31,290 research outputs found
Hybrid Functional Study Rationalizes the Simple Cubic Phase of Calcium at High Pressures
Simple cubic (SC) phase has been long experimentally determined as the
high-pressure phase III of elemental calcium (Ca) since 1984. However, recent
density functional calculations within semi-local approximation showed that
this SC phase is structurally unstable by exhibiting severely imaginary
phonons, and is energetically unstable with respect to a theoretical
body-centered tetragonal I41/amd structure over the pressure range of phase
III. These calculations generated extensive debates on the validity of SC
phase. Here we have re-examined the SC structure by performing more precise
density functional calculations within hybrid functionals of
Heyd-Scuseria-Erhzerhof (HSE) and PBE0. Our calculations were able to
rationalize fundamentally the phase stability of SC structure over all other
known phases by evidence of its actual energetic stability above 33 GPa and its
intrinsically dynamical stability without showing any imaginary phonons in the
entire pressure range studied. We further established that the long-thought
theoretical I41/amd structure remains stable in a narrow pressure range before
entering SC phase and is actually the structure of experimental Ca-III'
synthesized recently at low temperature 14 K as supported by the excellent
agreement between our simulated X-ray diffraction patterns and the experimental
data. Our results shed strong light on the crucial role played by the precise
electron exchange energy in a proper description of the potential energy of Ca.Comment: submitted to Physical Review
Pickering emulsions responsive to COâ‚‚/Nâ‚‚ and light dual stimuli at ambient temperature
A dual stimulus-responsive n-octane-in-water Pickering emulsion with CO₂/N₂ and light triggers is prepared using negatively charged silica nanoparticles in combination with a trace amount of dual switchable surfactant, 4-butyl-4-(4-N,N-dimethylbutoxyamine) azobenzene bicarbonate (AZO-B₄) as stabilizers. On one hand, the emulsion can be transformed between stable and unstable at ambient temperature rapidly via the N₂/CO₂ trigger, and on the other hand a change in droplet size of the emulsion can occur upon light irradiation/re-homogenization cycles without changing the particle/surfactant concentration. The dual responsiveness thus allows for a precise control of emulsion properties. Compared with emulsions stabilised by specially synthesized stimuli-responsive particles or by stimuli-responsive surfactants, the method reported here is much easier and requires relatively low concentration of surfactant (≈1/10 cmc), which is important for potential applications
Gluon GPDs and Exclusive Photoproduction of a Quarkonium in Forward Region
Forward photoproduction of can be used to extract Generalized Parton
Distributions(GPD's) of gluons. We analyze the process at twist-3 level and
study relevant classifications of twist-3 gluon GPD's. At leading power or
twist-2 level the produced is transversely polarized. We find that at
twist-3 the produced is longitudinally polarized. Our study shows that
in high energy limit the twist-3 amplitude is only suppressed by the inverse
power of the heavy quark mass relatively to the twist-2 amplitude. This
indicates that the power correction to the cross-section of unpolarized
can have a sizeable effect. We have also derived the amplitude of the
production of at twist-3, but the result contains end-point
singularities. The production of other quarkonia has been briefly discussed.Comment: Discussions of results are adde
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