28,692 research outputs found
An accurate determination of the Avogadro constant by counting the atoms in a 28Si crystal
The Avogadro constant links the atomic and the macroscopic properties of
matter. Since the molar Planck constant is well known via the measurement of
the Rydberg constant, it is also closely related to the Planck constant. In
addition, its accurate determination is of paramount importance for a
definition of the kilogram in terms of a fundamental constant. We describe a
new approach for its determination by "counting" the atoms in 1 kg
single-crystal spheres, which are highly enriched with the 28Si isotope. It
enabled isotope dilution mass spectroscopy to determine the molar mass of the
silicon crystal with unprecedented accuracy. The value obtained, 6.02214084(18)
x 10^23 mol^-1, is the most accurate input datum for a new definition of the
kilogram.Comment: 4 pages, 5 figures, 3 table
The wavefunction reconstruction effects in calculation of DM-induced electronic transition in semiconductor targets
The physics of the electronic excitation in semiconductors induced by sub-GeV
dark matter (DM) have been extensively discussed in literature, under the
framework of the standard plane wave (PW) and pseudopotential calculation
scheme. In this paper, we investigate the implication of the all-electron (AE)
reconstruction on estimation of the DM-induced electronic transition event
rates. As a benchmark study, we first calculate the wavefunctions in silicon
and germanium bulk crystals based on both the AE and pseudo (PS) schemes within
the projector augmented wave (PAW) framework, and then make comparisons between
the calculated excitation event rates obtained from these two approaches. It
turns out that in process where large momentum transfer is kinetically allowed,
the two calculated event rates can differ by a factor of a few. Such
discrepancies are found to stem from the high-momentum components neglected in
the PS scheme. It is thus implied that the correction from the AE wavefunction
in the core region is necessary for an accurate estimate of the DM-induced
transition event rate in semiconductors.Comment: A missing factor associated with the Fourier
transformation is added to both the AE and PS event rates in this version.
The ratio between the AE and PS event rates is not affecte
Theory of ice premelting in porous media
Premelting describes the confluence of phenomena that are responsible for the
stable existence of the liquid phase of matter in the solid region of its bulk
phase diagram. Here we develop a theoretical description of the premelting of
water ice contained in a porous matrix, made of a material with a melting
temperature substantially larger than ice itself, to predict the amount of
liquid water in the matrix at temperatures below its bulk freezing point. Our
theory combines the interfacial premelting of ice in contact with the matrix,
grain boundary melting in the ice, and impurity and curvature induced
premelting, the latter occurring in regions which force the ice-liquid
interface into a high curvature configuration. These regions are typically
found at points where the matrix surface is concave, along contact lines of a
grain boundary with the matrix, and in liquid veins. Both interfacial
premelting and curvature induced premelting depend on the concentration of
impurities in the liquid, which, due to the small segregation coefficient of
impurities in ice are treated as homogeneously distributed in the premelted
liquid. Our principal result is an equation for the fraction of liquid in the
porous medium as a function of the undercooling, which embodies the combined
effects of interfacial premelting, curvature induced premelting, and
impurities. The result is analyzed in detail and applied to a range of
experimentally relevant settings.Comment: 14 pages, 10 figures, accepted for publication in Physical Review
The compressibility and high pressure structure of diopside from first principles simulation
The structure of diopside (CaMgSi2O6) has been calculated at pressures between 0 and 25 GPa using the planewaves and pseudopotentials approach to density functional theory. After applying a pressure correction of 4.66 GPa to allow for the under-binding usually associated with the generalized gradient approximation, cell parameters are in good agreement with experiment. Fitting to the third-order Birch-Murnaghan equation of state yields values of 122 GPa and 4.7 for the bulk modulus and its pressure derivative. In addition to cell parameters, our calculations provide all atomic positional parameters to pressures considerably beyond those currently available from experiment. We have analyzed these data in terms of polyhedral rigidity and regularity and find that the most compressible Ca polyhedron becomes markedly less anisotropic above 10 GPa
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