664 research outputs found
Determining Absorption, Emissivity Reduction, and Local Suppression Coefficients inside Sunspots
The power of solar acoustic waves is reduced inside sunspots mainly due to
absorption, emissivity reduction, and local suppression. The coefficients of
these power-reduction mechanisms can be determined by comparing time-distance
cross-covariances obtained from sunspots and from the quiet Sun. By analyzing
47 active regions observed by SOHO/MDI without using signal filters, we have
determined the coefficients of surface absorption, deep absorption, emissivity
reduction, and local suppression. The dissipation in the quiet Sun is derived
as well. All of the cross-covariances are width corrected to offset the effect
of dispersion. We find that absorption is the dominant mechanism of the power
deficit in sunspots for short travel distances, but gradually drops to zero at
travel distances longer than about 6 degrees. The absorption in sunspot
interiors is also significant. The emissivity-reduction coefficient ranges from
about 0.44 to 1.00 within the umbra and 0.29 to 0.72 in the sunspot, and
accounts for only about 21.5% of the umbra's and 16.5% of the sunspot's total
power reduction. Local suppression is nearly constant as a function of travel
distance with values of 0.80 and 0.665 for umbrae and whole sunspots
respectively, and is the major cause of the power deficit at large travel
distances.Comment: 14 pages, 21 Figure
Integration of molecular biology tools for identifying promoters and genes abundantly expressed in flowers of Oncidium Gower Ramsey
<p>Abstract</p> <p>Background</p> <p>Orchids comprise one of the largest families of flowering plants and generate commercially important flowers. However, model plants, such as <it>Arabidopsis thaliana </it>do not contain all plant genes, and agronomic and horticulturally important genera and species must be individually studied.</p> <p>Results</p> <p>Several molecular biology tools were used to isolate flower-specific gene promoters from <it>Oncidium </it>'Gower Ramsey' (<it>Onc</it>. GR). A cDNA library of reproductive tissues was used to construct a microarray in order to compare gene expression in flowers and leaves. Five genes were highly expressed in flower tissues, and the subcellular locations of the corresponding proteins were identified using lip transient transformation with fluorescent protein-fusion constructs. BAC clones of the 5 genes, together with 7 previously published flower- and reproductive growth-specific genes in <it>Onc</it>. GR, were identified for cloning of their promoter regions. Interestingly, 3 of the 5 novel flower-abundant genes were putative trypsin inhibitor (<it>TI</it>) genes (<it>OnTI1</it>, <it>OnTI2 </it>and <it>OnTI3</it>), which were tandemly duplicated in the same BAC clone. Their promoters were identified using transient GUS reporter gene transformation and stable <it>A. thaliana </it>transformation analyses.</p> <p>Conclusions</p> <p>By combining cDNA microarray, BAC library, and bombardment assay techniques, we successfully identified flower-directed orchid genes and promoters.</p
Comparative study of density functional theories of the exchange-correlation hole and energy in silicon
We present a detailed study of the exchange-correlation hole and
exchange-correlation energy per particle in the Si crystal as calculated by the
Variational Monte Carlo method and predicted by various density functional
models. Nonlocal density averaging methods prove to be successful in correcting
severe errors in the local density approximation (LDA) at low densities where
the density changes dramatically over the correlation length of the LDA hole,
but fail to provide systematic improvements at higher densities where the
effects of density inhomogeneity are more subtle. Exchange and correlation
considered separately show a sensitivity to the nonlocal semiconductor crystal
environment, particularly within the Si bond, which is not predicted by the
nonlocal approaches based on density averaging. The exchange hole is well
described by a bonding orbital picture, while the correlation hole has a
significant component due to the polarization of the nearby bonds, which
partially screens out the anisotropy in the exchange hole.Comment: 16 pages, 5 figures, RevTeX, added conten
Charge order and low frequency spin dynamics in lanthanum cuprates revealed by Nuclear Magnetic Resonance
We report detailed 17O, 139La, and 63Cu Nuclear Magnetic Resonance (NMR) and
Nuclear Quadrupole Resonance (NQR) measurements in a stripe ordered
La1.875Ba0.125CuO4 single crystal and in oriented powder samples of
La1.8-xEu0.2SrxCuO4. We observe a partial wipeout of the 17O NMR intensity and
a simultaneous drop of the 17O electric field gradient (EFG) at low
temperatures where the spin stripe order sets in. In contrast, the 63Cu
intensity is completely wiped out at the same temperature. The drop of the 17O
quadrupole frequency is compatible with a charge stripe order. The 17O spin
lattice relaxation rate shows a peak similar to that of the 139La, which is of
magnetic origin. This peak is doping dependent and is maximal at x ~ 1/8.Comment: submitted to European Physical Journal Special Topic
Surface-focused Seismic Holography of Sunspots: I. Observations
We present a comprehensive set of observations of the interaction of p-mode
oscillations with sunspots using surface-focused seismic holography. Maps of
travel-time shifts, relative to quiet-Sun travel times, are shown for incoming
and outgoing p modes as well as their mean and difference. We compare results
using phase-speed filters with results obtained with filters that isolate
single p-mode ridges, and further divide the data into multiple temporal
frequency bandpasses. The f mode is removed from the data. The variations of
the resulting travel-time shifts with magnetic-field strength and with the
filter parameters are explored. We find that spatial averages of these shifts
within sunspot umbrae, penumbrae, and surrounding plage often show strong
frequency variations at fixed phase speed. In addition, we find that positive
values of the mean and difference travel-time shifts appear exclusively in
waves observed with phase-speed filters that are dominated by power in the
low-frequency wing of the p1 ridge. We assess the ratio of incoming to outgoing
p-mode power using the ridge filters and compare surface-focused holography
measurements with the results of earlier published p-mode scattering
measurements using Fourier-Hankel decomposition.Comment: Solar Physics, accepte
Weak antiferromagnetism due to Dzyaloshinskii-Moriya interaction in BaCuOCl
The antiferromagnetic insulating cuprate BaCuOCl contains
folded CuO chains with four magnetic copper ions () per unit cell.
