24 research outputs found
Flux Phase as a Dynamic Jahn-Teller Phase: Berryonic Matter in the Cuprates?
There is considerable evidence for some form of charge ordering on the
hole-doped stripes in the cuprates, mainly associated with the low-temperature
tetragonal phase, but with some evidence for either charge density waves or a
flux phase, which is a form of dynamic charge-density wave. These three states
form a pseudospin triplet, demonstrating a close connection with the E X e
dynamic Jahn-Teller effect, suggesting that the cuprates constitute a form of
Berryonic matter. This in turn suggests a new model for the dynamic Jahn-Teller
effect as a form of flux phase. A simple model of the Cu-O bond stretching
phonons allows an estimate of electron-phonon coupling for these modes,
explaining why the half breathing mode softens so much more than the full
oxygen breathing mode. The anomalous properties of provide a coupling
(correlated hopping) which acts to stabilize density wave phases.Comment: Major Revisions: includes comparisons with specific cuprate phonon
modes, 16 eps figures, revte
Van Hove Exciton-Cageons and High-T Superconductivity: VIIID Solitons and Nonlinear Dynamics
The low-temperature orthorhombic (LTO) phase transition in
LaSrCuO can be interpreted as a dynamic Jahn-Teller effect, in
which the degenerate electronic states are associated with the large densities
of states at the two van Hove singularities. The equations describing this
phase are strongly nonlinear. This paper illustrates some consequences of the
nonlinearity, by presenting a rich variety of exact nonlinear wave solutions
for the model.
Of particular interest are soliton lattice solutions: arrays of domain walls
separating regions of local low-temperature tetragonal (LTT) symmetry. These
arrays have a {\it macroscopic} average symmetry higher than LTT. These
lattices can display either orthorhombic (`orthons') or tetragonal (`tetrons')
symmetry, and can serve as models for a microscopic description of the dynamic
JT LTO and high-temperature tetragonal phases, respectively.Comment: 17 pages plain TeX, 14 figures available upon reques
Two-band Eliashberg equations and the experimental Tc of the diboride Mg1-xAlxB2
The variation of the superconducting critical temperature Tc as a function of
x in the diboride Mg1-xAlxB2 has been studied in the framework of the two-bands
Eliashberg theory and traditional phonon coupling mechanism. We have solved the
two-bands Eliashberg equations using first-principle calculations or simple
assumptions for the variation of the relevant physical quantities. We have
found that the experimental Tc curve can be explained only if the Coulomb
pseudopotential changes with x by tuning the Fermi level toward the sigma band
edge. In polycrystal samples the x dependence of the sigma and pi-band gap has
been found and is in agreement with experiments.Comment: 6 pages, 7 figure
Bonding, Moment Formation, and Magnetic Interactions in Ca14MnBi11 and Ba14MnBi11
The ``14-1-11'' phase compounds based on magnetic Mn ions and typified by
Ca14MnBi11 and Ba14MnBi11 show unusual magnetic behavior, but the large number
(104) of atoms in the primitive cell has precluded any previous full electronic
structure study. Using an efficient, local orbital based method within the
local spin density approximation to study the electronic structure, we find a
gap between a bonding valence band complex and an antibonding conduction band
continuum. The bonding bands lack one electron per formula unit of being
filled, making them low carrier density p-type metals. The hole resides in the
MnBi4 tetrahedral unit and partially compensates the high spin d^5 Mn moment,
leaving a net spin near 4 \mu_B that is consistent with experiment. These
manganites are composed of two disjoint but interpenetrating `jungle gym'
networks of spin 4/2 MnBi4^{9-} units with ferromagnetic interactions within
the same network, and weaker couplings between the networks whose sign and
magnitude is sensitive to materials parameters. Ca14MnBi11 is calculated to be
ferromagnetic as observed, while for Ba14MnBi11 (which is antiferromagnetic)
the ferro- and antiferromagnetic states are calculated to be essentially
degenerate. The band structure of the ferromagnetic states is very close to
half metallic.Comment: 17 pages, containing 10 postscript figures and 5 tables. Two
additional figures (Fig.8 and 11 of the paper) are provided in JPG format in
separate files. Submitted to Phys. Rev. B on September 20th 200
H-ATLAS/GAMA: magnification bias tomography. Astrophysical constraints above ~1 arcmin
An unambiguous manifestation of the magnification bias is the cross-correlation between two source samples with non-overlapping redshift distributions. In this work we measure and study the cross-correlation signal between a foreground sample of GAMA galaxies with spectroscopic redshifts in the range 0.2<z<0.8, and a background sample of H-ATLAS galaxies with photometric redshifts gsim1.2. It constitutes a substantial improvement over the cross-correlation measurements made by Gonzalez-Nuevo et al. (2014) with updated catalogues and wider area (with S/Ngsim 5 below 10 arcmin and reaching S/N~ 20 below 30 arcsec). The better statistics allow us to split the sample in different redshift bins and to perform a tomographic analysis (with S/Ngsim 3 below 10 arcmin and reaching S/N~ 15 below 30 arcsec). Moreover, we implement a halo model to extract astrophysical information about the background galaxies and the deflectors that are producing the lensing link between the foreground (lenses) and background (sources) samples. In the case of the sources, we find typical mass values in agreement with previous studies: a minimum halo mass to host a central galaxy, Mmin~ 1012.26 Mâ, and a pivot halo mass to have at least one sub-halo satellite, M1~ 1012.84 Mâ. However, the lenses are massive galaxies or even galaxy groups/clusters, with minimum mass of Mminlens~ 1013.06 Mâ. Above a mass of M1lens~ 1014.57 Mâ they contain at least one additional satellite galaxy which contributes to the lensing effect. The tomographic analysis shows that, while M1lens is almost redshift independent, there is a clear evolution of increase Mminlens with redshift in agreement with theoretical estimations. Finally, the halo modeling allows us to identify a strong lensing contribution to the cross-correlation for angular scales below 30 arcsec. This interpretation is supported by the results of basic but effective simulations
