1,253 research outputs found
Ni-In Synergy in CO2Hydrogenation to Methanol
Indium oxide (In2O3) is a promising catalyst for selective CH3OH synthesis from CO2but displays insufficient activity at low reaction temperatures. By screening a range of promoters (Co, Ni, Cu, and Pd) in combination with In2O3using flame spray pyrolysis (FSP) synthesis, Ni is identified as the most suitable first-row transition-metal promoter with similar performance as Pd-In2O3. NiO-In2O3was optimized by varying the Ni/In ratio using FSP. The resulting catalysts including In2O3and NiO end members have similar high specific surface areas and morphology. The main products of CO2hydrogenation are CH3OH and CO with CH4being only observed at high NiO loading (≥75 wt %). The highest CH3OH rate (∼0.25 gMeOH/(gcath), 250 °C, and 30 bar) is obtained for a NiO loading of 6 wt %. Characterization of the as-prepared catalysts reveals a strong interaction between Ni cations and In2O3at low NiO loading (≤6 wt %). H2-TPR points to a higher surface density of oxygen vacancy (Ov) due to Ni substitution. X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and electron paramagnetic resonance analysis of the used catalysts suggest that Ni cations can be reduced to Ni as single atoms and very small clusters during CO2hydrogenation. Supportive density functional theory calculations indicate that Ni promotion of CH3OH synthesis from CO2is mainly due to low-barrier H2dissociation on the reduced Ni surface species, facilitating hydrogenation of adsorbed CO2on Ov © 2021 The Authors. Published by American Chemical Societ
Quantum impurity in an antiferromagnet: non-linear sigma model theory
We present a new formulation of the theory of an arbitrary quantum impurity
in an antiferromagnet, using the O(3) non-linear sigma model. We obtain the low
temperature expansion for the impurity spin susceptibilities of
antiferromagnets with magnetic long-range order in the ground state. We also
consider the bulk quantum phase transition in d=2 to the gapped paramagnet (d
is the spatial dimension): the impurity is described solely by a topological
Berry phase term which is an exactly marginal perturbation to the critical
theory. The physical properties of the quantum impurity near criticality are
obtained by an expansion in (d-1).Comment: 14 pages, 7 figures; (v2) added re
Cosmic Rays during BBN as Origin of Lithium Problem
There may be non-thermal cosmic rays during big-bang nucleosynthesis (BBN)
epoch (dubbed as BBNCRs). This paper investigated whether such BBNCRs can be
the origin of Lithium problem or not. It can be expected that BBNCRs flux will
be small in order to keep the success of standard BBN (SBBN). With favorable
assumptions on the BBNCR spectrum between 0.09 -- 4 MeV, our numerical
calculation showed that extra contributions from BBNCRs can account for the
Li abundance successfully. However Li abundance is only lifted an order
of magnitude, which is still much lower than the observed value. As the
deuteron abundance is very sensitive to the spectrum choice of BBNCRs, the
allowed parameter space for the spectrum is strictly constrained. We should
emphasize that the acceleration mechanism for BBNCRs in the early universe is
still an open question. For example, strong turbulent magnetic field is
probably the solution to the problem. Whether such a mechanism can provide the
required spectrum deserves further studies.Comment: 34 pages, 21 figures, published versio
Long distance regularization in chiral perturbation theory with decuplet
We investigate the use of long distance regularization in SU(3) baryon chiral
perturbation theory with decuplet fields. The one-loop decuplet contributions
to the octet baryon masses, axial couplings, S-wave nonleptonic hyperon decays
and magnetic moments are evaluated in a chirally consistent fashion by
employing a cutoff to implement long distance regularization. The convergence
of the chiral expansions of these quantities is improved compared to the
dimensionally regularized version which indicates that the propagation of
Goldstone bosons over distances smaller than a typical hadronic size, which is
beyond the regime of chiral perturbation theory but included by dimensional
regularization, is removed by use of a cutoff.Comment: 31 page
Spin battery operated by ferromagnetic resonance
Precessing ferromagnets are predicted to inject a spin current into adjacent
conductors via Ohmic contacts, irrespective of a conductance mismatch with, for
example, doped semiconductors. This opens the way to create a pure spin source
spin battery by the ferromagnetic resonance. We estimate the spin current and
spin bias for different material combinations.Comment: The estimate for the magnitude of the spin bias is improved. We find
that it is feasible to get a measurable signal of the order of the microwave
frequency already for moderate rf intensitie
Spin-1/2 frustrated antiferromagnet on a spatially anisotopic square lattice: contribution of exact diagonalizations
The phase diagram of a spin-1/2 model is investigated by means of
exact diagonalizations on finite samples. This model is a generalization of the
model on the square lattice with two different nearest-neighbor
couplings and may be also viewed as an array of coupled Heisenberg
chains. The results suggest that the resonnating valence bond state predicted
by Nersesyan and Tsvelik [Phys. Rev. B {\bf 67}, 024422 (2003)] for is realized and extends beyond the limit of small interchain coupling
