722 research outputs found
The Origin of a Repose Angle: Kinetics of Rearrangements for Granular Materials
A microstructural theory of dense granular materials is presented, based on
two main ideas. Firstly, that macroscopic shear results form activated local
rearrangements at a mesoscopic scale. Secondly, that the update frequency of
microscopic processes is determined by granular temperature. In a shear cell,
the resulting constitutive equations account for Bagnold's scaling and for the
existence of a Coulomb criterion of yield. In the case of a granular flow down
an inclined plane, they account for the rheology observed in recent experiments
and for the temperature and velocity profiles measured numerically.Comment: submitted to PR
CMB anisotropies seen by an off-center observer in a spherically symmetric inhomogeneous universe
The current authors have previously shown that inhomogeneous, but spherically
symmetric universe models containing only matter can yield a very good fit to
the SNIa data and the position of the first CMB peak. In this work we examine
how far away from the center of inhomogeneity the observer can be located in
these models and still fit the data well. Furthermore, we investigate whether
such an off-center location can explain the observed alignment of the lowest
multipoles of the CMB map. We find that the observer has to be located within a
radius of 15 Mpc from the center for the induced dipole to be less than that
observed by the COBE satellite. But for such small displacements from the
center, the induced quadru- and octopoles turn out to be insufficiently large
to explain the alignment.Comment: 8 pages (REVTeX4), 7 figures; v2: minor changes, matches published
versio
Double Distribution of Dark Matter Halos with respect to Mass and Local Overdensity
We present a double distribution function of dark matter halos, with respect
to both object mass and local over- (or under-) density. This analytical tool
provides a statistical treatment of the properties of matter surrounding
collapsed objects, and can be used to study environmental effects on
hierarchical structure formation. The size of the "local environment" of a
collapsed object is defined to depend on the mass of the object. The
Press-Schechter mass function is recovered by integration of our double
distribution over the density contrast. We also present a detailed treatment of
the evolution of overdensities and underdensities in Einstein-deSitter and flat
LCDM universes, according to the spherical evolution model. We explicitly
distinguish between true and linearly extrapolated overdensities and provide
conversion relations between the two quantities.Comment: 25 pages, 10 figures, comments welcom
On-chip III-V monolithic integration of heralded single photon sources and beamsplitters
We demonstrate a monolithic III-V photonic circuit combining a heralded
single photon source with a beamsplitter, at room temperature and telecom
wavelength. Pulsed parametric down-conversion in an AlGaAs waveguide generates
counterpropagating photons, one of which is used to herald the injection of its
twin into the beamsplitter. We use this configuration to implement an
integrated Hanbury-Brown and Twiss experiment, yielding a heralded second-order
correlation that confirms single-photon
operation. The demonstrated generation and manipulation of quantum states on a
single III-V semiconductor chip opens promising avenues towards real-world
applications in quantum information
Can decaying modes save void models for acceleration?
The unexpected dimness of Type Ia supernovae (SNe), apparently due to
accelerated expansion driven by some form of dark energy or modified gravity,
has led to attempts to explain the observations using only general relativity
with baryonic and cold dark matter, but by dropping the standard assumption of
homogeneity on Hubble scales. In particular, the SN data can be explained if we
live near the centre of a Hubble-scale void. However, such void models have
been shown to be inconsistent with various observations, assuming the void
consists of a pure growing mode. Here it is shown that models with significant
decaying mode contribution today can be ruled out on the basis of the expected
cosmic microwave background spectral distortion. This essentially closes one of
the very few remaining loopholes in attempts to rule out void models, and
strengthens the evidence for Hubble-scale homogeneity.Comment: 11 pages, 3 figures; discussion expanded, appendix added; version
accepted to Phys. Rev.
Determination of the local concentrations of Mn interstitials and antisite defects in GaMnAs
We present a method for the determination of the local concentrations of
interstitial and substitutional Mn atoms and As antisite defects in GaMnAs. The
method relies on the sensitivity of the structure factors of weak reflections
to the concentrations and locations of these minority constituents. High
spatial resolution is obtained by combining structure factor measurement and
X-ray analysis in a transmission electron microscope. We demonstrate the
prevalence of interstitials with As nearest neighbors in as-grown layers.Comment: 10 pages, 4 figure
Magnetic properties and domain structure of (Ga,Mn)As films with perpendicular anisotropy
The ferromagnetism of a thin GaMnAs layer with a perpendicular easy
anisotropy axis is investigated by means of several techniques, that yield a
consistent set of data on the magnetic properties and the domain structure of
this diluted ferromagnetic semiconductor. The magnetic layer was grown under
tensile strain on a relaxed GaInAs buffer layer using a procedure that limits
the density of threading dislocations. Magnetometry, magneto-transport and
polar magneto-optical Kerr effect (PMOKE) measurements reveal the high quality
of this layer, in particular through its high Curie temperature (130 K) and
well-defined magnetic anisotropy. We show that magnetization reversal is
initiated from a limited number of nucleation centers and develops by easy
domain wall propagation. Furthermore, MOKE microscopy allowed us to
characterize in detail the magnetic domain structure. In particular we show
that domain shape and wall motion are very sensitive to some defects, which
prevents a periodic arrangement of the domains. We ascribed these defects to
threading dislocations emerging in the magnetic layer, inherent to the growth
mode on a relaxed buffer
Universal conductance fluctuations in epitaxial GaMnAs ferromagnets: structural and spin disorder
Mesoscopic transport measurements reveal a large effective phase coherence
length in epitaxial GaMnAs ferromagnets, contrary to usual 3d-metal
ferromagnets. Universal conductance fluctuations of single nanowires are
compared for epilayers with a tailored anisotropy. At large magnetic fields,
quantum interferences are due to structural disorder only, and an unusual
behavior related to hole-induced ferromagnetism is evidenced, for both quantum
interferences and decoherence. At small fields, phase coherence is shown to
persist down to zero field, even in presence of magnons, and an additional spin
disorder contribution to quantum interferences is observed under domain walls
nucleation.Comment: 15 pages, 4 figure
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