6,645 research outputs found
Theoretical investigation of magnetoelectric effects in Ba2CoGe2O7
A joint theoretical approach, combining macroscopic symmetry analysis with
microscopic methods (density functional theory and model cluster Hamiltonian),
is employed to shed light on magnetoelectricity in Ba2CoGe2O7. We show that the
recently reported experimental trend of polarization guided by magnetic field
can be predicted on the basis of phenomenological Landau theory. From the
microscopic side, Ba2CoGe2O7 emerges as a prototype of a class of
magnetoelectrics, where the cross coupling between magnetic and dipolar degrees
of freedom needs, as main ingredients, the on-site spin-orbit coupling and the
spin-dependent O p - Co d hybridization, along with structural constraints
related to the noncentrosymmetric structural symmetry and the peculiar
configuration of CoO4 tetrahedrons.Comment: 5 pages, 4 figures, submitted for publicatio
Local spin and charge properties of beta-Ag0.33V2O5 studied by 51V NMR
Local spin and charge properties were studied on beta-Ag0.33V2O5, a
pressure-induced superconductor, at ambient pressure using 51V-NMR and
zero-field-resonance (ZFR) techniques. Three inequivalent Vi sites (i=1, 2, and
3) were identified from 51V-NMR spectra and the principal axes of the
electric-field-gradient (EFG) tensor were determined in a metallic phase and
the following charge-ordering phase. We found from the EFG analysis that the V1
sites are in a similar local environment to the V3 sites. This was also
observed in ZFR spectra as pairs of signals closely located with each other.
These results are well explained by a charge-sharing model where a 3d1 electron
is shared within a rung in both V1-V3 and V2-V2 two-leg ladders.Comment: 12pages, 16figure
Toward a New Distance to the Active Galaxy NGC 4258: II. Centripetal Accelerations and Investigation of Spiral Structure
We report measurements of centripetal accelerations of maser spectral
components of NGC 4258 for 51 epochs spanning 1994 to 2004. This is the second
paper of a series, in which the goal is determination of a new geometric maser
distance to NGC 4258 accurate to possibly ~3%. We measure accelerations using a
formal analysis method that involves simultaneous decomposition of maser
spectra for all epochs into multiple, Gaussian components. Components are
coupled between epochs by linear drifts (accelerations) from their centroid
velocities at a reference epoch. For high-velocity emission, accelerations lie
in the range -0.7 to +0.7 km/s/yr indicating an origin within 13 degrees of the
disk midline (the perpendicular to the line-of-sight to the black hole).
Comparison of high-velocity emission projected positions in VLBI images, with
those derived from acceleration data, provides evidence that masers trace real
gas dynamics. High-velocity emission accelerations do not support a model of
trailing shocks associated with spiral arms in the disk. However, we find
strengthened evidence for spatial periodicity in high-velocity emission, of
wavelength 0.75 mas. This supports suggestions of spiral structure due to
density waves in the nuclear accretion disk of an active galaxy. Accelerations
of low-velocity (systemic) emission lie in the range 7.7 to 8.9 km/s/yr,
consistent with emission originating from a concavity where the thin, warped
disk is tangent to the line-of-sight. A trend in accelerations of low-velocity
emission as a function of Doppler velocity may be associated with disk geometry
and orientation, or with the presence of spiral structure.Comment: Accepted to ApJ, 48 pages and 20 figure
Open inflation in the landscape
Open inflation scenario is attracting a renewed interest in the context of
string landscape. Since there are a large number of metastable de Sitter vacua
in string landscape, tunneling transitions to lower metastable vacua through
the bubble nucleation occur quite naturally. Although the deviation of Omega_0
from unity is small by the observational bound, we argue that the effect of
this small deviation on the large angle CMB anisotropies can be significant for
tensor-type perturbation in open inflation scenario. We consider the situation
in which there is a large hierarchy between the energy scale of the quantum
tunneling and that of the slow-roll inflation in the nucleated bubble. If the
potential just after tunneling is steep enough, a rapid-roll phase appears
before the slow-roll inflation. In this case the power spectrum is basically
determined by the Hubble rate during the slow-roll inflation. If such
rapid-roll phase is absent, the power spectrum keeps the memory of the high
energy density there in the large angular components. The amplitude of large
angular components can be enhanced due to the effects of the wall fluctuation
mode if the bubble wall tension is small. Therefore, one can construct some
models in which the deviation of Omega_0 from unity is large enough to produce
measurable effects. We also consider a more general class of models, where the
false vacuum decay may occur due to Hawking-Moss tunneling, as well as the
models involving more than one scalar field. We discuss scalar perturbations in
these models and point out that a large set of such models is already ruled out
by observational data, unless there was a very long stage of slow-roll
inflation after the tunneling. These results show that observational data allow
us to test various assumptions concerning the structure of the string theory
potentials and the duration of the last stage of inflation.Comment: 14 pages, 11 figures, v2:minor corrections and a reference added,
v3:accepted for publication in PR
Charge Ordering and Ferroelectricity in Half-doped Manganites
By means of density-functional simulations for half-doped manganites, such as
pseudocubic Pr0.5Ca0.5MnO3 and bilayer PrCa2Mn2O7, we discuss the occurrence of
ferroelectricity and we explore its crucial relation to the crystal structure
and to peculiar charge/spin/orbital ordering effects. In pseudocubic
Pr0.5Ca0.5MnO3, ferroelectricity is induced in the Zener polaron type
structure, where Mn ions are dimerized. In marked contrast, in bilayer
PrCa2Mn2O7, it is the displacements of apical oxygens bonded to either Mn3+ or
Mn4+ ions that play a key role in the rising of ferroelectricity. Importantly,
local dipoles due to apical oxygens are also intimately linked to charge and
orbital ordering patterns in MnO2 planes, which in turn contribute to
polarization. Finally, an important outcome of our work consists in proposing
Born effective charges as a valid mean to quantify charge disproportionation
effects, in terms of anisotropy and size of electronic clouds around Mn ions.Comment: 5 pages, 2 figures, submitted for publicatio
First Detection of Millimeter/Submillimeter Extragalactic H2O Maser Emission
We report the first detection of an extragalactic millimeter wavelength H2O
maser at 183 GHz towards NGC 3079 using the Submillimeter Array (SMA), and a
tentative submillimeter wave detection of the 439 GHz maser towards the same
source using the James Clerk Maxwell Telescope (JCMT). These H2O transitions
are known to exhibit maser emission in star-forming regions and evolved stars.
