645 research outputs found
Spin-state transition and spin-polaron physics in cobalt oxide perovskites: ab initio approach based on quantum chemical methods
A fully ab initio scheme based on quantum chemical wavefunction methods is
used to investigate the correlated multiorbital electronic structure of a
3d-metal compound, LaCoO3. The strong short-range electron correlations,
involving both Co and O orbitals, are treated by multireference techniques. The
use of effective parameters like the Hubbard U and interorbital U', J terms and
the problems associated with their explicit calculation are avoided with this
approach. We provide new insight into the spin-state transition at about 90 K
and the nature of charge carriers in the doped material. Our results indicate
the formation of a t4e2 high-spin state in LaCoO3 for T>90 K. Additionally, we
explain the paramagnetic phase in the low-temperature lightly doped compound
through the formation of Zhang-Rice-like O hole states and ferromagnetic
clusters
Octahedral Tilting in ACu3Ru4O12 (A=Na,Ca,Sr,La,Nd)
The perovskite-like compounds ACu3Ru4O12 (A=Na,Ca,Sr,La,Nd) are studied by
means of density functional theory based electronic structure calculations
using the augmented spherical wave (ASW) method. The electronic properties are
strongly influenced by covalent type bonding between transition metal d and
oxygen p states. The characteristic tilting of the RuO6 octahedra arises mainly
from the Cu--O bonding, allowing for optimal bond lengths between these two
atoms. Our results provide a deeper understanding of octahedral tilting as a
universal mechanism, applicable to a large variety of multinary compounds.Comment: 9 pages, 4 eps figure
Orbital ordering and enhanced magnetic frustration of strained BiMnO3 thin films
Epitaxial thin films of multiferroic perovskite BiMnO3 were synthesized on
SrTiO3 substrates, and orbital ordering and magnetic properties of the thin
films were investigated. The ordering of the Mn^{3+} e_g orbitals at a wave
vector (1/4 1/4 1/4) was detected by Mn K-edge resonant x-ray scattering. This
peculiar orbital order inherently contains magnetic frustration. While bulk
BiMnO3 is known to exhibit simple ferromagnetism, the frustration enhanced by
in-plane compressive strains in the films brings about cluster-glass-like
properties.Comment: 8 pages, 4 figures, accepted to Europhysics Letter
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Dark Matter Annihilation in The Galactic Center As Seen by the Fermi Gamma Ray Space Telescope
We analyze the first two years of data from the Fermi Gamma Ray Space Telescope from the direction of the inner 10{sup o} around the Galactic Center with the intention of constraining, or finding evidence of, annihilating dark matter. We find that the morphology and spectrum of the emission between 1.25{sup o} and 10{sup o} from the Galactic Center is well described by a the processes of decaying pions produced in cosmic ray collisions with gas, and the inverse Compton scattering of cosmic ray electrons in both the disk and bulge of the Inner Galaxy, along with gamma rays from known points sources in the region. The observed spectrum and morphology of the emission within approximately 1.25{sup o} ({approx}175 parsecs) of the Galactic Center, in contrast, cannot be accounted for by these processes or known sources. We find that an additional component of gamma ray emission is clearly present which is highly concentrated around the Galactic Center, but is not point-like in nature. The observed morphology of this component is consistent with that predicted from annihilating dark matter with a cusped (and possibly adiabatically contracted) halo distribution ({rho} {proportional_to} r{sup -1.34{+-}0.04}). The observed spectrum of this component, which peaks at energies between 2-4 GeV (in E{sup 2} units), is well fit by that predicted for a 7.3-9.2 GeV dark matter particle annihilating primarily to tau leptons with a cross section in the range of <{sigma}{nu}> = 3.3 x 10{sup -27} to 1.5 x 10{sup -26} cm{sup 3}/s, depending on how the dark matter distribution is normalized. We discuss other possible sources for this component, but argue that they are unlikely to account for the observed emission
Strong anisotropy of superexchange in the copper-oxygen chains of La_{14-x}Ca_{x}Cu_{24}O_{41}
Electron spin resonance data of Cu^{2+} ions in La_{14-x}Ca_{x}Cu_{24}O_{41}
crystals (x=9,11,12) reveal a very large width of the resonance line in the
paramagnetic state. This signals an unusually strong anisotropy of ~10% of the
isotropic Heisenberg superexchange in the Cu-O chains of this compound. The
strong anisotropy can be explained by the specific geometry of two symmetrical
90 degree Cu-O-Cu bonds, which boosts the importance of orbital degrees of
freedom. Our data show the apparent limitations of the applicability of an
isotropic Heisenberg model to the low dimensional cuprates.Comment: 14 pages, 3 figures included, to be published in Phys. Rev. Let
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