844 research outputs found
Monte Carlo Simulations for the Magnetic Phase Diagram of the Double Exchange Hamiltonian
We have used Monte Carlo simulation techniques to obtain the magnetic phase
diagram of the double exchange Hamiltonian. We have found that the Berry's
phase of the hopping amplitude has a negligible effect in the value of the
magnetic critical temperature. To avoid finite size problems in our simulations
we have also developed an approximated expression for the double exchange
energy. This allows us to obtain the critical temperature for the ferromagnetic
to paramagnetic transition more accurately. In our calculations we do not
observe any strange behavior in the kinetic energy, chemical potential or
electron density of states near the magnetic critical temperature. Therefore,
we conclude that other effects, not included in the double exchange
Hamiltonian, are needed to understand the metal-insulator transition which
occurs in the manganites.Comment: 6 pages Revtex, 8 PS figure
Destructive attraction: factors that influence hunting pressure on the Blue Bird-of-paradise Paradisaea rudolphi
The Blue Bird-of-paradise Paradisaea rudolphi (BBOP) is a globally threatened species restricted to the montane rainforest of Papua New Guinea (PNG). Local inhabitants hunt the BBOP for its feathers, which is one of the main reasons for its population decline. The feathers are used for both traditional and commercial purposes. So far virtually nothing is known about which factors enhance or decrease hunting pressure, and how this is influenced by ongoing market integration of local communities. Field research in PNG suggests that the demand for BBOP feathers and, with that, the collection intensity, primarily stem from traditional uses, such as head decoration. Interest in the traditional use of the feathers is particularly high in remote villages, while the value attributed to living birds is higher in less remote villages, suggesting a decrease of BBOP collection intensity with increased market integration. However, increased access to 'outside influences' (namely tourism and Christianity) may both decrease as well as increase hunting pressures. Finally, we argue that awareness-raising among local people, government officials and tourists may well be the most sensible conservation measure.ASC – Publicaties niet-programma gebonde
Conductance as a Function of the Temperature in the Double Exchange Model
We have used the Kubo formula to calculate the temperature dependence of the
electrical conductance of the double exchange Hamiltonian. We average the
conductance over an statistical ensemble of clusters, which are obtained by
performing Monte Carlo simulations on the classical spin orientation of the
double exchange Hamiltonian. We find that for electron concentrations bigger
than 0.1, the system is metallic at all temperatures. In particular it is not
observed any change in the temperature dependence of the resistivity near the
magnetical critical temperature. The calculated resistivity near is
around ten times smaller than the experimental value. We conclude that the
double exchange model is not able to explain the metal to insulator transition
which experimentally occurs at temperatures near the magnetic critical
temperature.Comment: 6 pages, 5 figures included in the tex
A splitting line model for directional relations
Directional relations are fundamental to spatial data queries, analysis and reasoning. Consequently there has been a significant amount of effort to determine directional relations between two regions. However, many existing methods do not perform well when the regions are neighboring or intertwined. In this paper we introduce a new model for directional relations which is based on a splitting line separating the two regions in question. We identify essential quality criteria for directional relation models and translate them into measurable properties of a given splitting line. We present an efficient algorithm that computes an optimal splitting line for two regions and perform extensive experiments. Our results show that the splitting line model captures directional relations very well and that it clearly outperforms existing approaches on pairs of neighboring or intertwined regions
Raman spectroscopy-based identification of nosocomial outbreaks of the clonal bacterium Escherichia coli
DNA-based techniques are frequently used to confirm the relatedness of putative outbreak isolates. These techniques often lack the discriminatory power when analyzing closely related microbes such as E. coli. Here the value of Raman spectroscopy as a typing tool for E. coli in a clinical setting was retrospectively evaluated
Double Exchange Alone Does Not Explain the Resistivity of
The system with has
traditionally been modelled with a ``double exchange'' Hamiltonian, in which it
is assumed that the only relevant physics is the tendency of carrier hopping to
line up neighboring spins. We present a solution of the double exchange model,
show it is incompatible with many aspects of the resistivity data, and propose
that a strong electron-phonon interaction arising from a Jahn-Teller splitting
of the outer Mn d-level plays a crucial role.Comment: Figure available via concentional mail. Contact
[email protected]
Ultrafast photoinduced reflectivity transients in
The temperature dependence of ultrafast photoinduced reflectivity transients
is reported in NdSrMnO thin film. The photoinduced
reflectivity shows a complex response with very different temperature
dependences on different timescales. The response on the sub-ps timescale
appears to be only weakly sensitive to the 270K-metal-insulator phase
transition. Below K the sub-ps response displays a two component
behavior indicating inhomogeneity of the film resulting from the substrate
induced strain. On the other hand, the slower response on the 10-100 ps
timescale is sensitive only to the metal-insulator phase transition and is in
agreement with some previously published results. The difference in the
temperature dependences of the responses on nanosecond and s timescales
indicates that thermal equilibrium between the different degrees of fredom is
established relatively slowly - on a nanosecond timescale
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