along a curve nearly coincident with the line where the energy per spin is
maximum. This line is likely bordered on both side by a columnar dimer long
range order. This columnar order could extends for which correspond
to the model.Comment: 14 pages, 21 figures, final versio
Sensitivity of deexcitation energies of superdeformed secondary minima to the density dependence of symmetry energy with the relativistic mean-field theory
The relationship between deexcitation energies of superdeformed secondary
minima relative to ground states and the density dependence of the symmetry
energy is investigated for heavy nuclei using the relativistic mean field (RMF)
model. It is shown that the deexcitation energies of superdeformed secondary
minima are sensitive to differences in the symmetry energy that are mimicked by
the isoscalar-isovector coupling included in the model. With deliberate
investigations on a few Hg isotopes that have data of deexcitation energies, we
find that the description for the deexcitation energies can be improved due to
the softening of the symmetry energy. Further, we have investigated
deexcitation energies of odd-odd heavy nuclei that are nearly independent of
pairing correlations, and have discussed the possible extraction of the
constraint on the density dependence of the symmetry energy with the
measurement of deexcitation energies of these nuclei.Comment: 14 pages, 3 figure
A Naturally Narrow Positive Parity Theta^+
We present a consistent color-flavor-spin-orbital wave function for a
positive parity Theta^+ that naturally explains the observed narrowness of the
state. The wave function is totally symmetric in its flavor-spin part and
totally antisymmetric in its color-orbital part. If flavor-spin interactions
dominate, this wave function renders the positive parity Theta^+ lighter than
its negative parity counterpart. We consider decays of the Theta^+ and compute
the overlap of this state with the kinematically allowed final states. Our
results are numerically small. We note that dynamical correlations between
quarks are not necessary to obtain narrow pentaquark widths.Comment: 10 pages, 1 figure, Revtex4, two-column format, version to be
published in Phys. Rev. D, includes numerical estimates of decay width
Pulsar Timing and its Application for Navigation and Gravitational Wave Detection
Pulsars are natural cosmic clocks. On long timescales they rival the
precision of terrestrial atomic clocks. Using a technique called pulsar timing,
the exact measurement of pulse arrival times allows a number of applications,
ranging from testing theories of gravity to detecting gravitational waves. Also
an external reference system suitable for autonomous space navigation can be
defined by pulsars, using them as natural navigation beacons, not unlike the
use of GPS satellites for navigation on Earth. By comparing pulse arrival times
measured on-board a spacecraft with predicted pulse arrivals at a reference
location (e.g. the solar system barycenter), the spacecraft position can be
determined autonomously and with high accuracy everywhere in the solar system
and beyond. We describe the unique properties of pulsars that suggest that such
a navigation system will certainly have its application in future astronautics.
We also describe the on-going experiments to use the clock-like nature of
pulsars to "construct" a galactic-sized gravitational wave detector for
low-frequency (f_GW ~1E-9 - 1E-7 Hz) gravitational waves. We present the
current status and provide an outlook for the future.Comment: 30 pages, 9 figures. To appear in Vol 63: High Performance Clocks,
Springer Space Science Review
High-contrast imaging constraints on gas giant planet formation - The Herbig Ae/Be star opportunity
Planet formation studies are often focused on solar-type stars, implicitly
considering our Sun as reference point. This approach overlooks, however, that
Herbig Ae/Be stars are in some sense much better targets to study planet
formation processes empirically, with their disks generally being larger,
brighter and simply easier to observe across a large wavelength range. In
addition, massive gas giant planets have been found on wide orbits around early
type stars, triggering the question if these objects did indeed form there and,
if so, by what process. In the following I briefly review what we currently
know about the occurrence rate of planets around intermediate mass stars,
before discussing recent results from Herbig Ae/Be stars in the context of
planet formation. The main emphasis is put on spatially resolved polarized
light images of potentially planet forming disks and how these images - in
combination with other data - can be used to empirically constrain (parts of)
the planet formation process. Of particular interest are two objects, HD100546
and HD169142, where, in addition to intriguing morphological structures in the
disks, direct observational evidence for (very) young planets has been
reported. I conclude with an outlook, what further progress we can expect in
the very near future with the next generation of high-contrast imagers at 8-m
class telescopes and their synergies with ALMA.Comment: Accepted by Astrophysics and Space Science as invited short review in
special issue about Herbig Ae/Be stars; 12 pages incl. 5 figures, 2 tables
and reference
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