Spin-dependent transport in metal/semiconductor tunnel junctions

This paper describes a model as well as experiments on spin-polarized tunnelling with the aid of optical spin orientation. This involves tunnel junctions between a magnetic material and gallium arsenide (GaAs), where the latter is optically excited with circularly polarized light in order to generate spin-polarized carriers. A transport model is presented that takes account of carrier capture in the semiconductor surface states, and describes the semiconductor surface in terms of a spin-dependent energy distribution function. The so-called surface spin-splitting can be calculated from the balance of the polarized electron and hole flow in the semiconductor subsurface region, the polarized tunnelling current across the tunnel barrier between the magnetic material and the semiconductor surface, and the spin relaxation at the semiconductor surface. Measurements are presented of the circular-polarization-dependent photocurrent (the so-called helicity asymmetry) in thin-film tunnel junctions of Co/Al2O3/GaAs. In the absence of a tunnel barrier, the helicity asymmetry is caused by magneto-optical effects (magnetic circular dichroism). In the case where a tunnel barrier is present, the data cannot be explained by magneto-optical effects alone; the deviations provide evidence that spin-polarized tunnelling due to optical spin orientation occurs. In Co/τ-MnAl/AlAs/GaAs junctions no deviations from the magneto-optical effects are observed, most probably due to the weak spin polarization of τ-MnAl along the tunnelling direction; the latter is corroborated by bandstructure calculations. Finally, the application of photoexcited GaAs for spin-polarized tunnelling in a scanning tunnelling microscope is discussed.

Flexibility@Work 2013: yearly report on flexible labor and employment

There is no clear evidence that the strong growth in the share of flexible labor relations between 2002 and 2007 points at a worldwide trend towards a larger share of flexible labor at the expense of traditional open-ended labor contracts. The growth in flexible labor varies too much between countries and periods to draw such a conclusion. In most countries in Europe, in North America and Japan, the share of flexible labor has declined during the recent economic recession that started after 2007. There is also no structural trade-off between different forms of (formal) flexible labor. Observations in Europe suggest that growth or decline of different types of flexible labor can be attributed to changes in both local societal and economic structures and in institutions and legislation. Growth of flexible labor appears mainly in countries where the labor participation is increasing while it is declining where labor participation is relatively low or stagnating. This suggests an important role for flexible labor for new non-traditional labor market participants to enter the labor market. At the same time, there is a strong correlation between the share of flexible labor and economic growth, particularly with respect to fixed-term contracts and agency work. Flexible labor is the first form of employment effected by a decline in labor demand in an economic crisis, particularly when flexible workers are younger and lower educated. But at the same time, flexible work will be the first type of employment that recovers when the economy stabilizes after a crisis. The opportunity to offer flexible work may even accelerate economic growth. Therefore, a further growth in flexible work can be expected once the economies in most western countries start to grow significantly again. Agency work in particular has grown in nearly all markets over the last decade, which does indicate a structural component of growth of agency work in the modern labor market apart from the business cycle

Dependence of the Frequency of the Kilohertz Quasi-Periodic Oscillations on X-ray Count Rate and Colors in 4U 1608-52

We present new results based on observations carried out with the Rossi X-ray Timing Explorer during the decay of an outburst of the low-mass X-ray binary (LMXB) and atoll source 4U 1608-52. Our results appear to resolve, at least in 4U 1608-52, one of the long-standing issues about the phenomenology of the kilohertz quasi-periodic oscillations (kHz QPOs), namely, the lack of a unique relation between the frequency of the kHz QPOs and the X-ray flux. We show that despite its complex dependence on the X-ray flux, the frequency of the kHz QPOs is monotonically related to the position of the source in the color-color diagram. Our findings strengthen the idea that, as in the case of Z sources, in the atoll sources the X-ray flux is not a good indicator of $\dot M$, and that the observed changes in the frequency of the kHz QPOs in LMXBs are driven by changes in $\dot M$. These results raise some concern about the recently reported detection of the orbital frequency at the innermost stable orbit in 4U 1820-30.Comment: Accepted for publication in The Astrophysical Journal Letters. Uses AAS LaTex v4.0 (5 pages plus 4 postscript figures

Mitochondrial genome divergence supports an ancient origin of circatidal behaviour in the Anurida maritima (Collembola: Neanuridae) species group

