464 research outputs found
Impact de pollutions ponctuelles sur les phytocénoses des rivières acides à neutres du Limousin (Massif Central, France)
L'impact des pollutions ponctuelles sur les phytocénoses aquatiques est étudié autour des rejets de 12 agglomérations dont 9 sont équipées d'une station d'épuration. Un échantillonnage systématique avec segmentation du cours d'eau autour de chaque rejet est réalisé. Sur chaque secteur, des relevés de végétation sont pratiqués au niveau de faciès d'écoulements homogènes dont on caractérise le milieu physique parallèlement à une analyse physicochimique de l'eau.L'ensemble des rejets provoque globalement une élévation de la conductivité, des teneurs en ammonium, nitrates et orthophosphates.Cela ce traduit par la régression de la phytocénose à Callitriche hamulata et Myriophyllum alterniflorum, par le développement de Ranunculus peltatus, Callitriche platycarpa et d'espèces cryptogames telles que Leptodyctium riparium, ou Melosira sp.Une Analyse en Composantes Principales menée sur l'ensemble des données permet d'opposer des phytocénoses propres aux secteurs amonts (Scapania undulata, Chiloscyphus polyanthus) à d'autres situées au niveau de rejets (Callitriche platycarpa, Leptodictyum riparium, Melosira sp.,).Une Analyse Canonique de Correspondances valide le déterminisme de la qualité physicochimique de l'eau sur la végétation. La conductivité, les teneurs en ammonium, nitrates et orthophosphates deviennent prépondérants par rapport aux facteurs du milieu physique classiquement discriminants dans l'installation des phytocénoses dans les rivières limousines.The impact of located pollution on aquatic phytocénoses is studied around 12 cities discharge. Nine of them are fitted out purification plant.The sampling method is based on consecutive segments from upstream to downstream. On each sector, vegetation records are realized in homogeneous water runoff facies, which are characterized by physical factors as well as water value measures.The whole discharge leads globally to an increase of conductivity, ammonium amount, nitrates and orthophosphates. The consequence of that is a decrease of Callitriche hamulata and Myriophyllum alterniflorum phytocénoses, a development of Ranunculus peltatus, Callitriche platycarpa and cryptogams species like Leptodictyum riparium or Melosira sp.A Component Principal Analysis applied on data, distinguishes phytocénoses belonging to upstream sectors (Scapania undulata, Chiloscyphus polyanthus) from the ones of discharges (Callitriche platycarpa, Leptodictyum riparium, Melosira sp.).A Canonical Correspondence Analysis validates the impact of physico-chemical water quality on vegetation. Conductivity, ammonium amount, nitrates and orthophosphates become more preponderant in comparison with physical environments usually discriminant for phytocénoses installation in Limousin rivers
Observation of the spin-charge thermal isolation of ferromagnetic Ga_{0.94}Mn_{0.06}As by time-resolved magneto-optical measurement
The dynamics of magnetization under femtosecond optical excitation is studied
in a ferromagnetic semiconductor Ga_{0.94}Mn_{0.06}As with a time-resolved
magneto-optical Kerr effect measurement with two color probe beams. The
transient reflectivity change indicates the rapid rise of the carrier
temperature and relaxation to a quasi-thermal equilibrium within 1 ps, while a
very slow rise of the spin temperature of the order of 500ps is observed. This
anomalous behavior originates from the thermal isolation between the charge and
spin systems due to the spin polarization of carriers (holes) contributing to
ferromagnetism. This constitutes experimental proof of the half-metallic nature
of ferromagnetic Ga_{0.94}Mn_{0.06}As arising from double exchange type
mechanism originates from the d-band character of holes
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
Stability of trions in strongly spin-polarized two-dimensional electron gases
Low-temperature magneto-photoluminescence studies of negatively charged
excitons (X- trions) are reported for n-type modulation-doped ZnSe/Zn(Cd,Mn)Se
quantum wells over a wide range of Fermi energy and spin-splitting. The
magnetic composition is chosen such that these magnetic two-dimensional
electron gases (2DEGs) are highly spin-polarized even at low magnetic fields,
throughout the entire range of electron densities studied (5e10 to 6.5e11
cm^-2). This spin polarization has a pronounced effect on the formation and
energy of X-, with the striking result that the trion ionization energy (the
energy separating X- from the neutral exciton) follows the temperature- and
magnetic field-tunable Fermi energy. The large Zeeman energy destabilizes X- at
the nu=1 quantum limit, beyond which a new PL peak appears and persists to 60
Tesla, suggesting the formation of spin-triplet charged excitons.Comment: 5 pages (RevTex), 4 embedded EPS figs. Submitted to PRB-R
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
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
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
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
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