2,312 research outputs found
Spin wave dispersion based on the quasiparticle self-consistent method: NiO, MnO and -MnAs
We present spin wave dispersions in MnO, NiO, and -MnAs based on the
quasiparticle self-consistent method (\qsgw), which determines an optimum
quasiparticle picture. For MnO and NiO, \qsgw results are in rather good
agreement with experiments, in contrast to the LDA and LDA+U description. For
-MnAs, we find a collinear ferromagnetic ground state in \qsgw, while
this phase is unstable in the LDA.Comment: V2: add another figure for SW life time. Formalism is detaile
Periodicity and criticality in the Olami-Feder-Christensen model of earthquakes
Characteristic versus critical features of earthquakes are studied on the
basis of the Olami-Feder-Christensen model. It is found that the local
recurrence-time distribution exhibits a sharp -function-like peak
corresponding to rhythmic recurrence of events with a fixed ``period'' uniquely
determined by the transmission parameter of the model, together with a
power-law-like tail corresponding to scale-free recurrence of events. The model
exhibits phenomena closely resembling the asperity known in seismology
Gamma-ray variability from wind clumping in HMXBs with jets
In the subclass of high-mass X-ray binaries known as "microquasars",
relativistic hadrons in the jets launched by the compact object can interact
with cold protons from the star's radiatively driven wind, producing pions that
then quickly decay into gamma rays. Since the resulting gamma-ray emissivity
depends on the target density, the detection of rapid variability in
microquasars with GLAST and the new generation of Cherenkov imaging arrays
could be used to probe the clumped structure of the stellar wind. We show here
that the fluctuation in gamma rays can be modeled using a "porosity length"
formalism, usually applied to characterize clumping effects. In particular, for
a porosity length defined by h=l/f, i.e. as the ratio of the characteristic
size l of clumps to their volume filling factor f, we find that the relative
fluctuation in gamma-ray emission in a binary with orbital separation a scales
as sqrt(h/pi a) in the "thin-jet" limit, and is reduced by a factor 1/sqrt(1 +
phi a/(2 l)) for a jet with a finite opening angle phi. For a thin jet and
quite moderate porosity length h ~ 0.03 a, this implies a ca. 10 % variation in
the gamma-ray emission. Moreover, the illumination of individual large clumps
might result in isolated flares, as has been recently observed in some massive
gamma-ray binaries.Comment: Accepted for publication in ApJ; 5 pages, 1 figur
Low Energy, Coherent, Stoner-like Excitations in CaFeAs
Using linear-response density-functional theory, magnetic excitations in the
striped phase of CaFeAs are studied as a function of local moment
amplitude. We find a new kind of excitation: sharp resonances of Stoner-like
(itinerant) excitations at energies comparable to the N{\'{e}}el temperature,
originating largely from a narrow band of Fe states near the Fermi level,
and coexist with more conventional (localized) spin waves. Both kinds of
excitations can show multiple branches, highlighting the inadequacy of a
description based on a localized spin model
Positive Lyapunov Exponents for Quasiperiodic Szego cocycles
In this paper we first obtain a formula of averaged Lyapunov exponents for
ergodic Szego cocycles via the Herman-Avila-Bochi formula. Then using
acceleration, we construct a class of analytic quasi-periodic Szego cocycles
with uniformly positive Lyapunov exponents. Finally, a simple application of
the main theorem in [Y] allows us to estimate the Lebesgue measure of support
of the measure associated to certain class of C1 quasiperiodic 2- sided
Verblunsky coefficients. Using the same method, we also recover the [S-S]
results for Schrodinger cocycles with nonconstant real analytic potentials and
obtain some nonuniform hyperbolicity results for arbitrarily fixed Brjuno
frequency and for certain C1 potentials.Comment: 27 papge
Theoretical study of resonant x-ray emission spectroscopy of Mn films on Ag
We report a theoretical study on resonant x-ray emission spectra (RXES) in
the whole energy region of the Mn white lines for three prototypical
Mn/Ag(001) systems: (i) a Mn impurity in Ag, (ii) an adsorbed Mn monolayer on
Ag, and (iii) a thick Mn film. The calculated RXES spectra depend strongly on
the excitation energy. At excitation, the spectra of all three systems
are dominated by the elastic peak. For excitation energies around , and
between and , however, most of the spectral weight comes from
inelastic x-ray scattering. The line shape of these inelastic ``satellite''
structures changes considerably between the three considered Mn/Ag systems, a
fact that may be attributed to changes in the bonding nature of the Mn-
orbitals. The system-dependence of the RXES spectrum is thus found to be much
stronger than that of the corresponding absorption spectrum. Our results
suggest that RXES in the Mn region may be used as a sensitive probe
of the local environment of Mn atoms.Comment: 9 pages, 11 figure
Impulse balance and framing effects in threshold public good games
In this paper, we revisit the evidence for framing effects in threshold public good games. Our particular focus is on why the probability of providing the public good appears to be higher in positive, give frames compared with negative, take frames. We show that the impulse balance theory can explain this effect. We also report a new experiment designed to test the predictions of the impulse balance theory. The results of the experiment fit well, both in quantitative and qualitative terms, with our predictions.</p
Asperity characteristics of the Olami-Feder-Christensen model of earthquakes
Properties of the Olami-Feder-Christensen (OFC) model of earthquakes are
studied by numerical simulations. The previous study indicated that the model
exhibits ``asperity''-like phenomena, {\it i.e.}, the same region ruptures many
times near periodically [T.Kotani {\it et al}, Phys. Rev. E {\bf 77}, 010102
(2008)]. Such periodic or characteristic features apparently coexist with
power-law-like critical features, {\it e.g.}, the Gutenberg-Richter law
observed in the size distribution. In order to clarify the origin and the
nature of the asperity-like phenomena, we investigate here the properties of
the OFC model with emphasis on its stress distribution. It is found that the
asperity formation is accompanied by self-organization of the highly
concentrated stress state. Such stress organization naturally provides the
mechanism underlying our observation that a series of asperity events repeat
with a common epicenter site and with a common period solely determined by the
transmission parameter of the model. Asperity events tend to cluster both in
time and in space
A Massive Jet Ejection Event from the Microquasar SS 433 Accompanying Rapid X-Ray Variability
Microquasars occasionally exhibit massive jet ejections which are distinct
from the continuous or quasi-continuous weak jet ejections. Because those
massive jet ejections are rare and short events, they have hardly been observed
in X-ray so far. In this paper, the first X-ray observation of a massive jet
ejection from the microquasar SS 433 with the Rossi X-ray Timing Explorer
(RXTE) is reported. SS 433 undergoing a massive ejection event shows a variety
of new phenomena including a QPO-like feature near 0.1 Hz, rapid time
variability, and shot-like activities. The shot-like activity may be caused by
the formation of a small plasma bullet. A massive jet may be consist of
thousands of those plasma bullets ejected from the binary system. The size,
mass, internal energy, and kinetic energy of the bullets and the massive jet
are estimated.Comment: 21 pages including 5 figures, submitted to Ap
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