1,974 research outputs found
Arbuscular Mycorrhiza Symbiosis of \u3ci\u3eDactylis glomerata\u3c/i\u3e L. and \u3ci\u3eAnthoxanthum odoratum\u3c/i\u3e L. in an Acidic Pasture
The asymptotic quasi-stationary states of the two-dimensional magnetically confined plasma and of the planetary atmosphere
We derive the differential equation governing the asymptotic quasi-stationary
states of the two dimensional plasma immersed in a strong confining magnetic
field and of the planetary atmosphere. These two systems are related by the
property that there is an intrinsic constant length: the Larmor radius and
respectively the Rossby radius and a condensate of the vorticity field in the
unperturbed state related to the cyclotronic gyration and respectively to the
Coriolis frequency. Although the closest physical model is the
Charney-Hasegawa-Mima (CHM) equation, our model is more general and is related
to the system consisting of a discrete set of point-like vortices interacting
in plane by a short range potential. A field-theoretical formalism is developed
for describing the continuous version of this system. The action functional can
be written in the Bogomolnyi form (emphasizing the role of Self-Duality of the
asymptotic states) but the minimum energy is no more topological and the
asymptotic structures appear to be non-stationary, which is a major difference
with respect to traditional topological vortex solutions. Versions of this
field theory are discussed and we find arguments in favor of a particular form
of the equation. We comment upon the significant difference between the CHM
fluid/plasma and the Euler fluid and respectively the Abelian-Higgs vortex
models.Comment: Latex 126 pages, 7 eps figures included. Discussion on various forms
of the equatio
Finite Temperature Effects in One-dimensional Mott-Hubbard Insulator: Angle-Resolved Photoemission Study of Na_{0.96}V_{2}O_{5}
We have made an angle-resolved photoemission study of a one-dimensional (1D)
Mott-Hubbard insulator Na_{0.96}V_{2}O_{5} and found that the spectra of the V
3d lower Hubbard band are strongly dependent on the temperature. We have
calculated the one-particle spectral function of the one-dimensional t-J model
at finite temperatures by exact diagonalization and compared them with the
experimental results. Good overall agreement is obtained between experiment and
theory. The strong finite temperature effects are discussed in terms of the
existence of the ``Fermi surface'' of the spinon band.Comment: 4 pages, 3 figure
Can dark matter be a Bose-Einstein condensate?
We consider the possibility that the dark matter, which is required to
explain the dynamics of the neutral hydrogen clouds at large distances from the
galactic center, could be in the form of a Bose-Einstein condensate. To study
the condensate we use the non-relativistic Gross-Pitaevskii equation. By
introducing the Madelung representation of the wave function, we formulate the
dynamics of the system in terms of the continuity equation and of the
hydrodynamic Euler equations. Hence dark matter can be described as a
non-relativistic, Newtonian Bose-Einstein gravitational condensate gas, whose
density and pressure are related by a barotropic equation of state. In the case
of a condensate with quartic non-linearity, the equation of state is polytropic
with index . To test the validity of the model we fit the Newtonian
tangential velocity equation of the model with a sample of rotation curves of
low surface brightness and dwarf galaxies, respectively. We find a very good
agreement between the theoretical rotation curves and the observational data
for the low surface brightness galaxies. The deflection of photons passing
through the dark matter halos is also analyzed, and the bending angle of light
is computed. The bending angle obtained for the Bose-Einstein condensate is
larger than that predicted by standard general relativistic and dark matter
models. Therefore the study of the light deflection by galaxies and the
gravitational lensing could discriminate between the Bose-Einstein condensate
dark matter model and other dark matter models.Comment: 20 pages, 7 figures, accepted for publication in JCAP, references
adde
Charge Ordering and Ferroelectricity in Half-doped Manganites
By means of density-functional simulations for half-doped manganites, such as
pseudocubic Pr0.5Ca0.5MnO3 and bilayer PrCa2Mn2O7, we discuss the occurrence of
ferroelectricity and we explore its crucial relation to the crystal structure
and to peculiar charge/spin/orbital ordering effects. In pseudocubic
Pr0.5Ca0.5MnO3, ferroelectricity is induced in the Zener polaron type
structure, where Mn ions are dimerized. In marked contrast, in bilayer
PrCa2Mn2O7, it is the displacements of apical oxygens bonded to either Mn3+ or
Mn4+ ions that play a key role in the rising of ferroelectricity. Importantly,
local dipoles due to apical oxygens are also intimately linked to charge and
orbital ordering patterns in MnO2 planes, which in turn contribute to
polarization. Finally, an important outcome of our work consists in proposing
Born effective charges as a valid mean to quantify charge disproportionation
effects, in terms of anisotropy and size of electronic clouds around Mn ions.Comment: 5 pages, 2 figures, submitted for publicatio
