4,787 research outputs found
Synthesis of strontium ferrite/iron oxide exchange coupled nano-powders with improved energy product for rare earth free permanent magnet applications
We present a simple, scalable synthesis route for producing exchange coupled
soft/hard magnetic composite powder that outperforms pure soft and hard phase
constituents. Importantly, the composites is iron oxide based (SrFe12O19 and
Fe3O4) and contain no rare earth or precious metal. The two step synthesis
process consists of first precipitating, an Iron oxide/hydroxide precursor
directly on top of SrFe12O19 nano-flakes, ensuring a very fine degree of mixing
between the hard and the soft magnetic phases. We then use a second step that
serves to reduce the precursor to create the proper soft magnetic phase and
create the intimate interface necessary for exchange coupling. We establish a
clear processing window; at temperatures below this window the desired soft
phase is not produced, while higher temperatures result in deleterious reaction
at the soft/hard phase interfaces, causing an improper ratio of soft to hard
phases. Improvements of Mr, Ms, and (BH)max are 42%, 29% and 37% respectively
in the SrFe12O19/Fe3O4 composite compared to pure hard phase (SrFe12O19). We
provide evidence of coupling (exchange spring behavior) with hysteresis curves,
first order reversal curve (FORC) analysis and recoil measurements.Comment: in J. Mater. Chem. C, 201
Pure States, Mixed States and Hawking Problem in Generalized Quantum Mechanics
This paper is the continuation of a study into the information paradox
problem started by the author in his earlier works. As previously, the key
instrument is a deformed density matrix in quantum mechanics of the early
universe. It is assumed that the latter represents quantum mechanics with
fundamental length. It is demonstrated that the obtained results agree well
with the canonical viewpoint that in the processes involving black holes pure
states go to the mixed ones in the assumption that all measurements are
performed by the observer in a well-known quantum mechanics. Also it is shown
that high entropy for Planck remnants of black holes appearing in the
assumption of the Generalized Uncertainty Relations may be explained within the
scope of the density matrix entropy introduced by the author previously. It is
noted that the suggested paradigm is consistent with the Holographic Principle.
Because of this, a conjecture is made about the possibility for obtaining the
Generalized Uncertainty Relations from the covariant entropy bound at high
energies in the same way as R. Bousso has derived Heisenberg uncertainty
principle for the flat space.Comment: 12 pages,no figures,some corrections,new reference
First principles derivation of a Rayleigh Gans Debye model for scattering from anisotropic inhomogeneities
Scattering problems are important in describing light propagation in wide
ranging media such as the atmosphere, colloidal solutions, metamaterials, glass
ceramic composites, transparent polycrystalline ceramics, and surfaces. The
Rayleigh Gans Debye (RGD) approximation has enjoyed great success in describing
a wide range of scattering phenomena. We derive a generalized RGD formulation
from the perturbation of Maxwell equations. In contrast to most treatments of
RGD scattering, our formulation can model any soft scattering phenomena in
linear media, including scattering by stochastic process, lossy media, and by
anisotropic inhomogeneities occurring at multiple length scales. Our
first-principles derivation makes explicit underlying assumptions and provides
jumping off points for more general treatments. The derivation also facilitates
a deeper understanding of soft scattering. It is demonstrated that sources of
scattering are not interfaces as is often presumed, but excess accelerating
charges emitting uncompensated radiation. Approximations to the equations are
also presented and discussed. For example, the scattering coefficient in the
large size RGD limit is shown to be proportional to the correlation length and
the variance of a projected phase shift
Dynamics for a 2-vertex Quantum Gravity Model
We use the recently introduced U(N) framework for loop quantum gravity to
study the dynamics of spin network states on the simplest class of graphs: two
vertices linked with an arbitrary number N of edges. Such graphs represent two
regions, in and out, separated by a boundary surface. We study the algebraic
structure of the Hilbert space of spin networks from the U(N) perspective. In
particular, we describe the algebra of operators acting on that space and
discuss their relation to the standard holonomy operator of loop quantum
gravity. Furthermore, we show that it is possible to make the restriction to
the isotropic/homogeneous sector of the model by imposing the invariance under
a global U(N) symmetry. We then propose a U(N) invariant Hamiltonian operator
and study the induced dynamics. Finally, we explore the analogies between this
model and loop quantum cosmology and sketch some possible generalizations of
it.Comment: 28 pages, v2: typos correcte
Monotonic Prefix Consistency in Distributed Systems
We study the issue of data consistency in distributed systems. Specifically,
we consider a distributed system that replicates its data at multiple sites,
which is prone to partitions, and which is assumed to be available (in the
sense that queries are always eventually answered). In such a setting, strong
consistency, where all replicas of the system apply synchronously every
operation, is not possible to implement. However, many weaker consistency
criteria that allow a greater number of behaviors than strong consistency, are
implementable in available distributed systems. We focus on determining the
strongest consistency criterion that can be implemented in a convergent and
available distributed system that tolerates partitions. We focus on objects
where the set of operations can be split into updates and queries. We show that
no criterion stronger than Monotonic Prefix Consistency (MPC) can be
implemented.Comment: Submitted pape
Deformed Density Matrix and Generalized Uncertainty Relation in Thermodynamics
A generalization of the thermodynamic uncertainty relations is proposed. It
is done by introducing of an additional term proportional to the interior
energy into the standard thermodynamic uncertainty relation that leads to
existence of the lower limit of inverse temperature. The authors are of the
opinion that the approach proposed may lead to proof of these relations. To
this end, the statistical mechanics deformation at Planck scale. The
statistical mechanics deformation is constructed by analogy to the earlier
quantum mechanical results. As previously, the primary object is a density
matrix, but now the statistical one. The obtained deformed object is referred
to as a statistical density pro-matrix. This object is explicitly described,
and it is demonstrated that there is a complete analogy in the construction and
properties of quantum mechanics and statistical density matrices at Plank scale
(i.e. density pro-matrices). It is shown that an ordinary statistical density
matrix occurs in the low-temperature limit at temperatures much lower than the
Plank's. The associated deformation of a canonical Gibbs distribution is given
explicitly.Comment: 15 pages,no figure
Quantum evolution according to real clocks
We characterize good clocks, which are naturally subject to fluctuations, in
statistical terms. We also obtain the master equation that governs the
evolution of quantum systems according to these clocks and find its general
solution. This master equation is diffusive and produces loss of coherence.
