1,520 research outputs found
First-Principles Computation of YVO3; Combining Path-Integral Renormalization Group with Density-Functional Approach
We investigate the electronic structure of the transition-metal oxide YVO3 by
a hybrid first-principles scheme. The density-functional theory with the
local-density-approximation by using the local muffin-tin orbital basis is
applied to derive the whole band structure. The electron degrees of freedom far
from the Fermi level are eliminated by a downfolding procedure leaving only the
V 3d t2g Wannier band as the low-energy degrees of freedom, for which a
low-energy effective model is constructed. This low-energy effective
Hamiltonian is solved exactly by the path-integral renormalization group
method. It is shown that the ground state has the G-type spin and the C-type
orbital ordering in agreement with experimental indications. The indirect
charge gap is estimated to be around 0.7 eV, which prominently improves the
previous estimates by other conventional methods
The Size of the Radio-Emitting Region in Low-luminosity Active Galactic Nuclei
We have used the VLA to study radio variability among a sample of 18 low
luminosity active galactic nuclei (LLAGNs), on time scales of a few hours to 10
days. The goal was to measure or limit the sizes of the LLAGN radio-emitting
regions, in order to use the size measurements as input to models of the radio
emission mechanisms in LLAGNs. We detect variability on typical time scales of
a few days, at a confidence level of 99%, in half of the target galaxies.
Either variability that is intrinsic to the radio emitting regions, or that is
caused by scintillation in the Galactic interstellar medium, is consistent with
the data. For either interpretation, the brightness temperature of the emission
is below the inverse-Compton limit for all of our LLAGNs, and has a mean value
of about 1E10 K. The variability measurements plus VLBI upper limits imply that
the typical angular size of the LLAGN radio cores at 8.5 GHz is 0.2
milliarcseconds, plus or minus a factor of two. The ~ 1E10 K brightness
temperature strongly suggests that a population of high-energy nonthermal
electrons must be present, in addition to a hypothesized thermal population in
an accretion flow, in order to produce the observed radio emission.Comment: 61 pages, 17 figures, 10 tables. Accepted for publication in the
Astrophysical Journa
Lattice Distortion and Magnetic Ground State of YTiO and LaTiO
Effects of lattice distortion on the magnetic ground state of YTiO and
LaiO are investigated on the basis accurate tight-binding parametrization
of the electronic structure extracted from the local-density
approximation. The complexity of these compounds is related with the fact that
the -level splitting, caused by lattice distortions, is comparable with
the energies of superexchange and spin-orbit interactions. Therefore, all these
interactions are equally important and should be treated on an equal footing.
The Hartree-Fock approximation fails to provide a coherent description
simultaneously for YTiO and LaTiO, and it is essential to go beyond.Comment: 4 pages, 3 figures (good quality figures are available via e-mail
An Essential Role for Thymic Mesenchyme in Early T Cell Development
We show that the mesenchymal cells that surround the 12-d mouse embryo thymus are necessary for T cell differentiation. Thus, epithelial lobes with attached mesenchyme generate all T cell populations in vitro, whereas lobes from which mesenchyme has been removed show poor lymphopoiesis with few cells progressing beyond the CD4−CD8− stage of development. Interestingly, thymic mesenchyme is derived from neural crest cells, and extirpation of the region of the neural crest involved results in impaired thymic development and craniofacial abnormalities similar to the group of clinical defects found in the DiGeorge syndrome
Development of a Time Projection Chamber Using Gas Electron Multipliers (GEM-TPC)
We developed a prototype time projection chamber using gas electron
multipliers (GEM-TPC) for high energy heavy ion collision experiments. To
investigate its performance, we conducted a beam test with 3 kinds of gases
(Ar(90%)-CH4(10%), Ar(70%)-C2H6(30%) and CF4). Detection efficiency of 99%, and
spatial resolution of 79 m in the pad-row direction and 313 m in the
drift direction were achieved. The test results show that the GEM-TPC meets the
requirements for high energy heavy ion collision experiments. The configuration
and performance of the GEM-TPC are described.Comment: 18 pages, 12 figures, published online in Nucl. Instr. and Meth.
