544 research outputs found
Impact of Sb degrees of freedom on the charge density wave phase diagram of the kagome metal CsVSb
Elucidating the microscopic mechanisms responsible for the charge density
wave (CDW) instability of the AVSb (A=Cs, K, Rb) family of kagome
metals is critical for understanding their unique properties, including
superconductivity. In these compounds, distinct CDW phases with wave-vectors at
the and points are energetically favorable, opening the possibility of
tuning the type of CDW order by appropriate external parameters. Here, we shed
light on the CDW landscape of CsVSb via a combination of
first-principles calculations and phenomenology, which consists of extracting
the coefficients of the CDW Landau free-energy expansion from density
functional theory. We find that while the main structural distortions of the
kagome lattice in the staggered tri-hexagonal CDW phase are along the
nearest-neighbor V-V bonds, distortions associated with the Sb ions play a
defining role in the energy gain in this and all other CDW states. Moreover,
the coupling between ionic displacements from different unit cells is small,
thus explaining the existence of multiple CDW instabilities with different
modulations along the c-axis. We also investigate how pressure and temperature
impact the CDW phase of CsVSb. Increasing pressure does not change the
staggered tri-hexagonal CDW ground state, even though the -point CDW
instability disappears before the -point one, a behavior that we attribute
to the large nonlinear coupling between the order parameters. Upon changing the
temperature, we find a narrow regime in which another transition can take
place, toward a tri-hexagonal Star-of-David CDW phase. We discuss the
implications of our results by comparing them with experiments on this
compound
Neutrino Spectral Split in the Exact Many Body Formalism
We consider the many-body system of neutrinos interacting with each other
through neutral current weak force. Emerging many-body effects in such a system
could play important roles in some astrophysical sites such as the core
collapse supernovae. In the literature this many-body system is usually treated
within the mean field approximation which is an effective one-body description
based on omitting entangled neutrino states. In this paper, we consider the
original many-body system in an effective two flavor mixing scenario under the
single angle approximation and present a solution without using the mean field
approximation. Our solution is formulated around a special class of many-body
eigenstates which do not undergo any level crossings as the neutrino self
interaction rate decreases while the neutrinos radiate from the supernova. In
particular, an initial state which consists of electron neutrinos and
antineutrinos of an orthogonal flavor can be entirely decomposed in terms of
those eigenstates. Assuming that the conditions are perfectly adiabatic so that
the evolution of these eigenstates follow their variation with the interaction
rate, we show that this initial state develops a spectral split at exactly the
same energy predicted by the mean field formulation.Comment: Published version. 30 pages, 11 figure
Coherence lifetimes of excitations in an atomic condensate due to the thin spectrum
We study the quantum coherence properties of a finite sized atomic condensate
using a toy-model and the thin spectrum model formalism. The decoherence time
for a condensate in the ground state, nominally taken as a variational symmetry
breaking state, is investigated for both zero and finite temperatures. We also
consider the lifetimes for Bogoliubov quasi-particle excitations, and contrast
them to the observability window determined by the ground state coherence time.
The lifetimes are shown to exhibit a general characteristic dependence on the
temperature, determined by the thin spectrum accompanying the spontaneous
symmetry breaking ground state
Orange maize in Zambia: crop development and delivery experience
Biofortified vitamin A “orange” maize can help address the adverse health effects of vitamin A deficiency. By 2016, HarvestPlus and its partners had developed six orange maize varieties and delivery efforts have reached more than 100,000 farming households in Zambia. HarvestPlus has established the proof of concept, that vitamin A maize varieties can be developed without compromising yield levels and that these varieties can deliver sufficient quantities of vitamin A to improve nutrition. The delivery program has also shown that farmers are willing to grow orange maize varieties and consumers are willing to buy and eat orange maize products. This paper summarizes the country’s nutritional and consumer backgrounds, the crop development and release of orange maize varieties, the delivery efforts in Zambia and impact measurement. It also synthesizes lessons learned and future challenges.Keywords: Biofortification, Vitamin A Deficiency, Orange Maize, Vitamin A Maize, Zambi
Electromagnon dispersion probed by inelastic X-ray scattering in LiCrO2
