1,629 research outputs found
Some Consequences of Dark Energy Density varying Exponentially with Scale Factor
In this paper we have explored the consequences of a model of dark energy
with its energy density varying exponentially with the scale factor. We first
consider the model with , where
is a constant. This is a kind of generalisation of the cosmological constant
model with . We show that such an exponentially varying dark energy
density with the scale factor naturally leads to an equivalent phantom field.
We also consider a model with and we show
that this also naturally leads to an equivalent phantom field.Comment: 22 pages, 6 figure
Structural and magnetic characterization of the complete delafossite solid solution (CuAlO2){1-x}(CuCrO2){x}
We have prepared the complete delafossite solid solution series between
diamagnetic CuAlO2 and the t2g^3 frustrated antiferromagnet CuCrO2. The
evolution with composition x in CuAl(1-x)Cr(x)O2 of the crystal structure and
magnetic properties has been studied and is reported here. The room-temperature
unit cell parameters follow the Vegard law and increase with x as expected. The
effective moment is equal to the Cr^3+ spin-only S = 3/2 value throughout the
entire solid solution. Theta is negative, indicating that the dominant
interactions are antiferromagnetic, and its magnitude increases with Cr
substitution. For dilute Cr compositions, J_BB was estimated by mean-field
theory to be 2.0 meV. Despite the sizable Theta, long-range antiferromagnetic
order does not develop until very large x, and is preceeded by glassy behavior.
Data presented here, and that on dilute Al-substitution from Okuda et al.,
suggest that the reduction in magnetic frustration due to the presence of
non-magnetic Al does not have as dominant an effect on magnetism as chemical
disorder and dilution of the magnetic exchange. For all samples, the 5 K
isothermal magnetization does not saturate in fields up to 5 T and minimal
hysteresis is observed. The presence of antiferromagnetic interactions is
clearly evident in the sub-Brillouin behavior with a reduced magnetization per
Cr atom. An inspection of the scaled Curie plot reveals that significant
short-range antiferromagnetic interactions occur in CuCrO2 above its Neel
temperature, consistent with its magnetic frustration. Uncompensated
short-range interactions are present in the Al-substituted samples and are
likely a result of chemical disorder
Structural Diversity and Magnetic Properties of Hybrid Ruthenium Halide Perovskites and Related Compounds
Structural disorder, magnetism, and electrical and thermoelectric properties of pyrochlore Nd2Ru2O7
Polycrystalline Nd2Ru2O7 samples have been prepared and examined using a
combination of structural, magnetic, and electrical and thermal transport
studies. Analysis of synchrotron X-ray and neutron diffraction patterns
suggests some site disorder on the A-site in the pyrochlore sublattice: Ru
substitutes on the Nd-site up to 7.0(3)%, regardless of the different
preparative conditions explored. Intrinsic magnetic and electrical transport
properties have been measured. Ru 4d spins order antiferromagnetically at 143 K
as seen both in susceptibility and specific heat, and there is a corresponding
change in the electrical resistivity behaviour. A second antiferromagnetic
ordering transition seen below 10 K is attributed to ordering of Nd 4f spins.
Nd2Ru2O7 is an electrical insulator, and this behaviour is believed to be
independent of the Ru-antisite disorder on the Nd site. The electrical
properties of Nd2Ru2O7 are presented in the light of data published on all
A2Ru2O7 pyrochlores, and we emphasize the special structural role that Bi3+
ions on the A-site play in driving metallic behaviour. High-temperature
thermoelectric properties have also been measured. When considered in the
context of known thermoelectric materials with useful figures-of-merit, it is
clear that Nd2Ru2O7 has excessively high electrical resistivity which prevents
it from being an effective thermoelectric. A method for screening candidate
thermoelectrics is suggested.Comment: 19 pages, 10 figure
Primordial Magnetic Field Limits from Cosmic Microwave Background Bispectrum of Magnetic Passive Scalar Modes
Primordial magnetic fields lead to non-Gaussian signals in the cosmic
microwave background (CMB) even at the lowest order, as magnetic stresses and
the temperature anisotropy they induce depend quadratically on the magnetic
field. In contrast, CMB non-Gaussianity due to inflationary scalar
perturbations arises only as a higher order effect. Apart from a compensated
scalar mode, stochastic primordial magnetic fields also produce scalar
anisotropic stress that remains uncompensated till neutrino decoupling. This
gives rise to an adiabatic-like scalar perturbation mode that evolves passively
thereafter (called the passive mode). We compute the CMB reduced bispectrum
() induced by this passive mode, sourced via the
Sachs-Wolfe effect, on large angular scales. For any configuration of
bispectrum, taking a partial sum over mode-coupling terms, we find a typical
value of , for a magnetic field of nG, assuming a nearly
scale-invariant magnetic spectrum . We also evaluate, in full, the bispectrum
for the squeezed collinear configuration over all angular mode-coupling terms
and find . These values are more than times larger than the
previously calculated magnetic compensated scalar mode CMB bispectrum.
