309 research outputs found
Large internal waves in Massachusetts Bay transport sediments offshore
This paper is not subject to U.S. copyright. The definitive version was published in Continental Shelf Research 26 (2006): 2029-2049, doi:10.1016/j.csr.2006.07.022.A field experiment was carried out in Massachusetts Bay in August 1998 to assess the role of large-amplitude internal waves (LIWs) in resuspending bottom sediments. The field experiment consisted of a four-element moored array extending from just west of Stellwagen Bank (90-m water depth) across Stellwagen Basin (85- and 50-m water depth) to the coast (24-m water depth). The LIWs were observed in packets of 5–10 waves, had periods of 5–10 min and wavelengths of 200–400 m, and caused downward excursions of the thermocline of as much as 30 m. At the 85-m site, the current measured 1 m above bottom (mab) typically increased from near 0 to 0.2 m/s offshore in a few minutes upon arrival of the LIWs. At the 50-m site, the near-bottom offshore flow measured 6 mab increased from about 0.1 to 0.4–0.6 m/s upon arrival of the LIWs and remained offshore in the bottom layer for 1–2 h. The near-bottom currents associated with the LIWs, in concert with the tidal currents, were directed offshore and sufficient to resuspend the bottom sediments at both the 50- and 85-m sites. When LIWs are present, they may resuspend sediments for as long as 5 hours each tidal cycle as they travel westward across Stellwagen Basin. At 85-m water depth, resuspension associated with LIWs is estimated to occur for about 0.4 days each summer, about the same amount of time as caused by surface waves.MBIWE98 was supported by the USGS and the Office of Naval Research (ONR). The long-term observations at LT-A and LT-B were conducted under a Joint Funding Agreement between the USGS and the Massachusetts Water Resources Authority and an Inter-Service Agreement with the US Coast Guard. A. Scotti received support from the WHOI Postdoctoral Scholar program, the Johnson Foundation, the USGS, and ONR through grant N00014-01-1-0172; R. Beardsley through ONR grants N00014-98-1-0059, N00014-00-1-0210 and the WHOI Smith Chair in Coastal Physical Oceanography; and S. Anderson through ONR grant N000140-97-1-0158
Light and electric field control of ferromagnetism in magnetic quantum structures
A strong influence of illumination and electric bias on the Curie temperature
and saturation value of the magnetization is demonstrated for semiconductor
structures containing a modulation-doped p-type Cd0.96Mn0.04Te quantum well
placed in various built-in electric fields. It is shown that both light beam
and bias voltage generate an isothermal and reversible cross-over between the
paramagnetic and ferromagnetic phases, in the way that is predetermined by the
structure design. The observed behavior is in quantitative agreement with the
expectations for systems, in which ferromagnetic interactions are mediated by
the weakly disordered two-dimensional hole liquid.Comment: 4 pages and 3 figure
Theory of Magnetic Properties and Spin-Wave Dispersion for Ferromagnetic (Ga,Mn)As
We present a microscopic theory of the long-wavelength magnetic properties of
the ferromagnetic diluted magnetic semiconductor (Ga,Mn)As. Details of the host
semiconductor band structure, described by a six-band Kohn-Luttinger
Hamiltonian, are taken into account. We relate our quantum-mechanical
calculation to the classical micromagnetic energy functional and determine
anisotropy energies and exchange constants. We find that the exchange constant
is substantially enhanced compared to the case of a parabolic heavy-hole-band
model.Comment: 9 pages, 4 figure
Interlayer coupling in ferromagnetic semiconductor superlattices
We develop a mean-field theory of carrier-induced ferromagnetism in diluted
magnetic semiconductors. Our approach represents an improvement over standard
RKKY model allowing spatial inhomogeneity of the system, free-carrier spin
polarization, finite temperature, and free-carrier exchange and correlation to
be accounted for self-consistently. As an example, we calculate the electronic
structure of a MnGaAs/GaAs superlattice with alternating
ferromagnetic and paramagnetic layers and demonstrate the possibility of
semiconductor magnetoresistance systems with designed properties.Comment: 4 pages, 4 figure
Theory of Magnetic Anisotropy in III_{1-x}Mn_{x}V Ferromagnets
We present a theory of magnetic anisotropy in diluted magnetic semiconductors with carrier-induced
ferromagnetism. The theory is based on four and six band envelope functions
models for the valence band holes and a mean-field treatment of their exchange
interactions with ions. We find that easy-axis reorientations
can occur as a function of temperature, carrier density , and strain. The
magnetic anisotropy in strain-free samples is predicted to have a
hole-density dependence at small , a dependence at large , and
remarkably large values at intermediate densities. An explicit expression,
