4,918 research outputs found
Molar volume of solid isotopic helium mixtures
Solid isotopic helium mixtures have been studied by path-integral Monte Carlo
simulations in the isothermal-isobaric ensemble. This method allowed us to
study the molar volume as a function of temperature, pressure, and isotopic
composition. At 25 K and 0.2 GPa, the relative difference between molar volumes
of isotopically-pure crystals of 3He and 4He is found to be about 3%. This
difference decreases under pressure, and for 12 GPa it is smaller than 1%. For
isotopically-mixed crystals, a linear relation between lattice parameters and
concentrations of helium isotopes is found, in agreement with Vegard's law. The
virtual crystal approximation, valid for isotopic mixtures of heavier atoms,
does not give reliable results for solid solutions of helium isotopes.Comment: 7 pages, 5 figure
Rare-gas solids under pressure: A path-integral Monte Carlo simulation
Rare-gas solids (Ne, Ar, Kr, and Xe) under hydrostatic pressure up to 30 kbar
have been studied by path-integral Monte Carlo simulations in the
isothermal-isobaric ensemble. Results of these simulations have been compared
with available experimental data and with those obtained from a quasiharmonic
approximation (QHA). This comparison allows us to quantify the overall
anharmonicity of the lattice vibrations and its influence on several structural
and thermodynamic properties of rare-gas solids. The vibrational energy
increases with pressure, but this increase is slower than that of the elastic
energy, which dominates at high pressures. In the PIMC simulations, the
vibrational kinetic energy is found to be larger than the corresponding
potential energy, and the relative difference between both energies decreases
as the applied pressure is raised. The accuracy of the QHA increases for rising
pressure.Comment: 9 pages, 6 figure
Structural changes at the semiconductor-insulator phase transition in the single layered La0.5Sr1.5MnO4 perovskite
The semiconductor-insulator phase transition of the single-layer manganite
La0.5Sr1.5MnO4 has been studied by means of high resolution synchrotron x-ray
powder diffraction and resonant x-ray scattering at the Mn K edge. We conclude
that a concomitant structural transition from tetragonal I4/mmm to orthorhombic
Cmcm phases drives this electronic transition. A detailed symmetry-mode
analysis reveals that condensation of three soft modes -Delta_2(B2u), X1+(B2u)
and X1+(A)- acting on the oxygen atoms accounts for the structural
transformation. The Delta_2 mode leads to a pseudo Jahn-Teller distortion (in
the orthorhombic bc-plane only) on one Mn site (Mn1) whereas the two X1+ modes
produce an overall contraction of the other Mn site (Mn2) and expansion of the
Mn1 one. The X1+ modes are responsible for the tetragonal superlattice
(1/2,1/2,0)-type reflections in agreement with a checkerboard ordering of two
different Mn sites. A strong enhancement of the scattered intensity has been
observed for these superlattice reflections close to the Mn K edge, which could
be ascribed to some degree of charge disproportion between the two Mn sites of
about 0.15 electrons. We also found that the local geometrical anisotropy of
the Mn1 atoms and its ordering originated by the condensed Delta_2 mode alone
perfectly explains the resonant scattering of forbidden (1/4,1/4,0)-type
reflections without invoking any orbital ordering.Comment: 3 tables and 10 figures; accepted in Phys. Rev.