Estimates of electronic interaction parameters for LaO compounds (=Ti-Ni) from ab-initio approaches
We have analyzed the ab-initio local density approximation band structure
calculations for the family of perovskite oxides, LaO with =Ti-Ni
within a parametrized nearest neighbor tight-binding model and extracted
various interaction strengths. We study the systematics in these interaction
parameters across the transition metal series and discuss the relevance of
these in a many-body description of these oxides. The results obtained here
compare well with estimates of these parameters obtained via analysis of
electron spectroscopic results in conjunction with the Anderson impurity model.
The dependence of the hopping interaction strength, t, is found to be
approximately .Comment: 18 pages; 1 tex file+9 postscript files (appeared in Phys Rev B Oct
15,1996
Ferromagnetic zigzag chains and properties of the charge ordered perovskite manganites
The low-temperature properties of the so-called ''charge ordered'' state in
50% doped perovskite manganites are described from the viewpoint of the
magnetic spin ordering. In these systems, the zigzag antiferromagnetic
ordering, combined with the double-exchange physics, effectively divides the
whole sample into the one-dimensional ferromagnetic zigzag chains and results
in the anisotropy of electronic properties. The electronic structure of one
such chain is described by an effective 33 Hamiltonian in the basis of
Mn() orbitals. We treat this problem analytically and consider the
following properties: (i) the nearest-neighbor magnetic interactions; (ii) the
distribution of the Mn() and Mn() states near the Fermi level, and
their contribution to the optical conductivity and the resonant x-ray
scattering near the Mn -absorption edge. We argue that the anisotropy of
magnetic interactions in the double-exchange limit, combined with the isotropic
superexchange interactions, readily explains both the local and the global
stability of the zigzag antiferromagnetic state. The two-fold degeneracy of
levels plays a very important role in the problem and explains the
insulating behavior of the zigzag chain, as well as the appearance of the
orbital ordering in the double-exchange model. Importantly, however, the charge
ordering itself is expected to play only a minor role and is incompatible with
the ferromagnetic coupling within the chain. We also discuss possible effects
of the Jahn-Teller distortion and compare the tight-binding picture with
results of band structure calculations in the local-spin-density approximation.Comment: 35 pages, 8 figure
Crumbs 3b promotes tight junctions in an ezrin-dependent manner in mammalian cells
AMT-L is supported by the School of Biology, University of St Andrews. AMT-L, PAR and FJGM were funded by the Anonymous Trust, University of St Andrews. PAR is supported by the Melville Trust for the Care and Cure of Cancer. The mass spectrometry work was supported by the Wellcome Trust [grant number 094476/Z/10/Z], which funded the purchase of the TripleTOF 5600 mass spectrometer at the BSRC Mass Spectrometry and Proteomics Facility, University of St Andrews. The clinical study was supported by the Department of Pathology, Albert Einstein College of Medicine/ Montefiore Medical Center.Crumbs3 (CRB3) is a component of epithelial junctions that has been implicated in apical-basal polarity, apical identity, apical stability, cell adhesion and cell growth. CRB3 undergoes alternative splicing to yield two variants: CRB3a and CRB3b. Here, we describe novel data demonstrating that as with previous studies on CRB3a, CRB3b also promotes the formation of tight junctions. However, significantly we demonstrate that the 4.1-ezrin-radixin-moesin (FERM) binding motif (FBM) of CRB3b is required for CRB3b functionality and that ezrin binds to the FBM of CRB3b. Furthermore, we show that ezrin contributes to CRB3b functionality and the correct distribution of tight junction proteins. We demonstrate that both CRB3 isoforms are required for the production of functionally mature tight junctions and also the localization of ezrin to the plasma membrane. Finally, we demonstrate that reduced CRB3b expression in head and neck squamous cell carcinoma (HNSCC) correlates with cytoplasmic ezrin, a biomarker for aggressive disease, and show evidence that whilst CRB3a expression has no effect, low CRB3b and high cytoplasmic ezrin expression combined may be prognostic for HNSCC.PostprintPeer reviewe
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