Inelastic X-ray scattering with meV energy resolution (IXS) is an ideal tool to measure collective excitations in solids and liquids. In non-resonant scattering condition, the cross-section is strongly dominated by lattice vibrations (phonons). However, it is possible to probe additional degrees of freedom such as magnetic fluctuations that are strongly coupled to the phonons. The IXS spectrum of the coupled system contains not only the phonon dispersion but also the so far undetected magnetic correlation function. Here we report the observation of strong magnon-phonon coupling in LiCrO2 that enables the measurement of magnetic correlations throughout the Brillouin zone via IXS. We find electromagnon excitations and electric dipole active two-magnon excitations in the magnetically ordered phase and heavily damped electromagnons in the paramagnetic phase of LiCrO2. We predict that several (frustrated) magnets with dominant direct exchange and non-collinear magnetism show surprisingly large IXS cross-section for magnons and multi-magnon processes
Magnetodielectric coupling and phonon properties of compressively strained EuTiO3 thin films deposited on LSAT
Compressively strained epitaxial (001) EuTiO3 thin films of tetragonal
symmetry have been deposited on (001) (LaAlO3)_0.29-(SrAl_{1/2}Ta_{1/2}O3)_0.71
(LSAT) substrates by reactive molecular-beam epitaxy. Enhancement of the Neel
temperature by 1 K with 0.9% compressive strain was revealed. The polar phonons
ofthe films have been investigated as a function of temperature and magnetic
field by means of infrared reflectance spectroscopy. All three infrared active
phonons show strongly stiffened frequencies compared to bulk EuTiO3 in
accordance with first principles calculations. The phonon frequencies exhibit
gradual softening on cooling leading to an increase in static permittivity. A
new polar phonon with frequency near the TO1 soft mode was detected below 150
K. The new mode coupled with the TO1 mode was assigned as the optical phonon
from the Brillouin zone edge, which is activated in infrared spectra due to an
antiferrodistortive phase transition and due to simultaneous presence of polar
and/or magnetic nanoclusters. In the antiferromagnetic phase we have observed a
remarkable softening of the lowest-frequency polar phonon under an applied
magnetic field, which qualitatively agrees with first principles calculations.
This demonstrates the strong spin-phonon coupling in EuTiO3, which is
responsible for the pronounced dependence of its static permittivity on
magnetic field in the antiferromagnetic phase.Comment: Submitted to Phys. Rev.
Microstructural evolution under low shear rates during Rheo processing of LM25 alloy
© ASM InternationalMicrostructural features of LM25 alloy processed by two different routes: (1) conventional casting, and(2)shear casting based on inclined heated surface are studied. The microstructures of the primary phase for the shear-cast samples show rosette or ellipsoidal morphologies. Heat transfer of contacting melt with the
inclined tube surface and shear stress exerted on the layers of the melt as result of gravitational force are crucial parameters for the microstructural evolution. Compared to those produced by conventional casting, shear-cast samples have a much improved tensile strength and ductility due to globular microstructure
Extrusion limits of magnesium alloys
Magnesium alloys are generally found to be slower to extrude than aluminum alloys; however, limited quantitative comparisons of the actual operating windows have been published. In this work, the extrusion limits are determined for a series of commercial magnesium alloys (M1, ZM21, AZ31, AZ61, and ZK60). These are compared with the limits established for aluminum alloy AA6063. The maximum extrusion speed of alloy M1 is shown to be similar to AA6063. Alloys ZM21, AZ31, ZK60, and AZ61 exhibit maximum extrusion speeds 44, 18, 4, and 3 pct, respectively, of the maximum measured for AA6063. For AZ31, the maximum extrusion speed is increased by 22 pct after homogenization and by 64 pct for repeat extrusions. The variation in the extrusion limits with changing alloy content is rationalized in terms of differences in the hot working flow stress and solidus temperature.<br /
Coherent Pair Production by Photons in the 20-170 GeV Energy Range Incident on Crystals and Birefringence
The cross section for coherent pair production by linearly polarised photons
in the 20-170 GeV energy range was measured for photon aligned incidence on
ultra-high quality diamond and germanium crystals. The theoretical description
of coherent bremsstrahlung and coherent pair production phenomena is an area of
active theoretical debate and development. However, under our experimental
conditions, the theory predicted the combined cross section and polarisation
experimental observables very well indeed. In macroscopic terms, our experiment
measured a birefringence effect in pair production in a crystal. This study of
this effect also constituted a measurement of the energy dependent linear
polarisation of photons produced by coherent bremsstrahlung in aligned
crystals. New technologies for manipulating high energy photon beams can be
realised based on an improved understanding of QED phenomena at these energies.
In particular, this experiment demonstrates an efficient new polarimetry
technique. The pair production measurements were done using two independent
methods simultaneously. The more complex method using a magnet spectrometer
showed that the simpler method using a multiplicity detector was also viable.Comment: 10 pages, 13 figures, 1 table, REVTeX4 two column, Version for
publicatio
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