449 research outputs found
Aluminophosphate molecular sieves comprised of hydrated triple crankshaft chains
We report the first synthesis of pure aluminophosphate hydrate H2 (AlPO4-H2) and its structure; AlPO4-H2 is constructed exclusively from a hydrated chain building unit that also builds the 18-ring VPI-5 structure and has one-dimensional channels circumscribed by highly elliptical rings consisting often oxygen atoms, implications from the existence of this building unit for the synthesis of novel aluminophosphate molecular sieves and for the synthesis of aluminosilicate and silicate analogues of AlPO4-H2 and VPI-5 are discussed
Electronic structure of PrCaMnO near the Fermi level studied by ultraviolet photoelectron and x-ray absorption spectroscopy
We have investigated the temperature-dependent changes in the near-
occupied and unoccupied states of PrCaMnO which shows the
presence of ferromagnetic and antiferromagnetic phases. The
temperature-dependent changes in the charge and orbital degrees of freedom and
associated changes in the Mn 3 - O 2 hybridization result in varied O
2 contributions to the valence band. A quantitative estimate of the charge
transfer energy () shows a larger value compared to the earlier
reported estimates. The charge localization causing the large is
discussed in terms of different models including the electronic phase
separation.Comment: 19 pages, 7 figures, To be published in Phy. Rev.
Pseudogap Formation in Models for Manganites
The density-of-states (DOS) and one-particle spectral function of the one- and two-orbital models for manganites, the latter with
Jahn-Teller phonons, are evaluated using Monte Carlo techniques. Unexpectedly
robust pseudogap (PG) features were found at low- and
intermediate-temperatures, particularly at or near regimes where
phase-separation occurs as 0. The PG follows the chemical potential
and it is caused by the formation of ferromagnetic metallic clusters in an
insulating background. It is argued that PG formation should be generic of
mixed-phase regimes. The results are in good agreement with recent
photoemission experiments for .Comment: Accepted for publication in Phys. Rev. Lett., 4 pages, Revtex, with 4
figures embedde
Competition between spin exchange and correlated hopping
The ground-state phase diagram is numerically studied for an electronic model
consisting of the spin exchange term (J) and the correlated hopping term (t_3:
the three-site term). This model has no single-particle hopping and the ratio
of the two terms is controlled by a parameter \alpha \equiv 4 t_3 / J. The case
of \alpha=1 corresponds to complete suppression of single-particle hopping in
the strong-coupling limit of the Hubbard model. In one dimension, phase
separation takes place below a critical value \alpha_c = 0.36-0.63 which
depends on the electron density. Spin gap opens in the whole region except the
phase-separated one. For \alpha \gsim 1.2 and low hole densities,
charge-density-wave correlations are the most dominant, whereas singlet-pairing
correlations are the most dominant in the remaining region. The possibility of
superconductivity in the two-dimensional case is also discussed, based on
equal-time pairing correlations.Comment: 4 pages including 5 figures. Proceedings of ISSP-Kashiwa 2001
(submitted to J. Phys. Chem. Solids
Theory for the Interdependence of High-T Superconductivity and Dynamical Spin Fluctuations
The doping dependence of the superconducting state for the 2D one-band
Hubbard Hamiltonian is determined. By using an Eliashberg-type theory, we find
that the gap function has a symmetry in momentum
space and T becomes maximal for doping. Since we determine the
dynamical excitations directly from real frequency axis calculations, we obtain
new structures in the angular resolved density of states related to the
occurrence of {\it shadow states} below T. Explaining the anomalous
behavior of photoemission and tunneling experiments in the cuprates, we find a
strong interplay between -wave superconductivity and dynamical spin
fluctuations.Comment: 4 pages (REVTeX) with 4 figures (Postscript
Collective Modes and the Superconducting State Spectral Function of Bi2212
Photoemission spectra of the high temperature superconductor Bi2212 near
(pi,0) show a dramatic change when cooling below Tc: the broad peak in the
normal state turns into a sharp low energy peak followed by a higher binding
energy hump. Recent experiments find that this low energy peak persists over a
significant range in momentum space. We show in this paper that these data are
well described by a simple model of electrons interacting with a collective
mode which appears only below Tc.Comment: 4 pages, revtex, 4 encapsulated postscript figure
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Structural basis for the membrane fusion step in Hantavirus entry
Hantaviruses are important emerging human pathogens and are the causative agents of serious diseases in humans with high mortality rates. Like other members in the Bunyaviridae family their M segment encodes two glycoproteins, GN and GC, which are responsible for the early events of the viral infection. Hantaviruses deliver their tripartite genome into the cytoplasm by fusion of the viral and endosomal membranes in response to the reduced pH of the endosome. Unlike phleboviruses (e.g. Rift valley fever virus), that have an icosahedral glycoprotein envelope, hantaviruses display a pleomorphic virion morphology as GN and GC assemble into spikes with apparent four- fold symmetry organized in a grid-like pattern on the viral membrane. We determined the crystal structure of glycoprotein C (GC) from Puumala virus (PUUV), a representative member of the Hantavirus genus. The crystal structure shows GC as the membrane fusion effector of PUUV and it presents a class II membrane fusion protein fold. Furthermore, GC was crystallized in its post-fusion trimeric conformation that until now had been observed only in Flavi- and Togaviridae family members. The PUUV G C structure together with our functional data provides new mechanistic insights into class II membrane fusion proteins and reveals new targets for membrane fusion inhibitors against these important pathogens. Both similarities and differences to other class II membrane fusion proteins implies a revise paradigm for the evolution of these unique proteins
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