Understanding adhesion at as-deposited interfaces from ab initio thermodynamics of deposition growth: thin-film alumina on titanium carbide
We investigate the chemical composition and adhesion of chemical vapour
deposited thin-film alumina on TiC using and extending a recently proposed
nonequilibrium method of ab initio thermodynamics of deposition growth (AIT-DG)
[Rohrer J and Hyldgaard P 2010 Phys. Rev. B 82 045415]. A previous study of
this system [Rohrer J, Ruberto C and Hyldgaard P 2010 J. Phys.: Condens. Matter
22 015004] found that use of equilibrium thermodynamics leads to predictions of
a non-binding TiC/alumina interface, despite the industrial use as a
wear-resistant coating. This discrepancy between equilibrium theory and
experiment is resolved by the AIT-DG method which predicts interfaces with
strong adhesion. The AIT-DG method combines density functional theory
calculations, rate-equation modelling of the pressure evolution of the
deposition environment and thermochemical data. The AIT-DG method was
previously used to predict prevalent terminations of growing or as-deposited
surfaces of binary materials. Here we extent the method to predict surface and
interface compositions of growing or as-deposited thin films on a substrate and
find that inclusion of the nonequilibrium deposition environment has important
implications for the nature of buried interfaces.Comment: 8 pages, 6 figures, submitted to J. Phys.: Condens. Matte
Morphology of axisymmetric vesicles with encapsulated filaments and impurities
The shape deformation of a three-dimensional axisymmetric vesicle with
encapsulated filaments or impurities is analyzed by integrating a dissipation
dynamics. This method can incorporate systematically the constraint of a fixed
surface area and/or a fixed volume. The filament encapsulated in a vesicle is
assumed to take a form of a rod or a ring so as to imitate cytoskeletons. In
both cases, results of the shape transition of the vesicle are summarized in
phase diagrams in the phase space of the vesicular volume and a rod length or a
ring radius.
We also study the dynamics of a vesicle with impurities coupled to the
membrane curvature. The phase separation and the associated shape deformation
in the early stage of the dynamical evolution can well be explained by the
linear stability analysis. Long runs of simulation demonstrate the nonlinear
coarsening of the wavy deformation of the vesicle in the late stage.Comment: 9 pages, 9 figure
Fermi Surface of 3d^1 Perovskite CaVO3 Near the Mott Transition
We present a detailed de Haas van Alphen effect study of the perovskite
CaVO3, offering an unprecedented test of electronic structure calculations in a
3d transition metal oxide. Our experimental and calculated Fermi surfaces are
in good agreement -- but only if we ignore large orthorhombic distortions of
the cubic perovskite structure. Subtle discrepancies may shed light on an
apparent conflict between the low energy properties of CaVO3, which are those
of a simple metal, and high energy probes which reveal strong correlations that
place CaVO3 on the verge of a metal-insulator transition.Comment: 4 pages, 4 figures (REVTeX
Point defects, ferromagnetism and transport in calcium hexaboride
The formation energy and local magnetic moment of a series of point defects
in CaB are computed using a supercell approach within the generalized
gradient approximation to density functional theory. Based on these results,
speculations are made as to the influence of these defects on electrical
transport. It is found that the substitution of Ca by La does not lead to the
formation of a local moment, while a neutral B vacancy carries a moment of
2.4 Bohr magnetons, mostly distributed over the six nearest-neighbour B atoms.
A plausible mechanism for the ferromagnetic ordering of these moments is
suggested. Since the same broken B-B bonds appear on the preferred (100)
cleavage planes of the CaB structure, it is argued that internal surfaces
in polycrystals as well as external surfaces in general will make a large
contribution to the observed magnetization.Comment: Calculated defect formation energies had to be corrected, due to the
use of a wrong reference energy for the perfect crystal in the original pape
Measurement of the 8B Solar Neutrino Flux with the KamLAND Liquid Scintillator Detector
We report a measurement of the neutrino-electron elastic scattering rate from
8B solar neutrinos based on a 123 kton-day exposure of KamLAND. The
background-subtracted electron recoil rate, above a 5.5 MeV analysis threshold
is 1.49+/-0.14(stat)+/-0.17(syst) events per kton-day. Interpreted as due to a
pure electron flavor flux with a 8B neutrino spectrum, this corresponds to a
spectrum integrated flux of 2.77+/-0.26(stat)+/-0.32(syst) x 10^6 cm^-2s^-1.
The analysis threshold is driven by 208Tl present in the liquid scintillator,
and the main source of systematic uncertainty is due to background from
cosmogenic 11Be. The measured rate is consistent with existing measurements and
with Standard Solar Model predictions which include matter enhanced neutrino
oscillation.Comment: 6 pages, 3 figure
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