5,192 research outputs found
Pressure-induced phase transition in the electronic structure of palladium nitride
We present a combined theoretical and experimental study of the electronic
structure and equation of state (EOS) of crystalline PdN2. The compound forms
above 58 GPa in the pyrite structure and is metastable down to 11 GPa. We show
that the EOS cannot be accurately described within either the local density or
generalized gradient approximations. The Heyd-Scuseria-Ernzerhof
exchange-correlation functional (HSE06), however, provides very good agreement
with experimental data. We explain the strong pressure dependence of the Raman
intensities in terms of a similar dependence of the calculated band gap, which
closes just below 11 GPa. At this pressure, the HSE06 functional predicts a
first-order isostructural transition accompanied by a pronounced elastic
instability of the longitudinal-acoustic branches that provides the mechanism
for the experimentally observed decomposition. Using an extensive Wannier
function analysis, we show that the structural transformation is driven by a
phase transition of the electronic structure, which is manifested by a
discontinuous change in the hybridization between Pd-d and N-p electrons as
well as a conversion from single to triple bonded nitrogen dimers. We argue for
the possible existence of a critical point for the isostructural transition, at
which massive fluctuations in both the electronic as well as the structural
degrees of freedom are expected.Comment: 9 pages, 12 figures. Revised version corrects minor typographical
error
Molecular characterization of vernalization loci VRN1 in wild and cultivated wheats
<p>Abstract</p> <p>Background</p> <p>Variability of the <it>VRN1 </it>promoter region of the unique collection of spring polyploid and wild diploid wheat species together with diploid goatgrasses (donor of B and D genomes of polyploid wheats) were investigated. Accessions of wild diploid (<it>T. boeoticum</it>, <it>T. urartu</it>) and tetraploid (<it>T. araraticum, T. timopheevii</it>) species were studied for the first time.</p> <p>Results</p> <p>Sequence analysis indicated great variability in the region from -62 to -221 nucleotide positions of the <it>VRN1 </it>promoter region. Different indels were found within this region in spring wheats. It was shown that <it>VRN1 </it>promoter region of B and G genome can also contain damages such as the insertion of the transposable element.</p> <p>Some transcription factor recognition sites including hybrid C/G-box for TaFDL2 protein known as the <it>VRN1 </it>gene upregulator were predicted inside the variable region. It was shown that deletions leading to promoter damage occurred in diploid and polyploid species independently. DNA transposon insertions first occurred in polyploid species. At the same time, the duplication of the promoter region was observed in A genomes of polyploid species.</p> <p>Conclusions</p> <p>We can conclude that supposed molecular mechanism of the <it>VRN1 </it>gene activating in cultivated diploid wheat species <it>T. monococcum </it>is common also for wild <it>T. boeoticum </it>and was inherited by <it>T. monococcum</it>. The spring polyploids are not related in their origin to spring diploids. The spring <it>T. urartu </it>and goatgrass accessions have another mechanism of flowering activation that is not connected with indels in <it>VRN1 </it>promoter region. All obtained data may be useful for detailed insight into origin of spring wheat forms in evolution and domestication process.</p
Inflationary spacetimes are not past-complete
Many inflating spacetimes are likely to violate the weak energy condition, a
key assumption of singularity theorems. Here we offer a simple kinematical
argument, requiring no energy condition, that a cosmological model which is
inflating -- or just expanding sufficiently fast -- must be incomplete in null
and timelike past directions. Specifically, we obtain a bound on the integral
of the Hubble parameter over a past-directed timelike or null geodesic. Thus
inflationary models require physics other than inflation to describe the past
boundary of the inflating region of spacetime.Comment: We improve the basic argument to apply to a wider class of
spacetimes, use a better title and add a discussion of cyclic models. 4
pages, 1 figure, RevTe
Pulsed laser Raman spectroscopy in the laser-heated diamond anvil cell
We describe the design and operation of a spatially-filtered Raman/fluorescence spectrometer that incorporates a pulsed 532 nm laser excitation source and a synchronized and electronically gated CCD detector. This system permits the suppression of undesired continuous radiation from various sources by a factor of up to 50,000 providing the possibility of acquiring Raman signals at temperatures exceeding 5,000 K. We present performance comparisons of this system with that of a state-of-the-art conventional CW system using a 458 nm excitation source. We also demonstrate that the pulsed system is capable of suppressing an impurity-induced (single nitrogen defects) fluorescence in diamond, and further suggest that this capability can be used to suppress the stress-induced fluorescence in diamond that may appear at pressures near or above 150 GPa
High-Pressure Amorphous Nitrogen
The phase diagram and stability limits of diatomic solid nitrogen have been
explored in a wide pressure--temperature range by several optical spectroscopic
techniques. A newly characterized narrow-gap semiconducting phase has
been found to exist in a range of 80--270 GPa and 10--510 K. The vibrational
and optical properties of the phase produced under these conditions
indicate that it is largely amorphous and back transforms to a new molecular
phase. The band gap of the phase is found to decrease with pressure
indicating possible metallization by band overlap above 280 GPa.Comment: 5 pages, 4 figure
Eternal inflation and the initial singularity
It is shown that a physically reasonable spacetime that is eternally
inflating to the future must possess an initial singularity.Comment: 11 pages, Tufts University cosmology preprin
On Brane Inflation With Volume Stabilization
The distance between BPS branes in string theory corresponds to a flat
direction in the effective potential. Small deviations from supersymmetry may
lead to a small uplifting of this flat direction and to brane inflation.
However, this scenario can work only if the BPS properties of the branes and
the corresponding flatness of the inflaton potential are preserved in the
theories with the stable volume compactification. We present an ``inflaton
trench'' mechanism that keeps the inflaton potential flat due to shift
symmetry, which is related to near BPS symmetry in our model.Comment: 12 pages, 2 figure
String Theory and Inflation
String theory abounds with light scalar fields (the dilaton and various
moduli) which create a host of observational problems, and notably some serious
cosmological difficulties similar to the ones associated with the Polonyi field
in the earliest versions of spontaneously broken supergravity. We show that all
these problems are naturally avoided if a recently introduced mechanism for
fixing the vacuum expectation values of the dilaton and/or moduli is at work.
We study both the classical evolution and the quantum fluctuations of such
scalar fields during a primordial inflationary era and find that the results
are naturally compatible with observational facts. In this model, dilatons or
moduli within a very wide range of masses (which includes the SUSY-breaking
favored 1 TeV value and extends up to the Planck scale) qualify to define a
novel type of essentially stable ultra-weakly interacting massive particles
able to provide enough mass density to close the universeComment: 25 page
Raman spectrum and lattice parameters of MgB2 as a function of pressure
We report Raman spectra and synchrotron x-ray diffraction measurements of
lattice parameters of polycrystalline MgB2 under hydrostatic pressure
conditions up to 15 GPa. An anomalously broadened Raman band at 620 cm-1 is
observed that exhibits a large linear pressure shift of its frequency. The
large mode damping and Gruneisen parameter indicate a highly anharmonic nature
of the mode, broadly consistent with theoretical predictions for the E2g
in-plane boron stretching mode. The results obtained may provide additional
constraints on the electron-phonon coupling in the system.Comment: 3 pages, 3 figure
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