valid at small , is given for the uniaxial contribution to the magnetic
anisotropy due to unrelaxed epitaxial growth lattice-matching strains. Results
of our numerical simulations are in agreement with magnetic anisotropy
measurements on samples with both compressive and tensile strains. We predict
that decreasing the hole density in current samples will lower the
ferromagnetic transition temperature, but will increase the magnetic anisotropy
energy and the coercivity.Comment: 15 pages, 15 figure
Vertical distribution and diel patterns of zooplankton abundance and biomass at Conch Reef, Florida Keys (USA)
Zooplankton play an important role in the trophic dynamics of coral reef ecosystems. Detailed vertical and temporal distribution and biomass of zooplankton were evaluated at four heights off the bottom and at six times throughout the diel cycle over a coral reef in the Florida Keys (USA). Zooplankton abundance averaged 4396 ± 1949 SD individuals m−3, but temporal and spatial distributions varied for individual zooplankton taxa by time of day and by height off the bottom. Copepods comprised 93–96% of the abundance in the samples. Taxon-based zooplankton CHN values paired with abundance data were used to estimate biomass. Average daily biomass ranged from 3.1 to 21.4 mg C m−3 and differed by both height off the bottom and by time of day. While copepods were the numerically dominant organisms, their contribution to biomass was only 35% of the total zooplankton biomass. Our findings provide important support for the new emerging paradigm of how zooplankton are distributed over reefs
A theory of ferromagnetism in planar heterostructures of (Mn,III)-V semiconductors
A density functional theory of ferromagnetism in heterostructures of compound
semiconductors doped with magnetic impurities is presented. The variable
functions in the density functional theory are the charge and spin densities of
the itinerant carriers and the charge and localized spins of the impurities.
The theory is applied to study the Curie temperature of planar heterostructures
of III-V semiconductors doped with manganese atoms. The mean-field,
virtual-crystal and effective-mass approximations are adopted to calculate the
electronic structure, including the spin-orbit interaction, and the magnetic
susceptibilities, leading to the Curie temperature. By means of these results,
we attempt to understand the observed dependence of the Curie temperature of
planar -doped ferromagnetic structures on variation of their
properties. We predict a large increase of the Curie Temperature by additional
confinement of the holes in a -doped layer of Mn by a quantum well.Comment: 8 pages, 7 figure
Mesoscale subduction at the Almeria-Oran front. Part 2: biophysical interactions.
This paper presents a detailed diagnostic analysis of hydrographic and current meter data from three, rapidly repeated, fine-scale surveys of the Almeria–Oran front. Instability of the frontal boundary, between surface waters of Atlantic and Mediterranean origin, is shown to provide a mechanism for significant heat transfer from the surface layers to the deep ocean in winter. The data were collected during the second observational phase of the EU funded OMEGA project on RRS Discovery cruise 224 during December 1996. High resolution hydrographic measurements using the towed undulating CTD vehicle, SeaSoar, traced the subduction of Mediterranean Surface Water across the Almeria–Oran front. This subduction is shown to result from a significant baroclinic component to the instability of the frontal jet. The Q-vector formulation of the omega equation is combined with a scale analysis to quantitatively diagnose vertical transport resulting from mesoscale ageostrophic circulation. The analyses are presented and discussed in the presence of satellite and airborne remotely sensed data; which provide the basis for a thorough and novel approach to the determination of observational error
Algebraic Comparison of Partial Lists in Bioinformatics
The outcome of a functional genomics pipeline is usually a partial list of
genomic features, ranked by their relevance in modelling biological phenotype
in terms of a classification or regression model. Due to resampling protocols
or just within a meta-analysis comparison, instead of one list it is often the
case that sets of alternative feature lists (possibly of different lengths) are
obtained. Here we introduce a method, based on the algebraic theory of
symmetric groups, for studying the variability between lists ("list stability")
in the case of lists of unequal length. We provide algorithms evaluating
stability for lists embedded in the full feature set or just limited to the
features occurring in the partial lists. The method is demonstrated first on
synthetic data in a gene filtering task and then for finding gene profiles on a
recent prostate cancer dataset
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