Moreover, real clocks can be described in terms of effective interactions that
are nonlocal in time. Alternatively, they can be modeled by an effective
thermal bath coupled to the system.Comment: RevTeX 3.01, 6 page
Sensitivity of Hawking radiation to superluminal dispersion relations
We analyze the Hawking radiation process due to collapsing configurations in
the presence of superluminal modifications of the dispersion relation. With
such superluminal dispersion relations, the horizon effectively becomes a
frequency-dependent concept. In particular, at every moment of the collapse,
there is a critical frequency above which no horizon is experienced. We show
that, as a consequence, the late-time radiation suffers strong modifications,
both quantitative and qualitative, compared to the standard Hawking picture.
Concretely, we show that the radiation spectrum becomes dependent on the
measuring time, on the surface gravities associated with different frequencies,
and on the critical frequency. Even if the critical frequency is well above the
Planck scale, important modifications still show up.Comment: 14 pages, 7 figures. Extensive paragraph added in conclusions to
clarify obtained result
A Multiwavelength Study of Young Massive Star-Forming Regions. III. Mid-Infrared Emission
We present mid-infrared (MIR) observations, made with the TIMMI2 camera on
the ESO 3.6 m telescope, toward 14 young massive star-forming regions. All
regions were imaged in the N band, and nine in the Q band, with an angular
resolution of ~ 1 arcsec. Typically, the regions exhibit a single or two
compact sources (with sizes in the range 0.008-0.18 pc) plus extended diffuse
emission. The Spitzer-Galactic Legacy Infrared Mid-Plane Survey Extraordinaire
images of these regions show much more extended emission than that seen by
TIMMI2, and this is attributed to polycyclic aromatic hydrocarbon (PAH) bands.
For the MIR sources associated with radio continuum radiation (Paper I) there
is a close morphological correspondence between the two emissions, suggesting
that the ionized gas (radio source) and hot dust (MIR source) coexist inside
the H II region. We found five MIR compact sources which are not associated
with radio continuum emission, and are thus prime candidates for hosting young
massive protostars. In particular, objects IRAS 14593-5852 II (only detected at
17.7 microns) and 17008-4040 I are likely to be genuine O-type protostellar
objects. We also present TIMMI2 N-band spectra of eight sources, all of which
are dominated by a prominent silicate absorption feature (~ 9.7 microns). From
these data we estimate column densities in the range (7-17)x10^22 cm^-2, in
good agreement with those derived from the 1.2 mm data (Paper II). Seven
sources show bright [Ne II] line emission, as expected from ionized gas
regions. Only IRAS 123830-6128 shows detectable PAH emission at 8.6 and 11.3
microns.Comment: Published in ApJ. 15 pages, 6 figures. Formatted with emulateapj; v2:
Minor language changes to match the published versio
Massive protostars as gamma-ray sources
Massive protostars have associated bipolar outflows with velocities of
hundreds of km s. Such outflows can produce strong shocks when interact
with the ambient medium leading to regions of non-thermal radio emission. We
aim at exploring under which conditions relativistic particles are accelerated
at the terminal shocks of the protostellar jets and can produce significant
gamma-ray emission. We estimate the conditions necessary for particle
acceleration up to very high energies and gamma-ray production in the
non-thermal hot spots of jets associated with massive protostars embedded in
dense molecular clouds. We show that relativistic Bremsstrahlung and
proton-proton collisions can make molecular clouds with massive young stellar
objects detectable by the {\it Fermi}{} satellite at MeV-GeV energies and by
Cherenkov telescope arrays in the GeV-TeV range. Gamma-ray astronomy can be
used to probe the physical conditions in star forming regions and particle
acceleration processes in the complex environment of massive molecular clouds.Comment: 10 pages, 5 figures, 2 tables, accepted for publication in Astronomy
and Astrophysic
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