8,252 research outputs found
Decays of and into vector and pseudoscalar meson and the pseudoscalar glueball- mixing
We introduce a parametrization scheme for where
the effects of SU(3) flavor symmetry breaking and doubly OZI-rule violation
(DOZI) can be parametrized by certain parameters with explicit physical
interpretations. This scheme can be used to clarify the glueball-
mixing within the pseudoscalar mesons. We also include the contributions from
the electromagnetic (EM) decays of and via
. Via study of the isospin violated
channels, such as , ,
and , reasonable constraints on the EM decay
contributions are obtained. With the up-to-date experimental data for
, and , etc, we arrive at a consistent description of the mentioned
processes with a minimal set of parameters. As a consequence, we find that
there exists an overall suppression of the form factors,
which sheds some light on the long-standing " puzzle". By determining
the glueball components inside the pseudoscalar and in
three different glueball- mixing schemes, we deduce that the lowest
pseudoscalar glueball, if exists, has rather small component, and it
makes the a preferable candidate for glueball.Comment: Revised version to appear on J. Phys. G; An error in the code was
corrected. There's slight change to the numerical results, while the
conclusion is intac
Hidden itinerant-spin phase in heavily-overdoped La2-xSrxCuO4 revealed by dilute Fe doping: A combined neutron scattering and angle-resolved photoemission study
We demonstrated experimentally a direct way to probe a hidden propensity to
the formation of spin density wave (SDW) in a non-magnetic metal with strong
Fermi surface nesting. Substituting Fe for a tiny amount of Cu (1%) induced an
incommensurate magnetic order below 20 K in heavily-overdoped La2-xSrxCuO4
(LSCO). Elastic neutron scattering suggested that this order cannot be ascribed
to the localized spins on Cu or doped Fe. Angle-resolved photoemission
spectroscopy (ARPES), combined with numerical calculations, revealed a strong
Fermi surface nesting inherent in the pristine LSCO that likely drives this
order. The heavily-overdoped Fe-doped LSCO thus represents the first plausible
example of the long-sought "itinerant-spin extreme" of cuprates, where the
spins of itinerant doped holes define the magnetic ordering ground state. This
finding complements the current picture of cuprate spin physics that highlights
the predominant role of localized spins at lower dopings. The demonstrated set
of methods could potentially apply to studying hidden density-wave
instabilities of other "nested" materials on the verge of density wave
ordering.Comment: Abstract and discussion revised; to appear in Phys. Rev. Let
Doping dependence of the shadow band in La-based cuprates studied by angle-resolved photoemission spectroscopy
The shadow band (SB) in La-based cuprate family (La214) was
studied by angle-resolved photoemission spectroscopy (ARPES) over a wide doping
range from to . Unlike the well-studied case of the Bi-based
cuprate family, an overall strong, monotonic doping dependence of the SB
intensity at the Fermi level () was observed. In contrast to a previous
report for the presence of the SB only close to , we found it exists in
a wide doping range, associated with a doping-independent wave
vector but strongly doping-dependent intensity: It is the strongest at and systematically diminishes as the doping increases until it becomes
negligible in the overdoped regime. This SB with the observed doping dependence
of intensity can in principle be caused by the antiferromagnetic fluctuations
or a particular form of low-temperature orthorhombic lattice distortion known
to persist up to in the system, with both being weakened with
increasing doping. However, a detailed binding energy dependent analysis of the
SB at does not appear to support the former interpretation, leaving
the latter as a more plausible candidate, despite a challenge in quantitatively
linking the doping dependences of the SB intensity and the magnitude of the
lattice distortion. Our finding highlights the necessity of a careful and
global consideration of the inherent structural complications for correctly
understanding the cuprate Fermiology and its microscopic implication.Comment: Note the revised conclusion and author list; To appear in New J. Phy
Controlling Style and Semantics in Weakly-Supervised Image Generation
We propose a weakly-supervised approach for conditional image generation of
complex scenes where a user has fine control over objects appearing in the
scene. We exploit sparse semantic maps to control object shapes and classes, as
well as textual descriptions or attributes to control both local and global
style. In order to condition our model on textual descriptions, we introduce a
semantic attention module whose computational cost is independent of the image
resolution. To further augment the controllability of the scene, we propose a
