32 research outputs found
Pressure-Driven Metal-Insulator Transition in Hematite from Dynamical Mean-Field Theory
The Local Density Approximation combined with Dynamical Mean-Field Theory
(LDA+DMFT method) is applied to the study of the paramagnetic and magnetically
ordered phases of hematite FeO as a function of volume. As the volume
is decreased, a simultaneous 1st order insulator-metal and high-spin to
low-spin transition occurs close to the experimental value of the critical
volume. The high-spin insulating phase is destroyed by a progressive reduction
of the charge gap with increasing pressure, upon closing of which the high spin
phase becomes unstable. We conclude that the transition in FeO at
50 GPa can be described as an electronically driven volume collapse.Comment: 5 pages, 4 figure
First principle computation of stripes in cuprates
We present a first principle computation of vertical stripes in
within the LDA+U method. We find that Cu centered
stripes are unstable toward O centered stripes. The metallic core of the stripe
is quite wide and shows reduced magnetic moments with suppressed
antiferromagnetic (AF) interactions. The system can be pictured as alternating
metallic and AF two-leg ladders the latter with strong AF interaction and a
large spin gap. The Fermi surface shows warping due to interstripe
hybridization. The periodicity and amplitude of the warping is in good
agreement with angle resolved photoemission experiment. We discuss the
connection with low-energy theories of the cuprates.Comment: 5 pages,4 figure
Coulomb Parameter U and Correlation Strength in LaFeAsO
First principles constrained density functional theory scheme in Wannier
functions formalism has been used to calculate Coulomb repulsion U and Hund's
exchange J parameters for iron 3d electrons in LaFeAsO. Results strongly depend
on the basis set used in calculations: when O-2p, As-4p, and Fe-3d orbitals and
corresponding bands are included, computation results in U=3-4 eV, however,
with the basis set restricted to Fe-3d orbitals and bands only, computation
gives parameters corresponding to F^0=0.8 eV, J=0.5 eV. LDA+DMFT (the Local
Density Approximation combined with the Dynamical Mean-Field Theory)
calculation with this parameters results in weakly correlated electronic
structure that is in agreement with X-ray experimental spectra
Calculation of the exchange constants of the Heisenberg model in the plane-wave based methods using the Green's function approach
An approach to compute exchange parameters of the Heisenberg model in
plane-wave-based methods is presented. This calculation scheme is based on the
Green's function method and Wannier function projection technique. It was
implemented in the framework of the pseudopotential method and tested on such
materials as NiO, FeO, Li2MnO3, and KCuF3. The obtained exchange constants are
in a good agreement with both the total energy calculations and experimental
estimations for NiO and KCuF3. In the case of FeO our calculations explain the
pressure dependence of the N\'eel temperature. Li2MnO3 turns out to be a Slater
insulator with antiferromagnetic nearest-neighbor exchange defined by the spin
splitting. The proposed approach provides a unique way to analyze magnetic
interactions, since it allows one to calculate orbital contributions to the
total exchange coupling and study the mechanism of the exchange coupling
Spin state transition and covalent bonding in LaCoO3
We use the dynamical mean-field theory to study a p-d Hubbard Hamiltonian for
LaCoO3 derived from ab initio calculations in local density approximation
(LDA+DMFT scheme). We address the origin of local moments observed above 100 K
and discuss their attribution to a particular atomic multiplet in the presence
of covalent Co-O bonding. We show that in solids such attribution, based on the
single ion picture, is in general not possible. We explain when and how the
single ion picture can be generalized to provide a useful approximation in
solids. Our results demonstrate that the apparent magnitude of the local moment
is not necessarily indicative of the underlying atomic multiplet. We conclude
that the local moment behavior in LaCoO3 arises from the high-spin state of Co
and explain the precise meaning of this statement
Coulomb correlation effects in LaOFeAs: LDA+DMFT(QMC) study
Effects of Coulomb correlation on LaOFeAs electronic structure have been
investigated by LDA+DMFT(QMC) method. The calculation results show that LaOFeAs
is in the regime of intermediate correlation strength with significant part of
the spectral density moved from the Fermi energy to Hubbard bands. However the
system is not on the edge of metal insulator-transition because increase of the
Coulomb interaction parameter value from =4.0 eV to =5.0 eV did not
result in insulator state. Correlations affect different d-orbitals not in the
same way. states ( and orbitals) have higher energy
due to crystal filed splitting and are nearly half-filled. Their spectral
functions have pseudogap with Fermi energy position on the higher sub-band
slope. Lower energy set of d-orbitals ( and ) have
significantly larger occupancy values with typically metallic spectral
functions.Comment: 4 pages, 4 figure