7,267 research outputs found
The electronic structure of the NaCoO surface
The idea that surface effects may play an important role in suppressing
Fermi surface pockets on NaCoO has been
frequently proposed to explain the discrepancy between LDA calculations
(performed on the bulk compound) which find ' hole pockets present and
ARPES experiments, which do not observe the hole pockets. Since ARPES is a
surface sensitive technique it is important to investigate the effects that
surface formation will have on the electronic structure of NaCoO in
order to more accurately compare theory and experiment. We have calculated the
band structure and Fermi surface of cleaved NaCoO and determined
that the surface non-trivially affects the fermiology in comparison to the
bulk. Additionally, we examine the likelihood of possible hydroxyl cotamination
and surface termination. Our results show that a combination of surface
formation and contamination effects could resolve the ongoing controversy
between ARPES experiments and theory.Comment: 4 pages, 2 figure
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
Effect of doping and pressure on magnetism and lattice structure of Fe-based superconductors
Using first principles calculations, we analyze structural and magnetic
trends as a function of charge doping and pressure in BaFeAs, and
compare to experimentally established facts. We find that density functional
theory, while accurately reproducing the structural and magnetic ordering at
ambient pressure, fails to reproduce some structural trends as pressure is
increased. Most notably, the Fe-As bondlength which is a gauge of the magnitude
of the magnetic moment, , is rigid in experiment, but soft in calculation,
indicating residual local Coulomb interactions. By calculating the magnitude of
the magnetic ordering energy, we show that the disruption of magnetic order as
a function of pressure or doping can be qualitatively reproduced, but that in
calculation, it is achieved through diminishment of , and therefore
likely does not reflect the same physics as detected in experiment. We also
find that the strength of the stripe order as a function of doping is strongly
site-dependent: magnetism decreases monotonically with the number of electrons
doped at the Fe site, but increases monotonically with the number of electrons
doped at the Ba site. Intra-planar magnetic ordering energy (the difference
between checkerboard and stripe orderings) and interplanar coupling both follow
a similar trend. We also investigate the evolution of the orthorhombic
distortion, as a function of , and find that in the
regime where experiment finds a linear relationship, our calculations are
impossible to converge, indicating that in density functional theory, the
transition is first order, signalling anomalously large higher order terms in
the Landau functional
Formation of an unconventional Ag valence state in Ag2NiO2
The Ag ion in the recently synthesized novel material Ag2NiO2 adopts an
extremely unusual valency of 1/2, leaving the Ni ion as 3+, rather than the
expected 2+. Using first principles calculations, we show that this mysterious
subvalent state emerges due to a strong bonding-antibonding interaction between
the two Ag layers which drives the lower band beneath the O p complex,
eliminating the possibility of a conventional Ag 1+ valence state. The strong
renormalization of the specific heat coefficient, gamma, is likely due to
strong spin fluctuations that stem from nearly complete compensation of the
ferro- (metallic double exchange and the 90 degree superexchange) and
antiferromagnetic (conventional superexchange via Ni-O-Ag-O-Ni path)
interactions
The challenge of unravelling magnetic properties in LaFeAsO
First principles calculations of magnetic and, to a lesser extent, electronic
properties of the novel LaFeAsO-based superconductors show substantial apparent
controversy, as opposed to most weakly or strongly correlated materials. Not
only do different reports disagree about quantitative values, there is also a
schism in terms of interpreting the basic physics of the magnetic interactions
in this system. In this paper, we present a systematic analysis using four
different first principles methods and show that while there is an unusual
sensitivity to computational details, well-converged full-potential
all-electron results are fully consistent among themselves. What makes results
so sensitive and the system so different from simple local magnetic moments
interacting via basic superexchange mechanisms is the itinerant character of
the calculated magnetic ground state, where very soft magnetic moments and
long-range interactions are characterized by a particular structure in the
reciprocal (as opposed to real) space. Therefore, unravelling the magnetic
interactions in their full richness remains a challenging, but utterly
important task
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