334 research outputs found
Deuteron Momentum Distribution in KD2HPO4
The momentum distribution in KD2PO4(DKDP) has been measured using neutron
Compton scattering above and below the weakly first order
paraelectric-ferroelectric phase transition(T=229K). There is very litte
difference between the two distributions, and no sign of the coherence over two
locations for the proton observed in the paraelectric phase, as in KH2PO4(KDP).
We conclude that the tunnel splitting must be much less than 20mev. The width
of the distribution indicates that the effective potential for DKDP is
significantly softer than that for KDP. As electronic structure calculations
indicate that the stiffness of the potential increases with the size of the
coherent region locally undergoing soft mode fluctuations, we conclude that
there is a mass dependent quantum coherence length in both systems.Comment: 6 pages 5 figure
High-pressure gas hydrates
It has long been known that crystalline hydrates are formed by many simple gases that do not interact strongly with water, and in most cases the gas molecules or atoms occupy 'cages' formed by a framework of water molecules. The majority of these gas hydrates adopt one of two cubic cage structures and are called clathrate hydrates. Notable exceptions are hydrogen and helium which form 'exotic' hydrates with structures based on ice structures, rather than clathrate hydrates, even at low pressures. Clathrate hydrates have been extensively studied because they occur widely in nature, have important industrial applications, and provide insight into water-guest hydrophobic interactions. Until recently, the expectation-based on calculations-had been that all clathrate hydrates were dissociated into ice and gas by the application of pressures of 1 GPa or so. However, over the past five years, studies have shown that this view is incorrect. Instead, all the systems so far studied undergo structural rearrangement to other, new types of hydrate structure that remain stable to much higher pressures than had been thought possible. In this paper we review work on gas hydrates at pressures above 0.5 GPa, identify common trends in transformations and structures, and note areas of uncertainty where further work is needed
Atomic structure and vibrational properties of icosahedral BC boron carbide
The atomic structure of icosahedral BC boron carbide is determined by
comparing existing infra-red absorption and Raman diffusion measurements with
the predictions of accurate {\it ab initio} lattice-dynamical calculations
performed for different structural models. This allows us to unambiguously
determine the location of the carbon atom within the boron icosahedron, a task
presently beyond X-ray and neutron diffraction ability. By examining the inter-
and intra-icosahedral contributions to the stiffness we show that, contrary to
recent conjectures, intra-icosahedral bonds are harder.Comment: 9 pages including 3 figures, accepted in Physical Review Letter
Theoretical and computational study of high pressure structures in barium
Recent high pressure work has suggested that elemental barium forms a high
pressure self-hosting structure (Ba IV) involving two `types' of barium atom.
Uniquely among reported elemental structures it cannot be described by a single
crystalline lattice, instead involving two interpenetrating incommensurate
lattices. In this letter we report pseudopotential calculations demonstrating
the stability and the potentially disordered nature of the `guest' structure.
Using band structures and nearly-free electron theory we relate the appearance
of Ba IV to an instability in the close-packed structure, demonstrate that it
has a zero energy vibrational mode, and speculate about the structure's
stability in other divalent elements.Comment: 4 pages and 5 figures. To appear in PR
Suppression of Tc in the (Y0.9Ca0.1)Ba2Cu4-xFexO8 system
In this paper, the effects produced by the iron substitutions in the
(Y0.9Ca0.1)Ba2Cu4-xFexO8 system on the superconducting and structural
properties are studied. The Rietveld fit of the crystal structure and Mossbauer
spectroscopy results of (Y0.9Ca0.1)Ba2Cu4-xFexO8 samples indicate that, the
iron atoms occupy the Cu(1) sites of the (Cu-O)2 double chain in fivefold
coordination at low iron concentrations. Besides at high iron concentrations
the iron atoms occupy the Cu(1) sites of single Cu-O chainss and Cu(2) sites in
the CuO2 planes of the (Y0.9Ca0.1)Ba2Cu4-xFexO8 phase with structural defects.
Simultaneouly, as iron concentration increases, a faster decrease of Tc is
observed in this material comapred with the YBa2Cu3-xFexO7-y system. According
to the charge transfer model proposed for YBa2Cu4O8 under pressure, the
decrease in the Cu(1)-O(4) bond length in parallel to the increase in the
Cu(2)-O(4) bond length may affect the charge transfer mechanism leading to the
suppression of Tc.Comment: submitted to Journal of Physics: Condensed Matter on 12 October 2001,
8 figures, 2 tables, 9 page
Pressure-induced metallization in solid boron
Different phases of solid boron under high pressure are studied by first
principles calculations. The -B structure is found to be stable
up to 270 GPa. Its semiconductor band gap (1.72 eV) decreases continuously to
zero around 160 GPa, where the material transforms to a weak metal. The
metallicity, as measured by the density of states at the Fermi level, enhances
as the pressure is further increased. The pressure-induced metallization can be
attributed to the enhanced boron-boron interactions that cause bands overlap.
These results are consist with the recently observed metallization and the
associated superconductivity of bulk boron under high pressure (M.I.Eremets et
al, Science{\bf 293}, 272(2001)).Comment: 14 pages, 5 figure
A rotator for single-crystal neutron diffraction at high pressure
The structure and dark matter halo core properties of dwarf spheroidal
galaxies (dSphs) are investigated. A double-isothermal model of an isothermal
stellar system, embedded in an isothermal dark halo core provides an excellent
fit to the various observed stellar surface density distributions. The stellar
system can be well characterised by King profiles with a broad distribution of
concentration parameters c. The core scale length of the stellar system a_* is
sensitive to the central dark matter density rho_0. In contrast to
single-component systems, the cut-off radius of the stellar system, rs_t,
however does not trace the tidal radius but the core radius r_c of its dark
matter halo. c is therefore sensitive to the ratio of the stellar to the dark
matter velocity dispersion, sigma_*/sigma_0. Simple empirical relationships are
derived that allow to calculate the dark halo core parameters rho_0, r_c and
sigma_0, given the observable quantities sigma_*, a_* and c. The DIS model is
applied to the Milky Way's dSphs. Their halo velocity dispersions lie in a
narrow range of 10km/s <= sigma_0 <= 18km/s with halo core radii of 280pc <=
r_c <= 1.3kpc and r_c=2a_*. All dSphs follow closely the same universal dark
halo core scaling relation rho_0*r_c=75 Msolar/pc^2 that characterises the
cores of more massive galaxies over several orders of magnitude in mass. The
dark matter core mass is a strong function of core radius. Inside a fixed
radius r_u, with r_u the logarithmic mean of the dSph's core radii, the total
enclosed mass M_u is however roughly constant, although outliers should exist.
For our dSphs we find r_u=400pc and M_u=2.6*10^7 Msolar. The core densities of
the Galaxy's dSphs are very high, with rho_0=0.2 Msolar/pc^3. They should
therefore be tidally undisturbed. Observational evidence for tidal effects
might then provide a serious challenge for the cold dark matter scenario.Comment: 21 pages, 5 figures, submitted to Ap
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