195 research outputs found
Boundary Energies and the Geometry of Phase Separation in Double--Exchange Magnets
We calculate the energy of a boundary between ferro- and antiferromagnetic
regions in a phase separated double-exchange magnet in two and three
dimensions. The orientation dependence of this energy can significantly affect
the geometry of the phase-separated state in two dimensions, changing the
droplet shape and possibly stabilizing a striped arrangement within a certain
range of the model parameters. A similar effect, albeit weaker, is also present
in three dimensions. As a result, a phase-separated system near the percolation
threshold is expected to possess intrinsic hysteretic transport properties,
relevant in the context of recent experimental findings.Comment: 6 pages, including 4 figures; expanded versio
Inhomogeneous Phases in a Double-Exchange Magnet with Long Range Coulomb Interactions
We consider a model with competing double-exchange (ferromagnetic) and
super-exchange (anti-ferromagnetic) interactions in the regime where phase
separation takes place. The presence of a long range Coulomb interaction
frustrates a macroscopic phase separation, and favors microscopically
inhomogeneous configurations. We use the variational Hartree-Fock approach, in
conjunction with Monte-Carlo simulations to study the geometry of such
configurations in a two-dimensional system. We find that an array of diamond
shaped ferromagnetic droplets is the preferred configuration at low electronic
densities, while alternating ferromagnetic and anti-ferromagnetic diagonal
stripes emerge at higher densities. These findings are expected to be relevant
for thin films of colossal magneto-resistive manganates.Comment: 15 pages, 9 figures. Journal Ref. added, errors correcte
Dvoretzky type theorems for multivariate polynomials and sections of convex bodies
In this paper we prove the Gromov--Milman conjecture (the Dvoretzky type
theorem) for homogeneous polynomials on , and improve bounds on
the number in the analogous conjecture for odd degrees (this case
is known as the Birch theorem) and complex polynomials. We also consider a
stronger conjecture on the homogeneous polynomial fields in the canonical
bundle over real and complex Grassmannians. This conjecture is much stronger
and false in general, but it is proved in the cases of (for 's of
certain type), odd , and the complex Grassmannian (for odd and even and
any ). Corollaries for the John ellipsoid of projections or sections of a
convex body are deduced from the case of the polynomial field conjecture
Resistivity of Mixed-Phase Manganites
The resistivity of manganites is studied using a
random-resistor-network, based on phase-separation between metallic and
insulating domains. When percolation occurs, both as chemical composition and
temperature vary, results in good agreement with experiments are obtained.
Similar conclusions are reached using quantum calculations and microscopic
considerations. Above the Curie temperature, it is argued that ferromagnetic
clusters should exist in Mn-oxides. Small magnetic fields induce large
changes and a bad-metal state with (disconnected) insulating
domains.Comment: 4 pages, 4 eps figure
Detection of X-ray galaxy clusters based on the Kolmogorov method
The detection of clusters of galaxies in large surveys plays an important
part in extragalactic astronomy, and particularly in cosmology, since cluster
counts can give strong constraints on cosmological parameters. X-ray imaging is
in particular a reliable means to discover new clusters, and large X-ray
surveys are now available. Considering XMM-Newton data for a sample of 40 Abell
clusters, we show that their analysis with a Kolmogorov distribution can
provide a distinctive signature for galaxy clusters. The Kolmogorov method is
sensitive to the correlations in the cluster X-ray properties and can therefore
be used for their identification, thus allowing to search reliably for clusters
in a simple way
Kolmogorov analysis detecting radio and Fermi gamma-ray sources in cosmic microwave background maps
The Kolmogorov stochasticity parameter is shown to act as a tool to detect
point sources in the cosmic microwave background (CMB) radiation temperature
maps. Kolmogorov CMB map constructed for the WMAP's 7-year datasets reveals
tiny structures which in part coincide with point radio and Fermi/LAT gamma-ray
sources. In the first application of this method, we identified several sources
not present in the then available 0FGL Fermi catalog. Subsequently they were
confirmed in the more recent and more complete 1FGL catalog, thus strengthening
the evidence for the power of this methodology.Comment: 4 pages, 3 figs, 1 Table; to match the published versio
The phase-separated states in antiferromagnetic semiconductors with polarizable lattice
The possibility of the slab or stripe phase separation (alternating
ferromagnetic highly- conductive and insulating antiferromagnetic layers) is
proved for isotropic degenerate antiferromagnetic semiconductors. This type of
phase separation competes with the droplet phase separation (ferromagnetic
droplets in the antiferromagnetic host or vice versa). The interaction of
electrons with optical phonons alone cannot cause phase-separated state with
alternating highly-conductive and insulating regions but it stabilizes the
magnetic phase separation. The magnetostriction deformation of the lattice in
the phase-separated state is investigated.Comment: 17 Pages, 1 EPS Figur
Limits on light-speed anisotropies from Compton scattering of high-energy electrons
The possibility of anisotropies in the speed of light relative to the
limiting speed of electrons is considered. The absence of sidereal variations
in the energy of Compton-edge photons at the ESRF's GRAAL facility constrains
such anisotropies representing the first non-threshold collision-kinematics
study of Lorentz violation. When interpreted within the minimal Standard-Model
Extension, this result yields the two-sided limit of 1.6 x 10^{-14} at 95%
confidence level on a combination of the parity-violating photon and electron
coefficients kappa_{o+} and c. This new constraint provides an improvement over
previous bounds by one order of magnitude.Comment: 4 pages, 4 figure
Lowering the Light Speed Isotropy Limit: European Synchrotron Radiation Facility Measurements
The measurement of the Compton edge of the scattered electrons in GRAAL
facility in European Synchrotron Radiation Facility (ESRF) in Grenoble with
respect to the Cosmic Microwave Background dipole reveals up to 10 sigma
variations larger than the statistical errors. We now show that the variations
are not due to the frequency variations of the accelerator. The nature of
Compton edge variations remains unclear, thus outlining the imperative of
dedicated studies of light speed anisotropy
A new limit on the light speed isotropy from the GRAAL experiment at the ESRF
When the electrons stored in the ring of the European Synchrotron Radiation
Facility (ESRF, Grenoble) scatter on a laser beam (Compton scattering in
flight) the lower energy of the scattered electron spectra, the Compton Edge
(CE), is given by the two body photon-electron relativistic kinematics and
depends on the velocity of light. A precision measurement of the position of
this CE as a function of the daily variations of the direction of the electron
beam in an absolute reference frame provides a one-way test of Relativistic
Kinematics and the isotropy of the velocity of light. The results of GRAAL-ESRF
measurements improve the previously existing one-way limits, thus showing the
efficiency of this method and the interest of further studies in this
direction.Comment: Proceed. MG12 meeting, Paris, July, 200
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