636 research outputs found
Mean-atom-trajectory model for the velocity autocorrelation function of monatomic liquids
We present a model for the motion of an average atom in a liquid or
supercooled liquid state and apply it to calculations of the velocity
autocorrelation function and diffusion coefficient . The model
trajectory consists of oscillations at a distribution of frequencies
characteristic of the normal modes of a single potential valley, interspersed
with position- and velocity-conserving transits to similar adjacent valleys.
The resulting predictions for and agree remarkably well with MD
simulations of Na at up to almost three times its melting temperature. Two
independent processes in the model relax velocity autocorrelations: (a)
dephasing due to the presence of many frequency components, which operates at
all temperatures but which produces no diffusion, and (b) the transit process,
which increases with increasing temperature and which produces diffusion.
Because the model provides a single-atom trajectory in real space and time,
including transits, it may be used to calculate all single-atom correlation
functions.Comment: LaTeX, 8 figs. This is an updated version of cond-mat/0002057 and
cond-mat/0002058 combined Minor changes made to coincide with published
versio
Electronic and Magnetic Properties of Nanographite Ribbons
Electronic and magnetic properties of ribbon-shaped nanographite systems with
zigzag and armchair edges in a magnetic field are investigated by using a tight
binding model. One of the most remarkable features of these systems is the
appearance of edge states, strongly localized near zigzag edges. The edge state
in magnetic field, generating a rational fraction of the magnetic flux (\phi=
p/q) in each hexagonal plaquette of the graphite plane, behaves like a
zero-field edge state with q internal degrees of freedom. The orbital
diamagnetic susceptibility strongly depends on the edge shapes. The reason is
found in the analysis of the ring currents, which are very sensitive to the
lattice topology near the edge. Moreover, the orbital diamagnetic
susceptibility is scaled as a function of the temperature, Fermi energy and
ribbon width. Because the edge states lead to a sharp peak in the density of
states at the Fermi level, the graphite ribbons with zigzag edges show
Curie-like temperature dependence of the Pauli paramagnetic susceptibility.
Hence, it is shown that the crossover from high-temperature diamagnetic to
low-temperature paramagnetic behavior of the magnetic susceptibility of
nanographite ribbons with zigzag edges.Comment: 13 pages including 19 figures, submitted to Physical Rev
Refining human palaeodietary reconstruction using amino acid delta N-15 values of plants, animals and humans.
An established method of estimating the trophic level of an organism is through stable isotope analysis of
its tissues and those of its diet. This method has been used in archaeology to reconstruct past human diet
from the stable nitrogen isotope (d15N) values of human and herbivore bone collagen. However, this
approach, using the 15N-enrichment of human bone collagen d15N values over associated herbivore bone
collagen d15N values to predict the relative importance of animal protein, relies on the assumptions that:
(i) the d15N values of plants consumed by humans and herbivores are identical, and (ii) the 15Nenrichment
between diet and consumer is consistent. Bone collagen amino acid d15N values have the
potential to tackle these uncertainties, as they constrain the factors influencing bone collagen d15N
values. In this study, the d15N values of glutamic acid and phenylalanine in human and herbivore bone
collagen isolates from Neolithic sites in Germany, Greece and Turkey were determined by gas
chromatography-combustion-isotope ratio mass spectrometry. The fraction of animal protein in total
dietary protein consumed by the humans was estimated by: (i) comparing bulk human and herbivore
collagen d15N values, (ii) comparing bulk human and herbivore collagen and ancient charred cereal grain
d15N values, (iii) comparing human bone collagen d15NGlutamic acid and d15NPhenylalanine values, and (iv)
comparing d15NGlutamic acid values of human and herbivore bone collagen and estimated d15NGlutamic acid
values of ancient charred cereal grains. Where determined cereal grain d15N values are higher than
estimated herbivore forage values, estimates of animal protein consumption are significantly lower,
emphasising the importance of the plant nitrogen contribution to human bone collagen. This study also
highlights the need for further investigation into: (i) the D15NConsumer-Diet values of glutamic acid and
phenylalanine in terrestrial ecosystems, and (ii) D15NGlutamic acid-Phenylalanine values of common plant foods
in order to improve the accuracy and more widespread applicability of amino acid-based methods for
palaeodietary reconstruction
Relativistic effects and quasipotential equations
We compare the scattering amplitude resulting from the several quasipotential
equations for scalar particles. We consider the Blankenbecler-Sugar, Spectator,
Thompson, Erkelenz-Holinde and Equal-Time equations, which were solved
numerically without decomposition into partial waves. We analyze both
negative-energy state components of the propagators and retardation effects. We
found that the scattering solutions of the Spectator and the Equal-Time
equations are very close to the nonrelativistic solution even at high energies.
The overall relativistic effect increases with the energy. The width of the
band for the relative uncertainty in the real part of the scattering
matrix, due to different dynamical equations, is largest for
backward-scattering angles where it can be as large as 40%.Comment: Accepted for publication in Phys. Rev.
Shadowing in photo-production : role of in-medium hadrons
We study the effects of in-medium hadronic properties on shadowing in
photon-nucleus interactions in Glauber model as well as in the multiple
scattering approach. A reasonable agreement with the experimental data is
obtained in a scenario of downward spectral shift of the hadrons. Shadowing is
found to be insensitive to the broadening of the spectral functions. An impact
parameter dependent analysis of shadowing might shed more light on the role of
in-medium properties of hadrons.Comment: Title modified; version to appear in PRC, Rapid Communication
Towards a population of HMXB/NS microquasars as counterparts of low-latitude unidentified EGRET sources
The discovery of the microquasar LS 5039 well within the 95% conficence
contour of the Unidentified EGRET Source (UES) 3EG J1824-1514 was a major step
towards the possible association between microquasars (MQs) and UESs. The
recent discovery of precessing relativistic radio jets in LS I +61 303, a
source associated for long time with 2CG 135+01 and with the UES 3EG
J0241+6103, has given further support to this idea. Finally, the very recently
proposed association between the microquasar candidate AX J1639.0-4642 and the
UES 3EG J1639-4702 points towards a population of High Mass X-ray Binary
(HMXB)/Neutron Star (NS) microquasars as counterparts of low-latitude
unidentified EGRET sources.Comment: 12 pages, 7 figures. Proceedings of the Conference "The
Multiwavelength Approach to Unidentified Gamma-ray Sources", to appear in the
journal Astrophysics and Space Scienc
Line shape and composition of the In 3d5/2 core-level photoemission for the interface analysis of In-containing III-V semiconductors
The In 3d5/2 photoelectron spectroscopy peak has been widely used to determine the interface structures of In-containing III-V device materials (e.g., oxidation states). However, an unclear parameter affecting the determination of the energy shifts and number of the core-level components, and therefore, the interpreted interface structure and composition, is still the intrinsic In 3d5/2 peak line shape. It is undecided whether the line shape is naturally symmetric or asymmetric for pure In-containing III-V compounds. By using high-resolution photoelectron spectroscopy, we show that the In 3d5/2 asymmetry arising from the emission at high binding-energy tail is not an intrinsic property of InAs, InP, InSb and InGaAs. Furthermore, it is shown that asymmetry of In 3d5/2 peaks of pure III-V’s originates from the natural surface reconstructions which cause the coexistence of slightly shifted In 3d5/2 components with the symmetric peak shape and dominant Lorentzian broadening.</p
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