10,470 research outputs found
Band structure and atomic sum rules for x-ray dichroism
Corrections to the atomic orbital sum rule for circular magnetic x-ray
dichroism in solids are derived using orthonormal LMTOs as a single-particle
basis for electron band states.Comment: 7 pages, no figure
Orbital fluctuations in the different phases of LaVO3 and YVO3
We investigate the importance of quantum orbital fluctuations in the
orthorhombic and monoclinic phases of the Mott insulators LaVO3 and YVO3.
First, we construct ab-initio material-specific t2g Hubbard models. Then, by
using dynamical mean-field theory, we calculate the spectral matrix as a
function of temperature. Our Hubbard bands and Mott gaps are in very good
agreement with spectroscopy. We show that in orthorhombic LaVO3, quantum
orbital fluctuations are strong and that they are suppressed only in the
monoclinic 140 K phase. In YVO3 the suppression happens already at 300 K. We
show that Jahn-Teller and GdFeO3-type distortions are both crucial in
determining the type of orbital and magnetic order in the low temperature
phases.Comment: 4 pages, 3 figures, final version. To appear in PR
Insights from ARPES for an undoped, four-layered, two-gap high-T_c superconductor
An undoped cuprate with apical fluorine and inner (i) and outer (o)
CuO2-layers is a 60 K superconductor whose Fermi surface (FS) has large n- and
p-doped sheets with the SC gap on the n-sheet twice that on the p -sheet (Y.
Chen et al.). The Fermi surface is not reproduced by the LDA, but the screening
must be substantially reduced due to electronic correlations, and oxygen in the
o-layers must be allowed to dimple outwards. This charges the i-layers by
0.01|e|, causes an 0.4 eV Madelung-potential difference between the i and o
-layers, quenches the i-o hopping, and localizes the n-sheets onto the
i-layers, thus protecting their d-wave pairs from being broken by scattering on
impurities in the BaF layers. The correlation-reduced screening strengthens the
coupling to z-axis phonons.Comment: 4 pages, 3 figure
Electron-phonon interaction in the t-J model
We derive a t-J model with electron-phonon coupling from the three-band
model, considering modulation of both hopping and Coulomb integrals by phonons.
While the modulation of the hopping integrals dominates, the modulation of the
Coulomb integrals cannot be neglected. The model explains the experimentally
observed anomalous softening of the half-breathing mode upon doping and a
weaker softening of the breathing mode. It is shown that other phonons are not
strongly influenced, and, in particular, the coupling to a buckling mode is not
strong in this model.Comment: 4 pages, RevTeX, 3 eps figures; final version with minor correction
The origin of a and e' orderings in NaCoO
It has often been suggested that correlation effects suppress the small e_g'
Fermi surface pockets of NaxCoO_2 that are predicted by LDA, but absent in
ARPES measurements. It appears that within the dynamical mean field theory
(DMFT) the ARPES can be reproduced only if the on-site energy of the eg'
complex is lower than that of the a1g complex at the one-electron level, prior
to the addition of local correlation effects. Current estimates regarding the
order of the two orbital complexes range from -200 meV to 315 meV in therms of
the energy difference. In this work, we perform density functional theory
calculations of this one-electron splitting \Delta= \epsilon_a1g-\epsilon_e_g'
for the full two-layer compound, Na2xCo2O4, accounting for the effects of Na
ordering, interplanar interactions and octahedral distortion. We find that
\epsilon a_1g-\epsilon e_g' is negative for all Na fillings and that this is
primarily due to the strongly positive Coulomb field created by Na+ ions in the
intercalant plane. This field disproportionately affects the a_1g orbital which
protrudes farther upward from the Co plane than the e_g' orbitals. We discuss
also the secondary effects of octahedral compression and multi-orbital filling
on the value of \Delta as a function of Na content. Our results indicate that
if the e_g' pockets are indeed suppressed that can only be due to nonlocal
correlation effects beyond the standard DMFT.Comment: 4 pages, 3 figure
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