Observational limits on the bispectrum from WMAP7 data allow us to set upper
limits of nG on the present value of the cosmic magnetic field of
primordial origin. This is over 10 times more stringent than earlier limits on
based on the compensated mode bispectrum.Comment: 9 page
Self interacting Brans Dicke cosmology and Quintessence
Recent cosmological observations reveal that we are living in a flat
accelerated expanding universe. In this work we have investigated the nature of
the potential compatible with the power law expansion of the universe in a self
interacting Brans Dicke cosmology with a perfect fluid background and have
analyzed whether this potential supports the accelerated expansion. It is found
that positive power law potential is relevant in this scenario and can drive
accelerated expansion for negative Brans Dicke coupling parameter . The
evolution of the density perturbation is also analyzed in this scenerio and is
seen that the model allows growing modes for negative .Comment: 8pages, 5 figures, PRD style, some changes are made, figures added,
reference added. To be published in Int. J. Mod. Phys.
Disordered periodic systems at the upper critical dimension
The effects of weak point-like disorder on periodic systems at their upper
critical dimension D_c for disorder are studied. The systems studied range from
simple elastic systems with D_c=4 to systems with long range interactions with
D_c=2 and systems with D_c=3 such as the vortex lattice with dispersive elastic
constants. These problems are studied using the Gaussian Variational method and
the Functional Renormalisation Group. In all the cases studied we find a
typical ultra-slow loglog(x) growth of the asymptotic displacement correlation
function, resulting in nearly perfect translational order. Consequences for the
Bragg glass phase of vortex lattices are discussed.Comment: 12 RevTex pages, uses epsfig, 2 figure
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Complementary Metagenomic Approaches Improve Reconstruction of Microbial Diversity in a Forest Soil.
Soil ecosystems harbor diverse microorganisms and yet remain only partially characterized as neither single-cell sequencing nor whole-community sequencing offers a complete picture of these complex communities. Thus, the genetic and metabolic potential of this "uncultivated majority" remains underexplored. To address these challenges, we applied a pooled-cell-sorting-based mini-metagenomics approach and compared the results to bulk metagenomics. Informatic binning of these data produced 200 mini-metagenome assembled genomes (sorted-MAGs) and 29 bulk metagenome assembled genomes (MAGs). The sorted and bulk MAGs increased the known phylogenetic diversity of soil taxa by 7.2% with respect to the Joint Genome Institute IMG/M database and showed clade-specific sequence recruitment patterns across diverse terrestrial soil metagenomes. Additionally, sorted-MAGs expanded the rare biosphere not captured through MAGs from bulk sequences, exemplified through phylogenetic and functional analyses of members of the phylum Bacteroidetes Analysis of 67 Bacteroidetes sorted-MAGs showed conserved patterns of carbon metabolism across four clades. These results indicate that mini-metagenomics enables genome-resolved investigation of predicted metabolism and demonstrates the utility of combining metagenomics methods to tap into the diversity of heterogeneous microbial assemblages.IMPORTANCE Microbial ecologists have historically used cultivation-based approaches as well as amplicon sequencing and shotgun metagenomics to characterize microbial diversity in soil. However, challenges persist in the study of microbial diversity, including the recalcitrance of the majority of microorganisms to laboratory cultivation and limited sequence assembly from highly complex samples. The uncultivated majority thus remains a reservoir of untapped genetic diversity. To address some of the challenges associated with bulk metagenomics as well as low throughput of single-cell genomics, we applied flow cytometry-enabled mini-metagenomics to capture expanded microbial diversity from forest soil and compare it to soil bulk metagenomics. Our resulting data from this pooled-cell sorting approach combined with bulk metagenomics revealed increased phylogenetic diversity through novel soil taxa and rare biosphere members. In-depth analysis of genomes within the highly represented Bacteroidetes phylum provided insights into conserved and clade-specific patterns of carbon metabolism
Magnetodielectric coupling in Mn3O4
We have investigated the dielectric anomalies associated with spin ordering
transitions in the tetragonal spinel MnO, using thermodynamic,
magnetic, and dielectric measurements. We find that two of the three magnetic
ordering transitions in MnO lead to decreases in the temperature
dependent dielectric constant at zero applied field. Applying a magnetic field
to the polycrystalline sample leaves these two dielectric anomalies practically
unchanged, but leads to an increase in the dielectric constant at the
intermediate spin-ordering transition. We discuss possible origins for this
magnetodielectric behavior in terms of spin-phonon coupling. Band structure
calculations suggest that in its ferrimagnetic state, MnO corresponds
to a semiconductor with no orbital degeneracy due to strong Jahn-Teller
distortion.Comment: 6 pages, 7 figure
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