1,492 research outputs found
Models of f(R) Cosmic Acceleration that Evade Solar-System Tests
We study a class of metric-variation f(R) models that accelerates the
expansion without a cosmological constant and satisfies both cosmological and
solar-system tests in the small-field limit of the parameter space.
Solar-system tests alone place only weak bounds on these models, since the
additional scalar degree of freedom is locked to the high-curvature
general-relativistic prediction across more than 25 orders of magnitude in
density, out through the solar corona. This agreement requires that the
galactic halo be of sufficient extent to maintain the galaxy at high curvature
in the presence of the low-curvature cosmological background. If the galactic
halo and local environment in f(R) models do not have substantially deeper
potentials than expected in LCDM, then cosmological field amplitudes |f_R| >
10^{-6} will cause the galactic interior to evolve to low curvature during the
acceleration epoch. Viability of large-deviation models therefore rests on the
structure and evolution of the galactic halo, requiring cosmological
simulations of f(R) models, and not directly on solar-system tests. Even small
deviations that conservatively satisfy both galactic and solar-system
constraints can still be tested by future, percent-level measurements of the
linear power spectrum, while they remain undetectable to cosmological-distance
measures. Although we illustrate these effects in a specific class of models,
the requirements on f(R) are phrased in a nearly model-independent manner.Comment: 13 pages, 10 figures. Submitted to Phys. Rev.
Critical sets of the total variance of state detect all SLOCC entanglement classes
We present a general algorithm for finding all classes of pure multiparticle
states equivalent under Stochastic Local Operations and Classsical
Communication (SLOCC). We parametrize all SLOCC classes by the critical sets of
the total variance function. Our method works for arbitrary systems of
distinguishable and indistinguishable particles. We also discuss the Morse
indices of critical points which have the interpretation of the number of
independent non-local perturbations increasing the variance and hence
entanglement of a state. We illustrate our method by two examples.Comment: 4 page
Entanglement of four qubit systems: a geometric atlas with polynomial compass I (the finite world)
We investigate the geometry of the four qubit systems by means of algebraic
geometry and invariant theory, which allows us to interpret certain entangled
states as algebraic varieties. More precisely we describe the nullcone, i.e.,
the set of states annihilated by all invariant polynomials, and also the so
called third secant variety, which can be interpreted as the generalization of
GHZ-states for more than three qubits. All our geometric descriptions go along
with algorithms which allow us to identify any given state in the nullcone or
in the third secant variety as a point of one of the 47 varieties described in
the paper. These 47 varieties correspond to 47 non-equivalent entanglement
patterns, which reduce to 15 different classes if we allow permutations of the
qubits.Comment: 48 pages, 7 tables, 13 figures, references and remarks added (v2
Control of Coercivities in (Ga,Mn)As Thin Films by Small Concentrations of MnAs Nanoclusters
We demonstrate that low concentrations of a secondary magnetic phase in
(Ga,Mn)As thin films can enhance the coercivity by factors up to ~100 without
significantly degrading the Curie temperature or saturation magnetisation.
Magnetic measurements indicate that the secondary phase consists of MnAs
nanoclusters, of average size ~7nm. This approach to controlling the coercivity
while maintaining high Curie temperature, may be important for realizing
ferromagnetic semiconductor based devices.Comment: 8 pages,4 figures. accepted for publication in Appl. Phys. Let
The DEEP2 Redshift Survey: Lyman Alpha Emitters in the Spectroscopic Database
We present the first results of a search for Lyman-alpha emitters (LAEs) in
the DEEP2 spectroscopic database that uses a search technique that is different
from but complementary to traditional narrowband imaging surveys. We have
visually inspected ~20% of the available DEEP2 spectroscopic data and have
found nine high-quality LAEs with clearly asymmetric line profiles and an
additional ten objects of lower quality, some of which may also be LAEs. Our
survey is most sensitive to LAEs at z=4.4-4.9 and that is indeed where all but
one of our high-quality objects are found. We find the number density of our
spectroscopically-discovered LAEs to be consistent with those found in
narrowband imaging searches. The combined, averaged spectrum of our nine
high-quality objects is well fit by a two-component model, with a second,
lower-amplitude component redshifted by ~420 km/s with respect to the primary
Lyman-alpha line, consistent with large-scale outflows from these objects. We
conclude by discussing the advantages and future prospects of blank-sky
spectroscopic surveys for high-z LAEs.Comment: Accepted for publication in Ap
Comparison of Two Common Maximum Power Point Trackers by Simulating of PV Generators
AbstractPower point tracker algorithms play an important role in the optimization of the power and the efficiency of a photovoltaic generator (PVG).We made the comparison between two algorithms currently implemented for the power optimization of PVG. These algorithms are based on the Perturb Observe and the Conductance-Increment methods allowing the Maximum Power Point Tracking, MPPT, principle. The study leads us to conclude that these algorithms are not well adapted for PVG exposures in very unfavorable but realistic external conditions
Search For Hole Mediated Ferromagnetism In Cubic (Ga,Mn)N
Results of magnetisation measurements on p-type zincblende-(Ga,Mn)N are
reported. In addition to a small high temperature ferromagnetic signal, we
detect ferromagnetic correlation among the remaining Mn ions, which we assign
to the onset of hole-mediated ferromagnetism in (Ga,Mn)N.Comment: 2 pages, 1 figure, proc. ICPS 27, Flagstaff '0
Detailed transport investigation of the magnetic anisotropy of (Ga,Mn)As
This paper discusses transport methods for the investigation of the (Ga,Mn)As
magnetic anisotropy. Typical magnetoresistance behaviour for different
anisotropy types is discussed, focusing on an in depth discussion of the
anisotropy fingerprint technique and extending it to layers with primarily
uniaxial magnetic anisotropy. We find that in all (Ga,Mn)As films studied,
three anisotropy components are always present. The primary biaxial along
([100] and [010]) along with both uniaxial components along the [110] and [010]
crystal directions which are often reported separately. Various fingerprints of
typical (Ga,Mn)As transport samples at 4 K are included to illustrate the
variation of the relative strength of these anisotropy terms. We further
investigate the temperature dependence of the magnetic anisotropy and the
domain wall nucleation energy with the help of the fingerprint method
When is a pure state of three qubits determined by its single-particle reduced density matrices?
Using techniques from symplectic geometry, we prove that a pure state of
three qubits is up to local unitaries uniquely determined by its one-particle
reduced density matrices exactly when their ordered spectra belong to the
boundary of the, so called, Kirwan polytope. Otherwise, the states with given
reduced density matrices are parameterized, up to local unitary equivalence, by
two real variables. Given inevitable experimental imprecisions, this means that
already for three qubits a pure quantum state can never be reconstructed from
single-particle tomography. We moreover show that knowledge of the reduced
density matrices is always sufficient if one is given the additional promise
that the quantum state is not convertible to the Greenberger--Horne--Zeilinger
(GHZ) state by stochastic local operations and classical communication (SLOCC),
and discuss generalizations of our results to an arbitary number of qubits.Comment: 19 page
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