7,842 research outputs found
First-Principles Structural, Mechanical, and Thermodynamic Calculations of the Negative Thermal Expansion Compound Zr2(WO4)(PO4)2
The negative thermal expansion (NTE) material Zr2(WO4)(PO4)2 has been investigated for the first time within the framework of the density functional perturbation theory (DFPT). The structural, mechanical, and thermodynamic properties of this material have been predicted using the Perdew, Burke and Ernzerhof for solid (PBEsol) exchange–correlation functional, which showed superior accuracy over standard functionals in previous computational studies of the NTE material α-ZrW2O8. The bulk modulus calculated for Zr2(WO4)(PO4)2 using the Vinet equation of state at room temperature is K0 = 63.6 GPa, which is in close agreement with the experimental estimate of 61.3(8) at T = 296 K. The computed mean linear coefficient of thermal expansion is −3.1 × 10–6 K−1 in the temperature range ∼0–70 K, in line with the X-ray diffraction measurements. The mean Grüneisen parameter controlling the thermal expansion of Zr2(WO4)(PO4)2 is negative below 205 K, with a minimum of −2.1 at 10 K. The calculated standard molar heat capacity and entropy are CP0 = 287.6 and S0 = 321.9 J·mol–1·K–1, respectively. The results reported in this study demonstrate the accuracy of DFPT/PBEsol for assessing or predicting the relationship between structural and thermomechanical properties of NTE materials
Efficient CRISPR-rAAV engineering of endogenous genes to study protein function by allele-specific RNAi.
Gene knockout strategies, RNAi and rescue experiments are all employed to study mammalian gene function. However, the disadvantages of these approaches include: loss of function adaptation, reduced viability and gene overexpression that rarely matches endogenous levels. Here, we developed an endogenous gene knockdown/rescue strategy that combines RNAi selectivity with a highly efficient CRISPR directed recombinant Adeno-Associated Virus (rAAV) mediated gene targeting approach to introduce allele-specific mutations plus an allele-selective siRNA Sensitive (siSN) site that allows for studying gene mutations while maintaining endogenous expression and regulation of the gene of interest. CRISPR/Cas9 plus rAAV targeted gene-replacement and introduction of allele-specific RNAi sensitivity mutations in the CDK2 and CDK1 genes resulted in a >85% site-specific recombination of Neo-resistant clones versus ∼8% for rAAV alone. RNAi knockdown of wild type (WT) Cdk2 with siWT in heterozygotic knockin cells resulted in the mutant Cdk2 phenotype cell cycle arrest, whereas allele specific knockdown of mutant CDK2 with siSN resulted in a wild type phenotype. Together, these observations demonstrate the ability of CRISPR plus rAAV to efficiently recombine a genomic locus and tag it with a selective siRNA sequence that allows for allele-selective phenotypic assays of the gene of interest while it remains expressed and regulated under endogenous control mechanisms
Multidisciplinary research from ERTS-1 data
There are no author-identified significant results in this report
Explaining the entropy excess in clusters and groups of galaxies without additional heating
The X-ray luminosity and temperature of clusters and groups of galaxies do
not scale in a self-similar manner. This has often been interpreted as a sign
that the intracluster medium has been substantially heated by non-gravitational
sources. In this paper, we propose a simple model which, instead, uses the
properties of galaxy formation to explain the observations. Drawing on
available observations, we show that there is evidence that the efficiency of
galaxy formation was higher in groups than in clusters. If confirmed, this
would deplete the low-entropy gas in groups, increase their central entropy and
decrease their X-ray luminosity. A simple, empirical, hydrostatic model appears
to match both the luminosity-temperature relation of clusters and properties of
their internal structure as well.Comment: 5 pages, 4 figures, accepted in ApJL; added one reference, otherwise
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Summary report of work on ten tasks
There are no author-identified significant results in this report
Cosmological Simulations of the Preheating Scenario for Galaxy Cluster Formation: Comparison to Analytic Models and Observations
We perform a set of non--radiative cosmological simulations of a preheated
intracluster medium in which the entropy of the gas was uniformly boosted at
high redshift. The results of these simulations are used first to test the
current analytic techniques of preheating via entropy input in the smooth
accretion limit. When the unmodified profile is taken directly from
simulations, we find that this model is in excellent agreement with the results
of our simulations. This suggests that preheated efficiently smoothes the
accreted gas, and therefore a shift in the unmodified profile is a good
approximation even with a realistic accretion history. When we examine the
simulation results in detail, we do not find strong evidence for entropy
amplification, at least for the high-redshift preheating model adopted here. In
the second section of the paper, we compare the results of the preheating
simulations to recent observations. We show -- in agreement with previous work
-- that for a reasonable amount of preheating, a satisfactory match can be
found to the mass-temperature and luminosity-temperature relations. However --
as noted by previous authors -- we find that the entropy profiles of the
simulated groups are much too flat compared to observations. In particular,
while rich clusters converge on the adiabatic self--similar scaling at large
radius, no single value of the entropy input during preheating can
simultaneously reproduce both the core and outer entropy levels. As a result,
we confirm that the simple preheating scenario for galaxy cluster formation, in
which entropy is injected universally at high redshift, is inconsistent with
observations.Comment: 11 pages, 13 figures, accepted for publication in Ap
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