16,523 research outputs found
Injection and detection of spin in a semiconductor by tunneling via interface states
Injection and detection of spin accumulation in a semiconductor having
localized states at the interface is evaluated. Spin transport from a
ferromagnetic contact by sequential, two-step tunneling via interface states is
treated not in itself, but in parallel with direct tunneling. The spin
accumulation induced in the semiconductor channel is not suppressed, as
previously argued, but genuinely enhanced by the additional spin current via
interface states. Spin detection with a ferromagnetic contact yields a weighted
average of the spin accumulation in the channel and in the localized states. In
the regime where the spin accumulation in the localized states is much larger
than that in the channel, the detected spin signal is insensitive to the spin
accumulation in the localized states and the ferromagnet probes the spin
accumulation in the semiconductor channel.Comment: 7 pages, 2 figures. Theory onl
Thermal spin current and magnetothermopower by Seebeck spin tunneling
The recently observed Seebeck spin tunneling, the thermoelectric analog of
spin-polarized tunneling, is described. The fundamental origin is the spin
dependence of the Seebeck coefficient of a tunnel junction with at least one
ferromagnetic electrode. Seebeck spin tunneling creates a thermal flow of
spin-angular momentum across a tunnel barrier without a charge tunnel current.
In ferromagnet/insulator/semiconductor tunnel junctions this can be used to
induce a spin accumulation (\Delta \mu) in the semiconductor in response to a
temperature difference (\Delta T) between the electrodes. A phenomenological
framework is presented to describe the thermal spin transport in terms of
parameters that can be obtained from experiment or theory. Key ingredients are
a spin-polarized thermoelectric tunnel conductance and a tunnel spin
polarization with non-zero energy derivative, resulting in different Seebeck
tunnel coefficients for majority and minority spin electrons. We evaluate the
thermal spin current, the induced spin accumulation and \Delta\mu/\Delta T,
discuss limiting regimes, and compare thermal and electrical flow of spin
across a tunnel barrier. A salient feature is that the thermally-induced spin
accumulation is maximal for smaller tunnel resistance, in contrast to the
electrically-induced spin accumulation that suffers from the impedance mismatch
between a ferromagnetic metal and a semiconductor. The thermally-induced spin
accumulation produces an additional thermovoltage proportional to \Delta\mu,
which can significantly enhance the conventional charge thermopower. Owing to
the Hanle effect, the thermopower can also be manipulated with a magnetic
field, producing a Hanle magnetothermopower.Comment: 10 pages, 3 figures, 1 tabl
Atomic quasi-Bragg diffraction in a magnetic field
We report on a new technique to split an atomic beam coherently with an
easily adjustable splitting angle. In our experiment metastable helium atoms in
the |{1s2s}^3S_1 M=1> state diffract from a polarization gradient light field
formed by counterpropagating \sigma^+ and \sigma^- polarized laser beams in the
presence of a homogeneous magnetic field. In the near-adiabatic regime, energy
conservation allows the resonant exchange between magnetic energy and kinetic
energy. As a consequence, symmetric diffraction of |M=0> or |M=-1> atoms in a
single order is achieved, where the order can be chosen freely by tuning the
magnetic field. We present experimental results up to 6th order diffraction (24
\hbar k momentum splitting, i.e., 2.21 m/s in transverse velocity) and present
a simple theoretical model that stresses the similarity with conventional Bragg
scattering. The resulting device constitutes a flexible, adjustable,
large-angle, three-way coherent atomic beam splitter with many potential
applications in atom optics and atom interferometry.Comment: 4 pages, 5 figure
Comparison of trace metal bioavailabilities in European coastal waters using mussels from Mytilus edulis
Mussels from Mytilus edulis complex were used as biomonitors of the trace metals Fe, Mn, Pb, Zn, and Cu at 17 sampling sites to assess the relative bioavailability of metals in coastal waters around the European continent. Because accumulated metal concentrations in a given area can differ temporally, data were corrected for the effect of season before large-scale spatial comparisons were made. The highest concentration of Fe was noted in the North Sea and of Mn in the Baltic. Increased tissue concentrations of Pb were recorded in the mussels from the Bay of Biscay and the Baltic Sea. Low concentrations of metals were determined in the mussels from the Mediterranean Sea and the Northern Baltic. Relatively low geographic variations of Cu and Zn indicate that mussels are able to partially regulate accumulated body concentrations, which means Cu and Zn are, to some extent, independent of environmental concentrations
InAs-AlSb quantum wells in tilted magnetic fields
InAs-AlSb quantum wells are investigated by transport experiments in magnetic
fields tilted with respect to the sample normal. Using the coincidence method
we find for magnetic fields up to 28 T that the spin splitting can be as large
as 5 times the Landau splitting. We find a value of the g-factor of about 13.