An underlying multiorbital Hubbard model is formulated and the superexchange
theory is developed to derive an effective spin Hamiltonian for this cuprate.
The resulting spin Hamiltonian involves a Dzyaloshinskii-Moriya term and a more
weak symmetric anisotropic exchange term besides the isotropic exchange
interaction. The corresponding Dzyaloshinskii-Moriya vectors of each magnetic
Cu-Cu bond in the chain reveal a well defined spatial order. Both, the
superexchange theory and the complementary group theoretical consideration,
lead to the same conclusion on the character of this order. The analysis of the
ground-state magnetic properties of the derived model leads to the prediction
of an additional noncollinear modulation of the antiferromagnetic structure.
This weak antiferromagnetism is restricted to one of the Cu sublattices.Comment: 13 pages, 1 table, 4 figure
Enhancement of long-range magnetic order by magnetic field in superconducting La2CuO(4+y)
We report a detailed study, using neutron scattering, transport and
magnetization measurements, of the interplay between superconducting (SC) and
spin density wave (SDW) order in La2CuO(4+y). Both kinds of order set in below
the same critical temperature. However, the SDW order grows with applied
magnetic field, whereas SC order is suppressed. Most importantly, the field
dependence of the SDW Bragg peak intensity has a cusp at zero field, as
predicted by a recent theory of competing SDW and SC order. This leads us to
conclude that there is a repulsive coupling between the two order parameters.
The question of whether the two kinds of order coexist or microscopically phase
separate is discussed.Comment: Version accepted for publication in Phys. Rev. B. Improved discussion
in connection with the muSR result
Non-linear response of a Kondo system: Perturbation approach to the time dependent Anderson impurity model
Nonlinear tunneling current through a quantum dot
(an Anderson impurity system) subject to both constant and alternating
electric fields is studied in the Kondo regime. A systematic diagram technique
is developed for perturbation study of the current in physical systems out of
equilibrium governed by time - dependent Hamiltonians of the Anderson and the
Kondo models. The ensuing calculations prove to be too complicated for the
Anderson model, and hence, a mapping on an effective Kondo problem is called
for. This is achieved by constructing a time - dependent version of the
Schrieffer - Wolff transformation. Perturbation expansion of the current is
then carried out up to third order in the Kondo coupling J yielding a set of
remarkably simple analytical expressions for the current. The zero - bias
anomaly of the direct current differential conductance is shown to be
suppressed by the alternating field while side peaks develop at finite source -
drain voltage. Both the direct component and the first harmonics of the time -
dependent response are equally enhanced due to the Kondo effect, while
amplitudes of higher harmonics are shown to be relatively small. A zero
alternating bias anomaly is found in the alternating current differential
conductance, that is, it peaks around zero alternating bias. This peak is
suppressed by the constant bias. No side peaks show up in the differential
alternating - conductance but their counterpart is found in the derivative of
the alternating current with respect to the direct bias. The results pertaining
to nonlinear response are shown to be valid also below the Kondo temperature.Comment: 55 latex pages 11 ps figure
Non Linear Current Response of a Many-Level Tunneling System: Higher Harmonics Generation
The fully nonlinear response of a many-level tunneling system to a strong
alternating field of high frequency is studied in terms of the
Schwinger-Keldysh nonequilibrium Green functions. The nonlinear time dependent
tunneling current is calculated exactly and its resonance structure is
elucidated. In particular, it is shown that under certain reasonable conditions
on the physical parameters, the Fourier component is sharply peaked at
, where is the spacing between
two levels. This frequency multiplication results from the highly nonlinear
process of photon absorption (or emission) by the tunneling system. It is
also conjectured that this effect (which so far is studied mainly in the
context of nonlinear optics) might be experimentally feasible.Comment: 28 pages, LaTex, 7 figures are available upon request from
[email protected], submitted to Phys.Rev.
How to extract reliable core-volume fractions from core-shell polycrystalline microstructures using cross sectional TEM micrographs
A reliable method of extracting core-volume fraction from TEM micrographs of core-shell polycrystalline
microstructures is presented. Three commonly used averaging methods based on a simple spherical
model are shown to consistently underestimate the core-volume fraction due to the interpretation of
a 3D structure from a 2D slice. The same trend is also revealed using Voronoi tessellated structures to
mimic polycrystalline ceramics. In some cases the underestimate is less than half the true core-volume
fraction.
We show that using a new maximum core-volume fraction methodology can improve the extracted
value to a consistent error of less than 5%. This approach uses a value taken from the largest core-volume
fraction measured from 10 grains that exhibit a core-shell microstructure. This provides increasing accuracy
and improvements in the confidence of the measurement when extracting core-volume fractions of
polycrystalline ceramics from 2D TEM micrographs
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