two-step generation scheme that decomposes background and foreground. The label
maps used to train our model are produced by a large-vocabulary object
detector, which enables access to unlabeled data and provides structured
instance information. In such a setting, we report better FID scores compared
to fully-supervised settings where the model is trained on ground-truth
semantic maps. We also showcase the ability of our model to manipulate a scene
on complex datasets such as COCO and Visual Genome.Comment: European Conference on Computer Vision (ECCV) 2020, Spotlight. Code
at https://github.com/dariopavllo/style-semantic
Optical band edge shift of anatase cobalt-doped titanium dioxide
We report on the optical properties of magnetic cobalt-doped anatase phase
titanium dioxide Ti_{1-x}Co_{x}O_{2-d} films for low doping concentrations, 0
<= x <= 0.02, in the spectral range 0.2 to 5 eV. For well oxygenated films (d
<< 1) the optical conductivity is characterized by an absence of optical
absorption below an onset of interband transitions at 3.6 eV and a blue shift
of the optical band edge with increasing Co concentration. The absence of below
band gap absorption is inconsistent with theoretical models which contain
midgap magnetic impurity bands and suggests that strong on-site Coulomb
interactions shift the O-band to Co-level optical transitions to energies above
the gap.Comment: 5 pages, 4 figures, 1 table; Version 2 - major content revisio
Symmetry breaking orbital anisotropy on detwinned Ba(Fe1-xCox)2As2 above the spin density wave transition
Nematicity, defined as broken rotational symmetry, has recently been observed
in competing phases proximate to the superconducting phase in the cuprate high
temperature superconductors. Similarly, the new iron-based high temperature
superconductors exhibit a tetragonal to orthorhombic structural transition
(i.e. a broken C4 symmetry) that either precedes or is coincident with a
collinear spin density wave (SDW) transition in undoped parent compounds, and
superconductivity arises when both transitions are suppressed via doping.
Evidence for strong in-plane anisotropy in the SDW state in this family of
compounds has been reported by neutron scattering, scanning tunneling
microscopy, and transport measurements. Here we present an angle resolved
photoemission spectroscopy study of detwinned single crystals of a
representative family of electron-doped iron-arsenide superconductors,
Ba(Fe1-xCox)2As2 in the underdoped region. The crystals were detwinned via
application of in-plane uniaxial stress, enabling measurements of single domain
electronic structure in the orthorhombic state. At low temperatures, our
results clearly demonstrate an in-plane electronic anisotropy characterized by
a large energy splitting of two orthogonal bands with dominant dxz and dyz
character, which is consistent with anisotropy observed by other probes. For
compositions x>0, for which the structural transition (TS) precedes the
magnetic transition (TSDW), an anisotropic splitting is observed to develop
above TSDW, indicating that it is specifically associated with TS. For
unstressed crystals, the band splitting is observed close to TS, whereas for
stressed crystals the splitting is observed to considerably higher
temperatures, revealing the presence of a surprisingly large in-plane nematic
susceptibility in the electronic structure.Comment: final version published in PNAS, including supplementary informatio
Strong energy-momentum dispersion of phonon-dressed carriers in the lightly doped band insulator SrTiO
Much progress has been made recently in the study of the effects of
electron-phonon (el-ph) coupling in doped insulators using angle resolved
photoemission (ARPES), yielding evidence for the dominant role of el-ph
interactions in underdoped cuprates. As these studies have been limited to
doped Mott insulators, the important question arises how this compares with
doped band insulators where similar el-ph couplings should be at work. The
archetypical case is the perovskite SrTiO (STO), well known for its giant
dielectric constant of 10000 at low temperature, exceeding that of
LaCuO by a factor of 500. Based on this fact, it has been suggested
that doped STO should be the archetypical bipolaron superconductor. Here we
report an ARPES study from high-quality surfaces of lightly doped SrTiO.
Comparing to lightly doped Mott insulators, we find the signatures of only
moderate electron-phonon coupling: a dispersion anomaly associated with the low
frequency optical phonon with a and an overall bandwidth
renormalization suggesting an overall coming from the higher
frequency phonons. Further, we find no clear signatures of the large pseudogap
or small polaron phenomena. These findings demonstrate that a large dielectric
constant itself is not a good indicator of el-ph coupling and highlight the
unusually strong effects of the el-ph coupling in doped Mott insulators
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