3,598 research outputs found
An accurate linear model for redshift space distortions in the void-galaxy correlation function
Redshift space distortions within voids provide a unique method to test for
environmental dependence of the growth rate of structures in low density
regions, where effects of modified gravity theories might be important. We
derive a linear theory model for the redshift space void-galaxy correlation
that is valid at all pair separations, including deep within the void, and use
this to obtain expressions for the monopole and quadrupole
contributions. Our derivation highlights terms that have previously been
neglected but are important within the void interior. As a result our model
differs from previous works and predicts new physical effects, including a
change in the sign of the quadrupole term within the void radius. We show how
the model can be generalised to include a velocity dispersion. We compare our
model predictions to measurements of the correlation function using mock void
and galaxy catalogues modelled after the BOSS CMASS galaxy sample using the Big
MultiDark -body simulation, and show that the linear model with dispersion
provides an excellent fit to the data at all scales, Mpc. While the RSD model matches simulations, the linear bias
approximation does not hold within voids, and care is needed in fitting for the
growth rate. We show that fits to the redshift space correlation recover the
growth rate to a precision of using the simulation volume
of .Comment: 16 pages, 12 figures. v3: updated to match version published in
MNRAS. Several minor changes to text for better explanations, with reference
to subsequent results (arXiv:1805.09349). No changes to theory, results or
conclusion
Electrical and magnetic properties of the complete solid solution series between SrRuO3 and LaRhO3: Filling t2g versus tilting
A complete solid solution series between the t2g^4 perovskite ferromagnet
SrRuO3 and the diamagnetic t2g^6 perovskite LaRhO3 has been prepared. The
evolution with composition x in (SrRuO3)(1-x)(LaRhO3)(x) of the crystal
structure and electrical and magnetic properties has been studied and is
reported here. As x increases, the octahedral tilt angle gradually increases,
along with the pseudocubic lattice parameter and unit cell volume. Electrical
resistivity measurements reveal a compositionally driven metal to insulator
transition between x = 0.1 and 0.2. Ferromagnetic ordering gives over to glassy
magnetism for x > 0.3 and no magnetic ordering is found above 2 K for x > 0.5.
M_sat and Theta_CW decrease with increasing x and remain constant after x =
0.5. The magnetism appears poised between localized and itinerant behavior, and
becomes more localized with increasing x as evidenced by the evolution of the
Rhodes-Wohlfarth ratio. mu_eff per Ru is equal to the quenched spin-only S
value across the entire solid solution. Comparisons with Sr(1-x)Ca(x)RuO3
reinforce the important role of structural distortions in determining magnetic
ground state. It is suggested that electrical transport and magnetic properties
are not strongly coupled in this system
Beyond BAO: improving cosmological constraints from BOSS with measurement of the void-galaxy cross-correlation
We present a measurement of the anisotropic void-galaxy cross-correlation
function in the CMASS galaxy sample of the BOSS DR12 data release. We perform a
joint fit to the data for redshift space distortions (RSD) due to galaxy
peculiar velocities and anisotropies due to the Alcock-Paczynski (AP) effect,
for the first time using a velocity field reconstruction technique to remove
the complicating effects of RSD in the void centre positions themselves. Fits
to the void-galaxy function give a 1% measurement of the AP parameter
combination at redshift , where
is the angular diameter distance and the Hubble parameter, exceeding the
precision obtainable from baryon acoustic oscillations (BAO) by a factor of
~3.5 and free of systematic errors. From voids alone we also obtain a 10%
measure of the growth rate, . The parameter
degeneracies are orthogonal to those obtained from galaxy clustering. Combining
void information with that from BAO and galaxy RSD in the same CMASS sample, we
measure (at 0.8% precision),
kmsMpc (1%) and
(4.9%), consistent with cosmic microwave background
(CMB) measurements from Planck. These represent a factor \sim2 improvement in
precision over previous results through the inclusion of void information.
