1,095 research outputs found
Neutron resonances in planar waveguides
Results of experimental investigations of a neutron resonances width in
planar waveguides using the time-of-flight reflectometer REMUR of the IBR-2
pulsed reactor are reported and comparison with theoretical calculations is
presented. The intensity of the neutron microbeam emitted from the waveguide
edge was registered as a function of the neutron wavelength and the incident
beam angular divergence. The possible applications of this method for the
investigations of layered nanostructures are discussed
Electronic correlations and crystal structure distortions in BaBiO3
BaBiO3 is a material where formally Bi4+ ions with the half-filled 6s-states
form the alternating set of Bi3+ and Bi5+ ions resulting in a charge ordered
insulator. The charge ordering is accompanied by the breathing distortion of
the BiO6 octahedra (extension and contraction of the Bi-O bond lengths).
Standard Density Functional Theory (DFT) calculations fail to obtain the
crystal structure instability caused by the pure breathing distortions.
Combining effects of the breathing distortions and tilting of the BiO6
octahedra allows DFT to reproduce qualitatively experimentally observed
insulator with monoclinic crystal structure but gives strongly underestimate
breathing distortion parameter and energy gap values. In the present work we
reexamine the BaBiO3 problem within the GGA+U method using a Wannier functions
basis set for the Bi 6s-band. Due to high oxidation state of bismuth in this
material the Bi 6s-symmetry Wannier function is predominantly extended
spatially on surrounding oxygen ions and hence differs strongly from a pure
atomic 6s-orbital. That is in sharp contrast to transition metal oxides (with
exclusion of high oxidation state compounds) where the major part a of d-band
Wannier function is concentrated on metal ion and a pure atomic d-orbital can
serve as a good approximation. The GGA+U calculation results agree well with
experimental data, in particular with experimental crystal structure parameters
and energy gap values. Moreover, the GGA+U method allows one to reproduce the
crystal structure instability due to the pure breathing distortions without
octahedra tilting
Wannier functions and exchange integrals: The example of LiCuO
Starting from a single band Hubbard model in the Wannier function basis, we
revisit the problem of the ligand contribution to exchange and derive explicit
formulae for the exchange integrals in metal oxide compounds in terms of atomic
parameters that can be calculated with constrained LDA and LDA+U. The analysis
is applied to the investigation of the isotropic exchange interactions of
LiCuO, a compound where the Cu-O-Cu angle of the dominant exchange
path is close to 90. Our results show that the magnetic moments are
localized in Wannier orbitals which have strong contribution from oxygen atomic
orbitals, leading to exchange integrals that considerably differ from the
estimates based on kinetic exchange only. Using LSDA+U approach, we also
perform a direct {\it ab-initio} determination of the exchange integrals
LiCuO. The results agree well with those obtained from the Wannier
function approach, a clear indication that this modelization captures the
essential physics of exchange. A comparison with experimental results is also
included, with the conclusion that a very precise determination of the Wannier
function is crucial to reach quantitative estimates.Comment: 8 pages, 8 figure
Transition state method and Wannier functions
We propose a computational scheme for materials where standard Local Density
Approximation (LDA) fails to produce a satisfactory description of excitation
energies. The method uses Slater's "transition state" approximation and Wannier
functions basis set. We define a correction to LDA functional in such a way
that its variation produces one-electron energies for Wannier functions equal
to the energies obtained in "transition state" constrained LDA calculations. In
the result eigenvalues of the proposed functional could be interpreted as
excitation energies of the system under consideration. The method was applied
to MgO, Si, NiO and BaBiO and gave an improved agreement with experimental
data of energy gap values comparing with LDA.Comment: 13 pages, 6 figures, 1 tabl
Spin state of negative charge-transfer material SrCoO3
We employ the combination of the density functional and the dynamical
mean-field theory (LDA+DMFT) to investigate the electronic structure and
magnetic properties of SrCoO3, monocrystal of which were prepared recently. Our
calculations lead to a ferromagnetic metal in agreement with experiment. We
find that, contrary to some suggestions, the local moment in SrCoO3 does not
arise from intermediate spin state, but is a result of coherent superposition
of many different atomic states. We discuss how attribution of magnetic
response to different atomic states in solids with local moments can be
quantified.Comment: 5 pages, 5 figure
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