15,006 research outputs found
Charge ordering in doped manganese oxides: lattice dynamics and magnetic structure
Based on the Hamiltonian of small polarons with the strong nearest neighbor
repulsion, we have investigated the charge ordering phenomena observed in
half-doped manganites R_{1/2}A_{1/2}MnO_3. We have explored possible
consequences of the charge ordering phase in the half-doped manganites. First,
we have studied the renormalization of the sound velocity around ,
considering the acoustic phonons coupled to the electrons participating in the
charge ordering. Second, we have found a new antiferromagnetic phase induced by
the charge ordering, and discussed its role in connection with the specific
CE-type antiferromagnetic structure observed in half-doped manganites.Comment: 5 pages, 2 Postscript figures. To appear in Phys. Rev. B - Rapid
Comm. (01Jun97
Waiting time dynamics of priority-queue networks
We study the dynamics of priority-queue networks, generalizations of the
binary interacting priority queue model introduced by Oliveira and Vazquez
[Physica A {\bf 388}, 187 (2009)]. We found that the original AND-type protocol
for interacting tasks is not scalable for the queue networks with loops because
the dynamics becomes frozen due to the priority conflicts. We then consider a
scalable interaction protocol, an OR-type one, and examine the effects of the
network topology and the number of queues on the waiting time distributions of
the priority-queue networks, finding that they exhibit power-law tails in all
cases considered, yet with model-dependent power-law exponents. We also show
that the synchronicity in task executions, giving rise to priority conflicts in
the priority-queue networks, is a relevant factor in the queue dynamics that
can change the power-law exponent of the waiting time distribution.Comment: 5 pages, 3 figures, minor changes, final published versio
Correlation Assisted Phonon Softenings and the Mott-Peierls Transition in VO
To explore the driving mechanisms of the metal-insulator transition (MIT) and
the structural transition in VO2, we have investigated phonon dispersions of
rutile VO2 (R-VO2) in the DFT and the DFT+U (U : Coulomb correlation) band
calculations. We have found that the phonon softening instabilities occur in
both cases, but the softened phonon mode only in the DFT+U describes properly
both the MIT and the structural transition from R-VO2 to monoclinic VO2
(M1-VO2). This feature demonstrates that the Coulomb correlation effect plays
an essential role of assisting the Peierls transition in R-VO2. We have also
found from the phonon dispersion of M1-VO2 that M1 structure becomes unstable
under high pressure. We have predicted a new phase of VO2 at high pressure that
has a monoclinic CaCl2-type structure with metallic nature
Origin of the giant magnetic moments of Fe impurities on and in Cs films
To explore the origin of the observed giant magnetic moments ()
of Fe impurities on the surface and in the bulk of Cs films, we have performed
the relativistic LSDA + U calculations using the linearized muffin-tin orbital
(LMTO) band method. We have found that Fe impurities in Cs behave differently
from those in noble metals or in Pd. Whereas the induced spin polarization of
Cs atoms is negligible, the Fe ion itself is found to be the source of the
giant magnetic moment. The 3d electrons of Fe in Cs are localized as the 4f
electrons in rare-earth ions so that the orbital magnetic moment becomes as
large as the spin magnetic moment. The calculated total magnetic moment of , which comes mainly from Fe ion, is close to the experimentally
observed value.Comment: 4 pages including 3 figures and 1 table. Submitted to PR
Noise Characteristics of Molecular Oscillations in Simple Genetic Oscillatory Systems
We study the noise characteristics of stochastic oscillations in protein
number dynamics of simple genetic oscillatory systems. Using the
three-component negative feedback transcription regulatory system called the
repressilator as a prototypical example, we quantify the degree of fluctuations
in oscillation periods and amplitudes, as well as the noise propagation along
the regulatory cascade in the stable oscillation regime via dynamic Monte Carlo
simulations. For the single protein-species level, the fluctuation in the
oscillation amplitudes is found to be larger than that of the oscillation
periods, the distributions of which are reasonably described by the Weibull
distribution and the Gaussian tail, respectively. Correlations between
successive periods and between successive amplitudes, respectively, are
measured to assess the noise propagation properties, which are found to decay
faster for the amplitude than for the period. The local fluctuation property is
also studied.Comment: 7 pages, 6 figures, minor changes, final published versio
Chromatic transit light curves of disintegrating rocky planets
Context. Kepler observations have revealed a class of short period
exoplanets, of which Kepler-1520 b is the prototype, which have comet-like dust
tails thought to be the result of small, rocky planets losing mass. The shape
and chromaticity of the transits constrain the properties of the dust particles
originating from the planet's surface, offering a unique opportunity to probe
the composition and geophysics of rocky exoplanets.
Aims. We aim to approximate the average Kepler long-cadence light curve of
Kepler-1520 b and investigate how the optical thickness and transit
cross-section of a general dust tail can affect the observed wavelength
dependence and depth of transit light curves.
Methods. We developed a new 3D model that ejects sublimating particles from
the planet surface to build up a dust tail, assuming it to be optically thin,
and used 3D radiative transfer computations that fully treat scattering using
the distribution of hollow spheres (DHS) method, to generate transit light
curves between 0.45 and 2.5 m.