For small even-integer filling factors the corresponding minima in the
Shubnikov-de Haas oscillations cannot be tuned into maxima for arbitrary tilt
angles. This indicates the anti-crossing of neighboring Landau and spin levels.
Furthermore we find for particular tilt angles a crossover from even-integer
dominated Shubnikov-de Haas minima to odd-integer minima as a function of
magnetic field
Tracing Galaxy Assembly: Tadpole Galaxies in the Hubble Ultra Deep Field
In the Hubble Ultra Deep Field (HUDF) an abundance of galaxies is seen with a
knot at one end plus an extended tail, resembling a tadpole. These "tadpole
galaxies" appear dynamically unrelaxed--presumably in an early merging
state--where tidal interactions likely created the distorted knot-plus-tail
morphology. Here we systematically select tadpole galaxies from the HUDF and
study their properties as a function of their photometric redshifts. In a
companion HUDF variability study, Cohen et al. (2005) revealed a total of 45
variable objects believed to be Active Galactic Nuclei (AGN). Here we show that
this faint AGN sample has no overlap with the tadpole galaxy sample, as
predicted by theoretical work. The tadpole morphology--combined with the lack
of overlap with the variable objects--supports the idea that these galaxies are
in the process of an early-stage merger event, i.e., at a stage that likely
precedes the "turn-on" of any AGN component and the onset of any point-source
variability.Comment: 7 pages, 4 figures. Accepted for publication by Astrophysical Journa
Bond breaking in vibrationally excited methane on transition metal catalysts
The role of vibrational excitation of a single mode in the scattering of
methane is studied by wave packet simulations of oriented CH4 and CD4 molecules
from a flat surface. All nine internal vibrations are included. In the
translational energy range from 32 up to 128 kJ/mol we find that initial
vibrational excitations enhance the transfer of translational energy towards
vibrational energy and increase the accessibility of the entrance channel for
dissociation. Our simulations predict that initial vibrational excitations of
the asymmetrical stretch (nu_3) and especially the symmetrical stretch (nu_1)
modes will give the highest enhancement of the dissociation probability of
methane.Comment: 4 pages REVTeX, 2 figures (eps), to be published in Phys. Rev. B.
(See also arXiv:physics.chem-ph/0003031). Journal version at
http://publish.aps.org/abstract/PRB/v61/p1565
Comparison of PCBs and PAHs levels in European coastal waters using mussels from the <i>Mytilus edulis</i> complex as biomonitors
Mussels from the Mytilus edulis complex were used as biomonitors for two groups of organic pollutants: polychlorinated biphenyls (PCBs, congeners: 28, 52, 101, 118, 138, 153 and 180) and polycyclic aromatic hydrocarbons (PAHs, naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, benz(a)anthracene, chrysene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, indeno(1,2,3-cd)pyrene, dibenz(a,h)anthracene, benzo(g,h,i)perylene) at 17 sampling sites to assess their relative bioavailabilities in coastal waters around Europe. Because of the temporal differences in PCBs and PAHs concentrations, data were adjusted using Seasonal Variation Coefficients (SVC) before making large-scale spatial comparisons. The highest concentrations of PCBs were found near estuaries of large rivers flowing through urban areas and industrial regions. Elevated bioavailabilities of PAHs occurred in the vicinity of large harbors, urban areas, and regions affected by petroleum pollution as well as in some remote locations, which indicated long-range atmospheric deposition
Triatominae (Hemiptera, Reduviidae) in the Pantanal region: association with Trypanosoma cruzi, different habitats and vertebrate hosts.
The transmission cycle of Trypanosoma cruzi in the Brazilian Pantanal region has been studied during the last decade. Although considerable knowledge is available regarding the mammalian hosts infected by T. cruzi in this wetland, no studies have investigated its vectors in this region. This study aimed to investigate the presence of sylvatic triatomine species in different habitats of the Brazilian Pantanal region and to correlate their presence with the occurrences of vertebrate hosts and T. cruzi infection. Methods: The fieldwork involved passive search by using light traps and Noireau traps and active search by visual inspection. The light traps were placed at five selected points along forested areas for seven nights during each of the nine excursions. At each point where a light trap was set, eight Noireau traps were placed in palm trees and bromeliads. Results: In all, 88 triatomine bugs were collected: two and one individuals from light traps and Noireau traps, respectively; three from peridomestic areas; 23 in coati nests; and 59 in thornbird nests. In this study, active search in microhabitats showed higher efficiency than passive search, since 95% of the triatomine bugs were caught in nests. Further, triatomine bugs were only found to be infected by T. cruzi in coati nests. Conclusions: Coati nests might act as a point of convergence and dispersion for triatomine bugs and mammal hosts infected by T. cruzi, thereby playing an important role in the sylvatic cycle of T. cruzi in the Pantanal region
- …