Fitting a flat cosmological constant CDM model to these results in
combination with Planck CMB data, we find up to an 11% reduction in
uncertainties on and compared to use of the corresponding BOSS
consensus values. Constraints on extended models with non-flat geometry and a
dark energy of state that differs from show an even greater improvement.Comment: 22 pages, 15 figures. Accepted for publication in Phys.Rev.D. v2
corrects small error in likelihood analysis; minor changes to figures and
text, cosmological results unchanged. Reconstruction and void-finding code
available at https://github.com/seshnadathur/Revolver, likelihood analysis
code available at https://github.com/seshnadathur/void-galaxy-cosmo-fitte
Cosmic acceleration in a model of scalar-tensor gravitation
In this paper we consider a model of scalar-tensor theory of gravitation in
which the scalar field, determines the gravitational coupling G and has
a Lagrangian of the form, . We study the cosmological consequence
of this theory in the matter dominated era and show that this leads to a
transition from an initial decelerated expansion to an accelerated expansion
phase at the present epoch. Using observational constraints, we see that the
effective equation of state today for the scalar field turns out to be
, with and that the transition
to an accelerated phase happened at a redshift of about 0.3.Comment: 12 pages, 2 figures, matches published versio
Total scattering descriptions of local and cooperative distortions in the oxide spinel (Mg,Cu)Cr2O4 with dilute Jahn-Teller ions
The normal spinel oxide MgCr2O4 is cubic at room temperature while the normal
spinel CuCr2O4 is tetragonal as a consequence of the Jahn-Teller nature of Cu2+
on the tetrahedral sites. Despite different end-member structures, complete
solid solutions of Mg_{1-x}Cu_xCr2O4 can be prepared that display a first-order
structural transition with composition x = 0.43 at room temperature. Reverse
Monte Carlo analysis of total neutron scattering on data acquired between 300 K
and 15 K on samples with x = 0.10, 0.20, and 0.43 provides unbiased local and
average structure descriptions of the samples, including an understanding of
the transition from local Jahn-Teller distortions in the cubic phase to
cooperative distortions that result in a tetragonal structure. Distributions of
continuous symmetry measures help to understand and distinguish distorted and
undistorted coordination around the tetrahedral site in the solid solutions.
Magnetic exchange bias is observed in field-cooled hysteresis loops of samples
with dilute Cu2+ concentration and in samples with tetragonal--cubic phase
coexistence around 300 K.Comment: 10 pages, 14 figure
Exchange biasing of single-domain Ni nanoparticles spontaneously grown in an antiferromagnetic MnO matrix
Exchange biased composites of ferromagnetic single-domain Ni nanoparticles
embedded within large grains of MnO have been prepared by reduction of
NiMnO phases in flowing hydrogen. The Ni precipitates are 15-30
nm in extent, and the majority are completely encased within the MnO matrix.
The manner in which the Ni nanoparticles are spontaneously formed imparts a
high ferromagnetic- antiferromagnetic interface/volume ratio, which results in
substantial exchange bias effects. Exchange bias fields of up to 100 Oe are
observed, in cases where the starting Ni content in the precursor
NiMnO phase is small. For particles of approximately the same
size, the exchange bias leads to significant hardening of the magnetization,
with the coercive field scaling nearly linearly with the exchange bias field.Comment: 6 pages PDFLaTeX with 9 figure
Hydraulic Analogy for Isentropic Flow Through a Nozzle
Modelling aspects of isentropic compressible gas flow using hydraulic analogy are discussed. Subsonic and supersonic flows through a typical nozzle are simulated as free surface incompressible water flow in an equivalent 2-D model on a water table. The results are first compared for the well known classical analogy in order to estimate experimental errors. Correction factors for pressure and temperature, to account for non-ideal compressible gas flow are presented and the results obtained on the water table are modified and compared with gas dynamic solution. Within the experimental errors, it is shown that the hydraulic analogy can be used as an effective tool for the study of two dimensional isentropic flows of gases
A functorial construction of moduli of sheaves
We show how natural functors from the category of coherent sheaves on a
projective scheme to categories of Kronecker modules can be used to construct
moduli spaces of semistable sheaves. This construction simplifies or clarifies
technical aspects of existing constructions and yields new simpler definitions
of theta functions, about which more complete results can be proved.Comment: 52 pp. Dedicated to the memory of Joseph Le Potier. To appear in
Inventiones Mathematicae. Slight change in the definition of the Kronecker
algebra in Secs 1 (p3) and 2.2 (p6), with corresponding small alterations
elsewhere, to make the constructions work for non-reduced schemes. Section
6.5 rewritten. Remark 2.6 and new references adde
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