NGC 3079 is a well-studied nuclear H2O maser source at 22 GHz with a
time-variable peak flux density in the range 3 -- 12 Jy. The 183 GHz H2O maser
emission, with peak flux density 0.5 Jy (7 detection), also
originates from the nuclear region of NGC 3079 and is spatially coincident with
the dust continuum peak at 193 GHz (53 mJy integrated). Peak emission at both
183 and 439 GHz occurs in the same range of velocity as that covered by the 22
GHz spectrum. We estimate the gas to dust ratio of the nucleus of NGC 3079 to
be 150, comparable to the Galactic value of 160. Discovery of maser
emission in an active galactic nucleus beyond the long-known 22 GHz transition
opens the possibility of future position-resolved radiative transfer modeling
of accretion disks and outflows pc from massive black holes.Comment: 12 pages, 3 figures, ApJ Letters accepte
Evidence for a very slow X-ray pulsar in 2S0114+650 from RXTE All-Sky Monitor Observations
Rossi X-ray Timing Explorer (RXTE) All-Sky Monitor (ASM) observations of the
X-ray binary 2S0114+650 show modulations at periods close to both the optically
derived orbital period (11.591 days) and proposed pulse period (~ 2.7 hr). The
pulse period shows frequency and intensity variability during the more than 2
years of ASM observations analyzed. The pulse properties are consistent with
this arising from accretion onto a rotating neutron star and this would be the
slowest such period known. The shape of the orbital light curve shows
modulation over the course of the entire orbit and a comparison is made with
the orbital light curve of Vela X-1. However, the expected phase of eclipse,
based on an extrapolation of the optical ephemeris, does not correspond with
the observed orbital minimum. The orbital period derived from the ASM light
curve is also slightly longer than the optical period.Comment: To be published in the Astrophysical Journal, 1999, volume 511. 9
figure
X-Ray Spectral Variability of the Seyfert Galaxy NGC 4051 Observed with Suzaku
We report results from a Suzaku observation of the narrow-line Seyfert 1 NGC
4051. During our observation, large amplitude rapid variability is seen and the
averaged 2--10 keV flux is 8.1x10^-12 erg s^-1 cm^-2, which is several times
lower than the historical average. The X-ray spectrum hardens when the source
flux becomes lower, confirming the trend of spectral variability known for many
Seyfert 1 galaxies. The broad-band averaged spectrum and spectra in high and
low flux intervals are analyzed. The spectra are first fitted with a model
consisting of a power-law component, a reflection continuum originating in cold
matter, a blackbody component, two zones of ionized absorber, and several
Gaussian emission lines. The amount of reflection is rather large (R ~ 7, where
R=1 corresponds to reflection by an infinite slab), while the equivalent width
of the Fe-K line at 6.4 keV is modest (140 eV) for the averaged spectrum. We
then model the overall spectra by introducing partial covering for the
power-law component and reflection continuum independently. The column density
for the former is 1x10^23 cm^-2, while it is fixed at 1x10^24 cm-2 for the
latter. By comparing the spectra in different flux states, we identify the
causes of spectral variability. (abridged)Comment: 19 pages, 18 figures, accepted for publication in PASJ (Suzaku 3rd
special issue
Comparison and Mapping Facilitate Relation Discovery and Predication
Relational concepts play a central role in human perception and cognition, but little is known about how they are acquired. For example, how do we come to understand that physical force is a higher-order multiplicative relation between mass and acceleration, or that two circles are the same-shape in the same way that two squares are? A recent model of relational learning, DORA (Discovery of Relations by Analogy; Doumas, Hummel & Sandhofer, 2008), predicts that comparison and analogical mapping play a central role in the discovery and predication of novel higher-order relations. We report two experiments testing and confirming this prediction
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