Quantum atomic delocalization vs. structural disorder in amorphous silicon
Quantum effects on the atom delocalization in amorphous silicon have been
studied by path-integral Monte Carlo simulations from 30 to 800 K. The quantum
delocalization is appreciable vs. topological disorder, as seen from structural
observables such as the radial distribution function (RDF). At low
temperatures, the width of the first peak in the RDF increases by a factor of
1.5 due to quantum effects. The overall anharmonicity of the solid vibrations
at finite temperatures in amorphous silicon is clearly larger than in the
crystalline material. Low-energy vibrational modes are mainly located on
coordination defects in the amorphous material.Comment: 5 pages, 5 PS figures, REVTE
The Multitude of Unresolved Continuum Sources at 1.6 microns in Hubble Space Telescope images of Seyfert Galaxies
We examine 112 Seyfert galaxies observed by the Hubble Space Telescope (HST)
at 1.6 microns. We find that ~50% of the Seyfert 2.0 galaxies which are part of
the Revised Shapeley-Ames (RSA) Catalog or the CfA redshift sample contain
unresolved continuum sources at 1.6 microns. All but a couple of the Seyfert
1.0-1.9 galaxies display unresolved continuum sources. The unresolved sources
have fluxes of order a mJy, near-infrared luminosities of order 10^41 erg/s and
absolute magnitudes M_H ~-16. Comparison non-Seyfert galaxies from the RSA
Catalog display significantly fewer (~20%), somewhat lower luminosity nuclear
sources, which could be due to compact star clusters. We find that the
luminosities of the unresolved Seyfert 1.0-1.9 sources at 1.6 microns are
correlated with [OIII] 5007A and hard X-ray luminosities, implying that these
sources are non-stellar. Assuming a spectral energy distribution similar to
that of a Seyfert 2 galaxy, we estimate that a few percent of local spiral
galaxies contain black holes emitting as Seyferts at a moderate fraction, 10^-1
to 10^-4, of their Eddington luminosities. With increasing Seyfert type the
fraction of unresolved sources detected at 1.6 microns and the ratio of 1.6
microns to [OIII] fluxes tend to decrease. These trends are consistent with the
unification model for Seyfert 1 and 2 galaxies.Comment: accepted by Ap
Probing the Higgs mechanism via
We investigate the sensitivity of the reaction to
the Higgs sector based on the complete one-loop corrections in the minimal
Standard Model and the gauged non-linear -model. While this sensitivity
is very strong for the suppressed cross-section of equally polarized photons
and longitudinal W bosons, it is only marginal for the dominant mode of
transverse polarizations. The corrections within the -model turn out to
be UV-finite in accordance with the absence of \log\MH terms in the Standard
Model with a heavy Higgs boson.Comment: 12 pages uuencoded postscrip
Kinetic growth walks on complex networks
Kinetically grown self-avoiding walks on various types of generalized random
networks have been studied. Networks with short- and long-tailed degree
distributions were considered (, degree or connectivity), including
scale-free networks with . The long-range behaviour of
self-avoiding walks on random networks is found to be determined by finite-size
effects. The mean self-intersection length of non-reversal random walks, ,
scales as a power of the system size $N$: $ \sim N^{\beta}$, with an
exponent $\beta = 0.5$ for short-tailed degree distributions and $\beta < 0.5$
for scale-free networks with $\gamma < 3$. The mean attrition length of kinetic
growth walks, , scales as , with an exponent
which depends on the lowest degree in the network. Results of
approximate probabilistic calculations are supported by those derived from
simulations of various kinds of networks. The efficiency of kinetic growth
walks to explore networks is largely reduced by inhomogeneity in the degree
distribution, as happens for scale-free networks.Comment: 10 pages, 8 figure
Presupernova Evolution of Differentially Rotating Massive Stars Including Magnetic Fields
As a massive star evolves through multiple stages of nuclear burning on its
way to becoming a supernova, a complex, differentially rotating structure is
set up. Angular momentum is transported by a variety of classic instabilities,
and also by magnetic torques from fields generated by the differential
rotation. We present the first stellar evolution calculations to follow the
evolution of rotating massive stars including, at least approximately, all
these effects, magnetic and non-magnetic, from the zero-age main sequence until
the onset of iron-core collapse. The evolution and action of the magnetic
fields is as described by Spruit 2002 and a range of uncertain parameters is
explored. In general, we find that magnetic torques decrease the final rotation
rate of the collapsing iron core by about a factor of 30 to 50 when compared
with the non-magnetic counterparts. Angular momentum in that part of the
presupernova star destined to become a neutron star is an increasing function
of main sequence mass. That is, pulsars derived from more massive stars will
rotate faster and rotation will play a more dominant role in the star's
explosion. The final angular momentum of the core is determined - to within a
factor of two - by the time the star ignites carbon burning. For the lighter
stars studied, around 15 solar masses, we predict pulsar periods at birth near
15 ms, though a factor of two range is easily tolerated by the uncertainties.
Several mechanisms for additional braking in a young neutron star, especially
by fall back, are also explored.Comment: 32 pages, 3 figures (8 eps files), submitted to Ap
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