9,183 research outputs found
Low-temperature phases in Pb(Zr0.52Ti0.48)O3: A neutron powder diffraction study
A neutron powder diffraction study has been carried out on Pb(Zr0.52Ti0.48)O3
in order to resolve an ongoing controversy about the nature of the
low-temperature structure of this strongly-piezoelectric and
technologically-important material. The results of a detailed and systematic
Rietveld analysis at 20 K are consistent with the coexistence of two monoclinic
phases having space groups Cm and Ic respectively, in the approximate ratio
4:1, and thus support the findings of a recent electron diffraction study by
Noheda et al. [Phys. Rev. B 66, 060103 (2002)]. The results are compared to
those of two recent conflicting neutron powder diffraction studies of materials
of the same nominal composition by Hatch et al. [Phys. Rev. B 65, 212101
(2002)] and Frantti et al. [Phys. Rev. B 66, 064108 (2002)].Comment: RevTex4, 16 pages, 6 color figure
Construction of nonlocal light-cone operators with definite twist
A systematic procedure is introduced to uniquely decompose nonlocal
LC-operators into harmonic operators of well defined geometric twist. The
method will be demonstrated for (pseudo)scalar, (axial) vector and skew tensor
bilocal quark light-ray operatorsComment: 4 pages, AMSTeX, Contribution to 7th Int. Workshop on Deep Inelastic
Scatterin and QCD, Zeuthen, April 1999 change of formulas 25 and 2
Dynamics of the Narrow-Line Region in the Seyfert 2 Galaxy NGC 1068
We present dynamical models based on a study of high-resolution long-slit
spectra of the narrow-line region (NLR) in NGC 1068 obtained with the Space
Telescope Imaging Spectrograph (STIS) aboard The Hubble Space Telescope (HST).
The dynamical models consider the radiative force due to the active galactic
nucleus (AGN), gravitational forces from the supermassive black hole (SMBH),
nuclear stellar cluster, and galactic bulge, and a drag force due to the NLR
clouds interacting with a hot ambient medium. The derived velocity profile of
the NLR gas is compared to that obtained from our previous kinematic models of
the NLR using a simple biconical geometry for the outflowing NLR clouds. The
results show that the acceleration profile due to radiative line driving is too
steep to fit the data and that gravitational forces along cannot slow the
clouds down, but with drag forces included, the clouds can slow down to the
systemic velocity over the range 100--400 pc, as observed. However, we are not
able to match the gradual acceleration of the NLR clouds from ~0 to ~100 pc,
indicating the need for additional dynamical studies.Comment: Paper prepared by emulateapj version 10/09/06 and accepted for print
in Ap
Frustration of tilts and A-site driven ferroelectricity in KNbO_3-LiNbO_3 alloys
Density functional calculations for K_{0.5}Li_{0.5}NbO_3 show strong A-site
driven ferroelectricity, even though the average tolerance factor is
significantly smaller than unity and there is no stereochemically active A-site
ion. This is due to the frustration of tilt instabilities by A-site disorder.