Animals of the intertidal zone tolerate substantial environmental fuctuations. Survival under such unstable conditions requires specifc adaptations. Several intertidal species have evolved endogenous mechanisms that follow tidal rhythms permitting behavioural alignment with periodic inundation. For example, aggregation behaviour in the springtail Anurida maritima (Guérin-Méneville, 1836) (Collembola: Neanuridae) is controlled by a free-running clock with a period of~12.4 h. This cosmopolitan species is found in the upper intertidal zone where it forages during low tide. Before high tide, specimens aggregate in cracks in the substrate or under rocks to survive inundation. Here we report that the closely related intertidal species Anurida bisetosa Bagnall, 1949, displays a similar endogenously controlled circatidal behaviour. To obtain a minimum age estimate for this shared derived trait, we sequenced the full mitochondrial genome of A. bisetosa, which was then used for phylogenetic inference and molecular dating. The mitochondrial genomes of A. maritima and A. bisetosa are highly divergent. This divergence extends throughout the whole mitochondrial genome and is mirrored by a similar pattern in the nuclear internal transcribed spacer 2 (ITS2) region. Dating analyses suggest the species potentially split more than 40 million years ago. Under the assumption that the endogenously-controlled rhythmic behaviour evolved once in an ancestor of the two species, the trait must be of older age. Use of a single genetic marker and a limited number of fossil calibration points constrains accuracy of the age estimate, but nevertheless, it ofers a glimpse at otherwise ‘intangible’ palaeoecological and palaeoethological attributes

Validation of active forest fires detected by MSG-SEVIRI by means of MODIS hot spots and AWiFS images.

The detection of forest fires and the determination of their parameters have been usually carried out by polar-orbit sensors: AVHRR, (A)ATSR, BIRD and MODIS mainly. However, their time resolution prevents them from operating in real time. In contrast, the new geostationary sensors have very appropriate capacities for the observation of the Earth and monitoring of forest fires, as is being proved. GOES, MSG and MTSAT are already operative and they have led the international community to think that the global observation network in real time may become a reality. The implementation of this network is the aim of the Global Observations of Forest Cover and Land Cover Dynamics (GOFC/GOLD) FIRE Mapping and Monitoring program, focused internationally on taking decisions concerning the research of the Global Change. In this paper, the operation in real time by the MSG-SEVIRI sensor over the Iberian Peninsula is studied. On the other hand, the reliability of validation results by means of polar sensors, with better spatial resolution, is difficult to analyze due to errors caused by confused location of fires. This paper tries to find the most appropriate spatial scale to carry out comparison between Terra/Aqua-MODIS and Resourcesat1-AWiFS image

A conformational study of peptides with the general structure Ac-L-Xaa-Pro-D-Xaa-L-Xaa-NH_2: spectroscopic evidence for a peptide with significant β-turn character in water and in dimethyl sulfoxide

Several tetrapeptides, Ac-Val-Pro-D-Ser-His-NH_2, in particular, show significant type II β-turn character in water and in dimethyl sulfoxide. Evidence for this turn population is provided by 2D-rotating frame nuclear Overhauser effect (ROESY) spectroscopy, ^1H NMR amide temperature coefficients, and circular dichroism (CD) studies. To further investigate which residues specifically contribute to the integrity of the turn, studies on 10 tetrapeptides, having the general sequence AC-LXaa-Pro-D-Xaa-L-Xaa-NH_2, are described. The results show the effects of sequence variations on the type II β-turn forming propensity of these peptides in solution. Conclusions from these studies indicate that a cooperative effect between a sterically hindered, β-branched amino acid at the (i) position and a small, non-β-branched D-amino acid at the (i+2) position promotes turn formation. Implications for use of these sequences as structural nucleation elements in de novo protein design are discussed

Are neighbourhood social capital and availability of sports facilities related to sports participation among Dutch adolescents?

<p>Abstract</p> <p>Background</p> <p>The aim of this study is to explore whether availability of sports facilities, parks, and neighbourhood social capital (NSC) and their interaction are associated with leisure time sports participation among Dutch adolescents.</p> <p>Methods</p> <p>Cross-sectional analyses were conducted on complete data from the last wave of the YouRAction evaluation trial. Adolescents (n = 852) completed a questionnaire asking for sports participation, perceived NSC and demographics. Ecometric methods were used to aggregate perceived NSC to zip code level. Availability of sports facilities and parks was assessed by means of geographic information systems within the zip-code area and within a 1600 meter buffer. Multilevel logistic regression analyses, with neighborhood and individual as levels, were conducted to examine associations between physical and social environmental factors and leisure time sports participation. Simple slopes analysis was conducted to decompose interaction effects.</p> <p>Results</p> <p>NSC was significantly associated with sports participation (OR: 3.51 (95%CI: 1.18;10.41)) after adjustment for potential confounders. Availability of sports facilities and availability of parks were not associated with sports participation. A significant interaction between NSC and density of parks within the neighbourhood area (OR: 1.22 (90%CI: 1.01;1.34)) was found. Decomposition of the interaction term showed that adolescents were most likely to engage in leisure time sports when both availability of parks and NSC were highest.</p> <p>Conclusions</p> <p>The results of this study indicate that leisure time sports participation is associated with levels of NSC, but not with availability of parks or sports facilities. In addition, NSC and availability of parks in the zip code area interacted in such a way that leisure time sports participation is most likely among adolescents living in zip code areas with higher levels of NSC, and higher availability of parks. Hence, availability of parks appears only to be important for leisure time sports participation when NSC is high.</p