1,261 research outputs found
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
Toward one-band superconductivity in MgB2
The two-gap model for superconductivity in MgB2 predicts that interband
impurity scattering should be pair breaking, reducing the critical temperature.
This is perhaps the only prediction of the model that has not been confirmed
experimentally. It was previously shown theoretically that common
substitutional impurities lead to negligible interband scattering - if the
lattice is assumed not to distort. Here we report theoretical results showing
that certain impurities can indeed produce lattice distortions sufficiently
large to create measurable interband scattering. On this basis, we predict that
isoelectronic codoping with Al and Na will provide a decisive test of the
two-gap model.Comment: 4 pages, 2 figures, to appear in Phys. Rev.
Electronic structure and magnetism in the frustrated antiferromagnet LiCrO2
LiCrO2 is a 2D triangular antiferromagnet, isostructural with the common
battery material LiCoO2 and a well-known Jahn-Teller antiferromagnet NaNiO2. As
opposed to the latter, LiCrO2 exibits antiferromagnetic exchange in Cr planes,
which has been ascribed to direct Cr-Cr d-d overlap. Using LDA and LDA+U first
principles calculations I confirm this conjecture and show that (a) direct d-d
overlap is indeed enhanced compared to isostructural Ni and Cr compounds, (b)
p-d charge transfer gap is also enhanced, thus suppressing the ferromagnetic
superexchange, (c) the calculated magnetic Hamiltonian maps well onto the
nearest neighbors Heisenberg exchange model and (d) interplanar inteaction is
antiferromagnetic.Comment: 5 pages, 4 figure
Accounting for spin fluctuations beyond LSDA in the density functional theory
We present a method to correct the magnetic properties of itinerant systems
in local spin density approximation (LSDA) and we apply it to the
ferromagnetic-paramagnetic transition under pressure in a typical itinerant
system, NiAl. We obtain a scaling of the critical fluctuations as a
function of pressure equivalent to the one obtained within Moryia's theory.
Moreover we show that in this material the role of the bandstructure is crucial
in driving the transition. Finally we calculate the magnetic moment as a
function of pressure, and find that it gives a scaling of the Curie temperature
that is in good agreement with the experiment. The method can be easily
extended to the antiferromagnetic case and applied, for instance, to the
Fe-pnictides in order to correct the LSDA magnetic moment.Comment: 7 pages, 4 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
Spin-orbit driven Peierls transition and possible exotic superconductivity in CsWO
We study \textit{ab initio} a pyrochlore compound, CsWO, which
exhibits a yet unexplained metal-insulator transition. We find that (1) the
reported low- structure is likely inaccurate and the correct structure has a
twice larger cell; (2) the insulating phase is not of a Mott or dimer-singlet
nature, but a rare example of a 3D Peierls transition, with a simultaneous
condensation of three density waves; (3) spin-orbit interaction plays a crucial
role, forming well-nested bands. The high- (HT) phase, if stabilized, could
harbor a unique superconducting state that breaks the time
reversal symmetry, but is not chiral. This state was predicted in 1999, but
never observed. We speculate about possible ways to stabilize the HT phase
while keeping the conditions for superconductivity
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
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