a histidine containing dipeptide available through the diet, is an effective scavenger of each of the aforementioned reactive species. This results in doubling of insulin secretion from isolated mouse islets or INS-1 β-cells. Crucially, carnosine also reverses glucolipotoxic inhibition of insulin secretion and enhances glucose uptake into skeletal muscle cells. Thus, carnosine, or non-hydrolysable carnosine analogs, may represent a new class of therapeutic agent to fight type 2 diabetes
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