Toward an internally consistent astronomical distance scale
Accurate astronomical distance determination is crucial for all fields in
astrophysics, from Galactic to cosmological scales. Despite, or perhaps because
of, significant efforts to determine accurate distances, using a wide range of
methods, tracers, and techniques, an internally consistent astronomical
distance framework has not yet been established. We review current efforts to
homogenize the Local Group's distance framework, with particular emphasis on
the potential of RR Lyrae stars as distance indicators, and attempt to extend
this in an internally consistent manner to cosmological distances. Calibration
based on Type Ia supernovae and distance determinations based on gravitational
lensing represent particularly promising approaches. We provide a positive
outlook to improvements to the status quo expected from future surveys,
missions, and facilities. Astronomical distance determination has clearly
reached maturity and near-consistency.Comment: Review article, 59 pages (4 figures); Space Science Reviews, in press
(chapter 8 of a special collection resulting from the May 2016 ISSI-BJ
workshop on Astronomical Distance Determination in the Space Age
Lens Models of Herschel-selected Galaxies from High-resolution Near-IR Observations
We present Keck-Adaptive Optics and Hubble Space Telescope high resolution near-infrared (IR)
imaging for 500 m-bright candidate lensing systems identi ed by the Herschel Multi-tiered Extragalactic
Survey (HerMES) and Herschel Astrophysical Terahertz Survey (H-ATLAS). Out of 87 candidates
with near-IR imaging, 15 ( 17%) display clear near-IR lensing morphologies. We present
near-IR lens models to reconstruct and recover basic rest-frame optical morphological properties of
the background galaxies from 12 new systems. Sources with the largest near-IR magni cation factors
also tend to be the most compact, consistent with the size bias predicted from simulations and previous
lensing models for sub-millimeter galaxies. For four new sources that also have high-resolution
sub-mm maps, we test for di erential lensing between the stellar and dust components and nd that
the 880 m magni cation factor ( 880) is 1:5 times higher than the near-IR magni cation factor
( NIR), on average. We also nd that the stellar emission is 2 times more extended in size than
dust. The rest-frame optical properties of our sample of Herschel-selected lensed SMGs are consistent
with those of unlensed SMGs, which suggests that the two populations are similar
A Tentative Study on the Annotation of Evidentiality
This article aims at presenting our ongoing work on the construction of a Chinese corpus in which the credibility of textual information is annotated. The linguistic markers of evidentiality in each sentence are identified as cues of credibility. We annotated both the scale and scope of the evidential markers. The annotated corpus can serve as a data basis for the research of information credibility. In this article, we analyze the theoretical underpinnings of our preliminary annotation guideline and the considerations on the choice of texts. Also, we discuss the possible hierarchy of evidentiality annotation. ? 2013 Springer-Verlag.EI
The nature of faint Spitzer-selected dust-obscured galaxies
We use deep far-IR, submillimeter, radio, and X-ray imaging and mid-IR spectroscopy to explore the nature of a sample of Spitzer-selected dust-obscured galaxies (DOGs) in GOODS-N. A sample of 79 galaxies satisfy the criteria R-[24]>14 (Vega) down to S24>100 ÎŒJy (median flux density S24=180 ÎŒJy). Twelve of these galaxies have IRS spectra available, which we use to measure redshifts and classify these objects as being dominated by star formation or active galactic nucleus (AGN) activity in the mid-IR. The IRS spectra and Spitzer photometric redshifts confirm that the DOGs lie in a tight redshift distribution around z~2. Based on mid-IR colors, 80% of DOGs are likely dominated by star formation; the stacked X-ray emission from this subsample of DOGs is also consistent with star formation. Since only a small number of DOGs are individually detected at far-IR and submillimeter wavelengths, we use a stacking analysis to determine the average flux from these objects and plot a composite IR (8-1000 ÎŒm) spectral energy distribution (SED). The average luminosity of these star-forming DOGs is LIR~1Ă1012 Lsolar. We compare the average star-forming DOG to the average bright (S850>5 mJy) submillimeter galaxy (SMG); the S24>100 ÎŒJy DOGs are 3 times more numerous but 8 times less luminous in the IR. The far-IR SED shape of DOGs is similar to that of SMGs (average dust temperature of around 30 K), but DOGs have a higher mid-IR-to-far-IR flux ratio. The average star formation-dominated DOG has a star formation rate of 200 Msolar yr-1, which, given their space density, amounts to a contribution of 0.01 Msolar yr-1 Mpc-3 (or 5%-10%) to the star formation rate density at z~2
Identifying the pairing mechanism in FeâAs based superconductors: gaps and isotope effects
The temperature dependencies of the coupled superconducting gaps, observed in FeâAs based superconducting compounds is calculated and a universal temperature scaling observed which is only present if the coupled order parameters both have s-wave symmetry. Predictions for possible isotope effects on the superconducting transition temperature T c are made if phonons are involved in the pairing or polaronic effects are of importance. Comparison to experimental data is given where these are available