1,451 research outputs found
Nematic state stabilized by off-site Coulomb interaction in iron-based superconductors
Using a variational Monte Carlo method, we investigate the nematic state in
iron-base superconductors based on a three-band Hubbard model. Our results
demonstrate that the nematic state, formed by introducing an anisotropic
hopping order into the projected wave function, can arise in the underdoped
regime when a realistic off-site Coulomb interaction is considered. {\color
{red} We demonstrate that the off-site Coulomb interaction , which is
neglected so far in the analysis of iron-base superconductors, make a dominant
contribution to the stabilization of nematic state. We calculate the doping
dependencies of the anisotropic properties such as the unequal occupation of
and orbitals, anisotropies of kinetic energy and spin
correlations, and show that they are all suppressed upon electron doping, which
are consistent with the intrinsic anisotropies observed by optical spectrum
measurement and ARPES experiments.Comment: 5 pages, 4 figure
Band-filling and correlation controlling electronic properties and magnetism in KFeSe: A slave boson study
In this paper we investigate the electronic and magnetic properties of
KFeSe materials at different band fillings utilizing the
multi-orbital Kotliar-Ruckenstein's slave-boson mean field approach. We find
that at three-quarter filling, corresponding to KFeSe, the ground
state is a paramagnetic bad metal. Through band renormalization analysis and
comparison with the angle-resolved photoemission spectra data, we identify that
KFeSe is also an intermediate correlated system, similar to
iron-pnictide systems. At two-third filling, corresponding to the
Fe-based systems, the ground state is a striped antiferromagnetic (SAFM)
metal with spin density wave gap partially opened near the Fermi level. In
comparison, at half filling case, corresponding to the Fe-based
compounds, besides SAFM, a antiferromagnetic metallic ground
state without orbital ordering is observed in the intermediate correlation
range, and an orbital selective Mott phase (OSMP) accompanied with an
intermediate-spin to high-spin transition is also found. These results
demonstrate that the band filling and correlation control the electronic state,
Fermi surface topology and magnetism in KFeSe.Comment: 21 pages, 12 figure
Coexistence of localized and itinerant magnetism in newly discovered iron-selenide superconductor LiFeO2Fe2Se2
The electronic structure and magnetism of LiFeOFeSe are
investigated using the first-principle calculations. The ground state is
Nel antiferromagnetic (AFM) Mott insulating state for Fe1 with
localized magnetism in LiFeO layer and striped AFM metallic state for Fe2
with itinerant magnetism in FeSe layer, accompanied with a weak
interlayer AFM coupling between Fe1 and Fe2 ions, resulting in a coexistence of
localized and itinerant magnetism. Moreover, the layered LiFeO is found
to be more than an insulating block layer but responsible for enhanced AFM
correlation in FeSe layer through the interlayer magnetic coupling.
The interplay between the magnetisms of Fe1 and Fe2 introduces a control
mechanism for spin fluctuations associated with superconductivity in iron-based
superconductors.Comment: 5 pages, 7 figure
Monogamy relation of multi-qubit systems for squared Tsallis-\emph{q} entanglement
Tsallis- entanglement is a bipartite entanglement measure which is the
generalization of entanglement of formation for tending to 1. We first
expand the range of for the analytic formula of Tsallis-\emph{q}
entanglement. For \frac{5-\sqrt{13}}{2} \leq \emph{q} \leq
\frac{5+\sqrt{13}}{2}, we prove the monogamy relation in terms of the squared
Tsallis- entanglement for an arbitrary multi-qubit systems. It is shown that
the multipartite entanglement indicator based on squared Tsallis-
entanglement still works well even when the indicator based on the squared
concurrence loses its efficacy. We also show that the -th power of
Tsallis-\emph{q} entanglement satisfies the monogamy or polygamy inequalities
for any three-qubit state.Comment: This paper was submitted on 27th Marc
External Shock in a Multi-Bursting Gamma-ray Burst: Energy Injection Phase induced by the Later Launched Ejecta
Central engine of gamma-ray bursts (GRBs) may be intermittent and launch
several episodes of ejecta separated by a long quiescent interval. In this
scenario, an external shock is formed due to the propagation of the first
launched ejecta into the circum-burst medium and the later launched ejecta may
interact with the external shock at later period. Owing to the internal
dissipation, the later launched ejecta may be observed at a later time
(). In this paper, we study the relation of and
, where is the collision time of the later launched
ejecta with the formed external shock. It is found that the relation of
and depends on the bulk Lorentz factor
() of the later launched ejecta and the density () of
the circum-burst medium. If the value of or is low,
the would be significantly larger than . However,
the can be found if the value of
or is significantly large. Our results can explain
the large lag of the optical emission relative to the -ray/X-ray
emission in GRBs, e.g., GRB~111209A. For GRBs with a precursor, our results
suggest that the energy injection into the external shock and thus more than
one external-reverse shock may appear in the main prompt emission phase.