There are very large off-centerings of the Li ions, which contribute strongly
to the anisotropy between the tetragonal and rhombohedral ferroelectric states,
yielding a tetragonal ground state even without strain coupling.Comment: 4 pages, 5 figure
Recommended from our members
Observations of ozone and related species in the northeast Pacific during the PHOBEA campaigns 1. Ground-based observations at Cheeka Peak
The core flux of the brightest 10 micron galaxies in the southern sky
Aims. Near diffraction-limited images have been taken at 8.9, 11.9, and 12.9
micron for the brightest extragalactic sources in the southern sky, in order to
optimally plan N-band observations with MIDI (MID-infrared Interferometric
instrument) at the VLTI. Methods. We have assembled a sample of 21 objects
consisting of all the AGNs observable from Paranal observatory, Chile, plus
three non-AGN objects, with an estimated N-band flux greater than 400mJy. We
used the TIMMI2 Mid Infrared instrument mounted on the ESO's 3.6m telescope to
obtain near diffraction-limited images in order to establish the unresolved
core flux within < 0.5 arscsec. Results. Positions and core total fluxes were
obtained for all sources in our sample and compared with similar investigations
in the literature. We find that 15 AGN and the nuclear starburst in NGC 253
exhibit an unresolved core flux < 300mJy at 11.9 micron, making them promising
targets for MIDI at the VLTI. For extended sources, near diffraction-limited
images are presented and discussed.Comment: Accepted to A&
Gravitational Waves Probe the Coalescence Rate of Massive Black Hole Binaries
We calculate the expected nHz--Hz gravitational wave (GW) spectrum from
coalescing Massive Black Hole (MBH) binaries resulting from mergers of their
host galaxies. We consider detection of this spectrum by precision pulsar
timing and a future Pulsar Timing Array. The spectrum depends on the merger
rate of massive galaxies, the demographics of MBHs at low and high redshift,
and the dynamics of MBH binaries. We apply recent theoretical and observational
work on all of these fronts. The spectrum has a characteristic strain
, just below the detection limit from
recent analysis of precision pulsar timing measurements. However, the amplitude
of the spectrum is still very uncertain owing to approximations in the
theoretical formulation of the model, to our lack of knowledge of the merger
rate and MBH population at high redshift, and to the dynamical problem of
removing enough angular momentum from the MBH binary to reach a GW-dominated
regime.Comment: 31 Pages, 8 Figures, small changes to match the published versio
Spin structure and longitudinal polarization of hyperon in e+e- annihilation at high energies
Longitudinal polarizations of different kinds of hyperons produced in e+e-
annihilation at LEP I and LEP II energies in different event samples are
calculated using two different pictures for the spin structure of hyperon: that
drawn from polarized deep inelastic lepton-nucleon scattering data or that
using SU(6) symmetric wave functions. The result shows that measurements of
such polarizations should provide useful information to the question of which
picture is more suitable in describing the spin effects in the fragmentation
processes.Comment: 26 pages with 10 figures. Submitted to Phys. Rev.
Quark Masses: An Environmental Impact Statement
We investigate worlds that lie on a slice through the parameter space of the
Standard Model over which quark masses vary. We allow as many as three quarks
to participate in nuclei, while fixing the mass of the electron and the average
mass of the lightest baryon flavor multiplet. We classify as "congenial" worlds
that satisfy the environmental constraint that the quark masses allow for
stable nuclei with charges one, six, and eight, making organic chemistry
possible. Whether a congenial world actually produces observers depends on a
multitude of historical contingencies, beginning with primordial
nucleosynthesis, which we do not explore. Such constraints may be independently
superimposed on our results. Environmental constraints such as the ones we
study may be combined with information about the a priori distribution of quark
masses over the landscape of possible universes to determine whether the
measured values of the quark masses are determined environmentally, but our
analysis is independent of such an anthropic approach.
We estimate baryon masses as functions of quark masses and nuclear masses as
functions of baryon masses. We check for the stability of nuclei against
fission, strong particle emission, and weak nucleon emission. For two light
quarks with charges 2/3 and -1/3, we find a band of congeniality roughly 29 MeV
wide in their mass difference. We also find another, less robust region of
congeniality with one light, charge -1/3 quark, and two heavier, approximately
degenerate charge -1/3 and 2/3 quarks. No other assignment of light quark
charges yields congenial worlds with two baryons participating in nuclei. We
identify and discuss the region in quark-mass space where nuclei would be made
from three or more baryon species.Comment: 40 pages, 16 figures (in color), 4 tables. See paper for a more
detailed abstract. v4: Cleaning up minor typo
Parton distribution functions from nonlocal light-cone operators with definite twist
We introduce the chiral-even and chiral-odd quark distributions as forward
matrix elements of related bilocal quark operators with well-defined
(geometric) twist. Thereby, we achieve a Lorentz invariant classification of
these distributions which differ from the conventional ones by explicitly
taking into account the necessary trace terms. The relations between both kinds
of distribution functions are given and the mismatch between their different
definition of twist is discussed. Wandzura-Wilczek--like relations between the
conventional distributions (based on dynamical twist) are derived by means of
geometric twist distribution functions.Comment: 17 pages, REVTEX, Extended version, The Introduction has been
rewritten, Setion V "Wandzura-Wilczek--like relations" and App. B are added;
Sign errors are correcte
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