311 research outputs found
Interplay between the electrical transport properties of GeMn thin films and Ge substrates
We present evidence that electrical transport studies of epitaxial p-type
GeMn thin films fabricated on high resistivity Ge substrates are severely
influenced by parallel conduction through the substrate, related to the large
intrinsic conductivity of Ge due to its small bandgap. Anomalous Hall
measurements and large magneto resistance effects are completely understood by
taking a dominating substrate contribution as well as the measurement geometry
into account. It is shown that substrate conduction persists also for well
conducting, degenerate, p-type thin films, giving rise to an effective
two-layer conduction scheme. Using n-type Ge substrates, parallel conduction
through the substrate can be reduced for the p-type epi-layers, as a
consequence of the emerging pn-interface junction. GeMn thin films fabricated
on these substrates exhibit a negligible magneto resistance effect. Our study
underlines the importance of a thorough characterization and understanding of
possible substrate contributions for electrical transport studies of GeMn thin
films.Comment: 9 pages, 9 figure
Clustering in a precipitate free GeMn magnetic semiconductor
We present the first study relating structural parameters of precipitate free
Ge0.95Mn0.05 films to magnetisation data. Nanometer sized clusters - areas with
increased Mn content on substitutional lattice sites compared to the host
matrix - are detected in transmission electron microscopy (TEM) analysis. The
films show no overall spontaneous magnetisation at all down to 2K. The TEM and
magnetisation results are interpreted in terms of an assembly of
superparamagnetic moments developing in the dense distribution of clusters.
Each cluster individually turns ferromagnetic below an ordering temperature
which depends on its volume and Mn content.Comment: accepted for publication in Phys. Rev. Lett. (2006). High resolution
images ibide
Porcine and Human intestinal cells for profiling the capacity of colonization and infection of the foodborne pathogen Yersinia enterocolitica
Y. enterocolitica is the third bacterial cause of human enteritis in Europe. The species is divided into six biotypes (BT), BT1A regarded as nonpathogenic and pathogenic biotypes 1B, 2, 3, 4 and 5. Pigs, the principal reservoir for human pathogenic strains, do not develop clinical signs. The BT4 is the most frequently biotype isolated from pig and encountered in human yersiniosis. This study investigated the use of in vitro cultured cells to assess the ability of Y. enterocolitica to adhere and invade pig and human cells. We tested in vitro the adhesion and invasion abilities of a collection of 23 Y. enterocolitica on intestinal pork cells IPEC-J2 and on human intestinal cells Caco-2. The overall profile of adhesion / invasion was different in the both tests. Nevertheless, in the two tests, the BT1A and the BT5 strains, which are rarely isolated from pigs, show a low capacity to adhere and to invade. These strains were clustered in the class 1. The class 2, forming by strains having a greater efficiency of adhesion and/or a greater efficiency of invasion, contained predominantly strains of BT4. The results obtained in this study reflect the ability of BT4 to colonize pigs and the low capacity to BT1A and BT5 to colonize pigs and humans
Effect of the angle of attack of a rectangular wing on the heat transfer enhancement in channel flow at low Reynolds number
Convective heat transfer enhancement can be achieved by generating secondary flow structures that are added to the main flow to intensify the fluid exchange between hot and cold regions. One method involves the use of vortex generators to produce streamwise and transverse vortices superimposed to the main flow. This study presents numerical computation results of laminar convection heat transfer in a rectangular channel whose bottom wall is equipped with one row of rectangular wing vortex generators. The governing equations are solved using finite volume method by considering steady state, laminar regime and incompressible flow. Three-dimensional numerical simulations are performed to study the effect of the angle of attack α of the wing on heat transfer and pressure drop. Different values are taken into consideration within the range 0° < α < 30°. For all of these geometrical configurations the Reynolds number is maintained to Re = 456. To assess the effect of the angle of attack on the heat transfer enhancement, Nusselt number and the friction factor are studied on both local and global perspectives. Also, the location of the generated vortices within the channel is studied, as well as their effect on the heat transfer enhancement throughout the channel for all α values. Based on both local and global analysis, our results show that the angle of attack α has a direct impact on the heat transfer enhancement. By increasing its value, it leads to better enhancement until an optimal value is reached, beyond which the thermal performances decrease
Magnetic and structural properties of GeMn films: precipitation of intermetallic nanomagnets
We present a comprehensive study relating the nanostructure of Ge_0.95Mn_0.05
films to their magnetic properties. The formation of ferromagnetic nanometer
sized inclusions in a defect free Ge matrix fabricated by low temperature
molecular beam epitaxy is observed down to substrate temperatures T_S as low as
70 deg. Celsius. A combined transmission electron microscopy (TEM) and electron
energy-loss spectroscopy (EELS) analysis of the films identifies the inclusions
as precipitates of the ferromagnetic compound Mn_5Ge_3. The volume and amount
of these precipitates decreases with decreasing T_S. Magnetometry of the films
containing precipitates reveals distinct temperature ranges: Between the
characteristic ferromagnetic transition temperature of Mn_5Ge_3 at
approximately room temperature and a lower, T_S dependent blocking temperature
T_B the magnetic properties are dominated by superparamagnetism of the Mn_5Ge_3
precipitates. Below T_B, the magnetic signature of ferromagnetic precipitates
with blocked magnetic moments is observed. At the lowest temperatures, the
films show features characteristic for a metastable state.Comment: accepted for publication in Phys. Rev. B 74 (01.12.2006). High
resolution images ibide
A Correlation between the Emission Intensity of Self-Assembled Germanium Islands and the Quality Factor of Silicon Photonic Crystal Nanocavities
We present a comparative micro-photoluminescence study of the emission
intensity of self-assembled germanium islands coupled to the resonator mode of
two-dimensional silicon photonic crystal defect nanocavities. The emission
intensity is investigated for cavity modes of L3 and Hexapole cavities with
different cavity quality factors. For each of these cavities many nominally
identical samples are probed to obtain reliable statistics. As the quality
factor increases we observe a clear decrease in the average mode emission
intensity recorded under comparable optical pumping conditions. This clear
experimentally observed trend is compared with simulations based on a
dissipative master equation approach that describes a cavity weakly coupled to
an ensemble of emitters. We obtain evidence that reabsorption of photons
emitted into the cavity mode is responsible for the observed trend. In
combination with the observation of cavity linewidth broadening in power
dependent measurements, we conclude that free carrier absorption is the
limiting effect for the cavity mediated light enhancement under conditions of
strong pumping.Comment: 8 pages, 5 figure
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