According to our model, we estimate the Lorentz factor of the second launched
ejecta in GRB~160625B.Comment: 10 pages, 4 figures, accepted for publication in Ap
Steep Decay Phase Shaped by the Curvature Effect. II. Spectral Evolution
We derive a simple analytical formula to describe the evolution of spectral
index in the steep decay phase shaped by the curvature effect with
assumption that the spectral parameters and Lorentz factor of jet shell is the
same for different latitude. Here, the value of is estimated in
0.310keV energy band. For a spherical thin shell with a cutoff power law
(CPL) intrinsic radiation spectrum, the spectral evolution can be read as a
linear function of observer time. For the situation with Band function
intrinsic radiation spectrum, the spectral evolution may be complex. If the
observed break energy of radiation spectrum is larger than 10keV, the spectral
evolution is the same as that shaped by jet shells with a CPL spectrum. If the
observed break energy is less than 0.3keV, the value of would be a
constant. Others, the spectral evolution can be approximated as a logarithmal
function of the observer time in generally.Comment: 24 pages, 13 figures, accepted for publication in Ap
The Feasibility and Flexibility of Selecting Quasars by Variability Using Ensemble Machine Learning Algorithms
In this work we train three decision-tree based ensemble machine learning
algorithms (Random Forest Classifier, Adaptive Boosting and Gradient Boosting
Decision Tree respectively) to study quasar selection in the variable source
catalog in SDSS Stripe 82. We build training and test samples (both containing
1:1 of quasars and stars) using the spectroscopic confirmed sources in SDSS
DR14 (including 8330 quasars and 3966 stars). We find that, trained with
variation parameters alone, all three models can select quasars with similarly
and remarkably high precision and completeness ( 98.5% and 97.5%), even
better than trained with SDSS colors alone ( 97.2% and 96.5%), consistent
with previous studies. Through applying the trained models on the variable
sources without spectroscopic identifications, we estimate the
spectroscopically confirmed quasar sample in Stripe 82 variable source catalog
is 93% complete (95% for ). Using the Random Forest Classifier
we derive the relative importance of the observational features utilized for
classifications. We further show that even using one- or two-year time domain
observations, variability-based quasar selection could still be highly
efficient.Comment: 20 pages, 13 figures, accepted to RA
Steep Decay Phase Shaped by the Curvature Effect. I. Flux Evolution
The curvature effect may be responsible for the steep decay phase observed in
gamma-ray bursts. For testing the curvature effect with observations, the zero
time point adopted to plot observer time and flux on a logarithmic scale
should be appropriately selected. In practice, however, the true cannot
be directly constrained from the data. Then, we move to a certain time in
the steep decay phase, which can be easily identified. In this situation, we
derive an analytical formula to describe the flux evolution of the steep decay
phase. The analytical formula is read as with , where is the flux observed at frequency ,
is the observer time by setting zero time point at a
certain time in the steep decay phase, is the spectral index estimated
around , and is the decay timescale of the phase with
. We test the analytical formula with the data
from numerical calculations. It is found that the analytical formula presents a
well estimation about the evolution of flux shaped by the curvature effect. Our
analytical formula can be used to confront the curvature effect with
observations and estimate the decay timescale of the steep decay phase.Comment: 28 pages, 18 figures, accepted for publication in Ap
Polygamy relation for the R\'enyi- entanglement of assistance in multi-qubit systems
We prove a new polygamy relation of multi-party quantum entanglement in terms
of R\'{e}nyi- entanglement of assistance for . This class of
polygamy inequality reduces to the polygamy inequality based on entanglement of
assistance since R\'{e}nyi- entanglement is a generalization of
entanglement of formation. We further show that the polygamy inequality also
holds for the th power of R\'{e}nyi- entanglement of assistance.Comment: 6 pages, 7 figure
Correlations of Disk and Jet Emission Deviating from the Fundamental Plane
The variability of accretion rate, which is believed to induce the aperiodic
variability of X-ray emission from disk, may affect the energy injection into
the jet. In this spirit, a correlation between disk emission and jet emission
can be formed even if the mean luminosity of disk emission keeps constant. In
this work, these correlations are found in the situation that the luminosity of
disk emission is variable and kept with a constant mean value. The obtained
correlations may be shallower than that of the fundamental plane of black hole
activity. In addition, the slope of correlation may increase with increasing
observed frequency of jet emission. For the luminosities spacing with three
days, the slope of correlation decreases with increasing black hole mass. The
deviation of our found correlations from that of the fundamental plane is
related to the suppression of variability in the jet emission in comparison
with that in the disk emission. This mechanism may work in some sources in
which shallower correlations have been reported. Moreover, it implies that
luminosities used to estimate the relation of fundamental plane should cover an
appropriate timescale, in which the variability of jet emission is not
significantly suppressed.Comment: 19 pages, 5 figures, accepted for publication in Ap
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