Results. We show that the transit depth is wavelength independent for
optically thick tails, potentially explaining why only some observations
indicate a wavelength dependence. From the 3D nature of our simulated tails, we
show that their transit cross-sections are related to the component of particle
ejection velocity perpendicular to the planet's orbital plane and use this to
derive a minimum ejection velocity of 1.2 kms. To fit the average
transit depth of Kepler-1520 b of 0.87%, we require a high dust mas-loss rate
of 7 80 M Gyr which implies planet lifetimes that may be
inconsistent with the observed sample. Therefore, these mass-loss rates should
be considered to be upper limits.Comment: 22 pages, 22 figures, accepted for publication in A&
Monte Carlo modeling of spin injection through a Schottky barrier and spin transport in a semiconductor quantum well
We develop a Monte Carlo model to study injection of spin-polarized electrons
through a Schottky barrier from a ferromagnetic metal contact into a
non-magnetic low-dimensional semiconductor structure. Both mechanisms of
thermionic emission and tunneling injection are included in the model. Due to
the barrier shape, the injected electrons are non-thermalized. Spin dynamics in
the semiconductor heterostructure is controlled by the Rashba and Dresselhaus
spin-orbit interactions and described by a single electron spin density matrix
formalism. In addition to the linear term, the third order term in momentum for
the Dresselhaus interaction is included. Effect of the Schottky potential on
the spin dynamics in a 2 dimensional semiconductor device channel is studied.
It is found that the injected current can maintain substantial spin
polarization to a length scale in the order of 1 micrometer at room temperature
without external magnetic fields.Comment: 18 pages, 4 figures, J. Appl. Phys., accepted for publicatio
Density functional calculations of the electronic structure and magnetic properties of the hydrocarbon K3picene superconductor near the metal-insulator transition
We have investigated the electronic structures and magnetic properties of of
K3picene, which is a first hydrocarbon superconductor with high transition
temperature T_c=18K. We have shown that the metal-insulator transition (MIT) is
driven in K3picene by 5% volume enhancement with a formation of local magnetic
moment. Active bands for superconductivity near the Fermi level E_F are found
to have hybridized character of LUMO and LUMO+1 picene molecular orbitals.
Fermi surfaces of K3picene manifest neither prominent nesting feature nor
marked two-dimensional behavior. By estimating the ratio of the Coulomb
interaction U and the band width W of the active bands near E_F, U/W, we have
demonstrated that K3picene is located in the vicinity of the Mott transition.Comment: 5 pages, 5 figure
A Limit Relation between Black Hole Mass and H Width: Testing Super-Eddington Accretion in Active Galactic Nuclei
(abbreviated) We show that there is a limit relation between the black hole
mass and the width at the half maximum of H for active galactic nuclei
(AGNs) with super-Eddington accretion rates. When a black hole has a
super-Eddington accretion rate, the empirical relation of reverberation mapping
has two possible ways. First, it reduces to a relation between the black hole
mass and the size of the broad line region due to the photon trapping effects
inside the accretion disk. For the Kaspi et al.'s empirical reverberation
relation, we get the limit relation as , called as the
Eddington limit. Second, the Eddington limit luminosity will be relaxed if the
trapped photons can escape from the magnetized super-Eddington accretion disk
via the photon bubble instability, and the size of the broad line region will
be enlarged according to the empirical reverberation relation, leading to a
relatively narrow width of H. We call this the Begelman limit.
Super-Eddington accretions in a sample composed of 164 AGNs have been
searched by this limit relation. We find there are a handful of objects locate
between the Eddington and Begelman limit lines, they may be candidates of
super-Eddington accretors in a hybrid structure of photon trapping and photon
bubble instability. The maximum width of H is in the reange of km s for the maximum mass black holes with
super-Eddington accretion rates among AGNs. We suggest that this limit relation
is more reliable and convenient to test whether a source is super-Eddington and
useful to probe the structure of the super-Eddington accretion process.Comment: 5 pages (emulateapj5.sty), 1 figure. Astronomical Journal, 125 (June
Issue 2003) in pres
The black hole fundamental plane from a uniform sample of radio and X-ray emitting broad line AGNs
We derived the black hole fundamental plane relationship among the 1.4GHz
radio luminosity (L_r), 0.1-2.4keV X-ray luminosity (L_X), and black hole mass
(M) from a uniform broad line SDSS AGN sample including both radio loud and
radio quiet X-ray emitting sources. We found in our sample that the fundamental
plane relation has a very weak dependence on the black hole mass, and a tight
correlation also exists between the Eddington luminosity scaled X-ray and radio
luminosities for the radio quiet subsample. Additionally, we noticed that the
radio quiet and radio loud AGNs have different power-law slopes in the
radio--X-ray non-linear relationship. The radio loud sample displays a slope of
1.39, which seems consistent with the jet dominated X-ray model. However, it
may also be partly due to the relativistic beaming effect. For radio quiet
sample the slope of the radio--X-ray relationship is about 0.85, which is
possibly consistent with the theoretical prediction from the accretion flow
dominated X-ray model. We briefly discuss the reason why our derived
relationship is different from some previous works and expect the future
spectral studies in radio and X-ray bands on individual sources in our sample
to confirm our result.Comment: 23 pages, 7 figures, ApJ accepte
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