2,216 research outputs found
Calculations of the thermodynamic and kinetic properties of LiV3O8
The phase behavior and kinetic pathways of Li1+xV3O8 are investigated by
means of density functional theory (DFT) and a cluster expansion (CE)
methodology that approximates the system Hamiltonian in order to identify the
lowest energy configurations. Although DFT calculations predict the correct
ground state for a given composition, both GGA and LDA fail to obtain phase
stability consistent with experiment due to strongly localized vanadium 3d
electrons. A DFT+U methodology recovers the correct phase stability for an
optimized U value of 3.0eV. GGA+U calculations with this value of U predict
electronic structures that qualitatively agree with experiment. The resulting
calculations indicate solid solution behavior from LiV3O8 to Li2.5V3O8 and
two-phase coexistence between Li2.5V3O8 and Li4V3O8. Analysis of the lithiation
sequence from LiV3O8 to Li2.5V3O8 reveals the mechanism by which lithium
intercalation proceeds in this material. Calculations of lithium migration
energies for different lithium concentrations and configurations provides
insight into the relevant diffusion pathways and their relationship to
structural properties
Exact Kohn-Sham potential of strongly correlated finite systems
The dissociation of molecules, even the most simple hydrogen molecule, cannot
be described accurately within density functional theory because none of the
currently available functionals accounts for strong on-site correlation. This
problem has led to a discussion of properties that the local Kohn-Sham
potential has to satisfy in order to correctly describe strongly correlated
systems. We derive an analytic expression for this potential at the
dissociation limit and show that the numerical calculations for a
one-dimensional two electron model system indeed approach and reach this limit.
It is shown that the functional form of the potential is universal, i.e.
independent of the details of the system.Comment: 17 pages, 3 figures, submitted to JC
Orbital selective Mott transition in multi-band systems: slave-spin representation and dynamical mean-field theory
We examine whether the Mott transition of a half-filled, two-orbital Hubbard
model with unequal bandwidths occurs simultaneously for both bands or whether
it is a two-stage process in which the orbital with narrower bandwith localizes
first (giving rise to an intermediate `orbital-selective' Mott phase). This
question is addressed using both dynamical mean-field theory, and a
representation of fermion operators in terms of slave quantum spins, followed
by a mean-field approximation (similar in spirit to a Gutzwiller
approximation). In the latter approach, the Mott transition is found to be
orbital-selective for all values of the Coulomb exchange (Hund) coupling J when
the bandwidth ratio is small, and only beyond a critical value of J when the
bandwidth ratio is larger. Dynamical mean-field theory partially confirms these
findings, but the intermediate phase at J=0 is found to differ from a
conventional Mott insulator, with spectral weight extending down to arbitrary
low energy. Finally, the orbital-selective Mott phase is found, at
zero-temperature, to be unstable with respect to an inter-orbital
hybridization, and replaced by a state with a large effective mass (and a low
quasiparticle coherence scale) for the narrower band.Comment: Discussion on the effect of hybridization on the OSMT has been
extende
LDA+DMFT Spectral Functions and Effective Electron Mass Enhancement in Superconductor LaFePO
In this Letter we report the first LDA+DMFT results (method combining Local
Density Approximation with Dynamical Mean-Field Theory) for spectral properties
of superconductor LaFePO. Calculated {\bf k}-resolved spectral functions
reproduce recent angle-resolved photoemission spectroscopy (ARPES) data [D. H.
Lu {\it et al}., Nature {\bf 455}, 81 (2008)]. Obtained effective electron mass
enhancement values 1.9 -- 2.2 are in good agreement with
infrared and optical studies [M. M. Qazilbash {\it et al}., Nature Phys. {\bf
5}, 647 (2009)], de Haas--van Alphen, electrical resistivity, and electronic
specific heat measurements results, that unambiguously evidence for moderate
correlations strength in LaFePO. Similar values of were found in the
other Fe-based superconductors with substantially different superconducting
transition temperatures. Thus, the dynamical correlation effects are essential
in the Fe-based superconductors, but the strength of electronic correlations
does not determine the value of superconducting transition temperature.Comment: 4 pages, 3 figure
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
The influence of the rare earth ions radii on the Low Spin to Intermediate Spin state transition in lanthanide cobaltite perovskites: LaCoO3 vs. HoCoO3
We present first principles LDA+U calculations of electronic structure and
magnetic state for LaCoO3 and HoCoO3. Low Spin to Intermediate Spin state
transition was found in our calculations using experimental crystallographic
data for both materials with a much higher transition temperature for HoCoO3,
which agrees well with the experimental estimations. Low Spin state t6e0
(non-magnetic) to Intermediate Spin state t5e1 (magnetic) transition of Co(3+)
ions happens due to the competition between crystal field t_2g-e_g splitting
and effective exchange interaction between 3 spin-orbitals. We show that the
difference in crystal structure parameters for HoCoO3 and LaCoO3 due to the
smaller ionic radius of Ho ion comparing with La ion results in stronger
crystal field splitting for HoCoO3 (0.09 eV ~ 1000 K larger than for LaCoO3)
and hence tip the balance between the Low Spin and Intermediate Spin states to
the non-magnetic solution in HoCoO3.Comment: 13 pages, 6 figure
Orbital-spin order and the origin of structural distortion in MgTiO
We analyze electronic, magnetic, and structural properties of the spinel
compound MgTiO using the local density approximation+U method. We show
how MgTiO undergoes to a canted orbital-spin ordered state, where
charge, spin and orbital degrees of freedom are frozen in a geometrically
frustrated network by electron interactions. In our picture orbital order
stabilize the magnetic ground state and controls the degree of structural
distortions. The latter is dynamically derived from the cubic structure in the
correlated LDA+U potential. Our ground-state theory provides a consistent
picture for the dimerized phase of MgTiO, and might be applicable to
frustrated materials in general.Comment: 6 pages, 6 figure
Orbital Selective Pressure-Driven Metal-Insulator Transition in FeO from Dynamical Mean-Field Theory
In this Letter we report the first LDA+DMFT (method combining Local Density
Approximation with Dynamical Mean-Field Theory) results of magnetic and
spectral properties calculation for paramagnetic phases of FeO at ambient and
high pressures (HP). At ambient pressure (AP) calculation gave FeO as a Mott
insulator with Fe 3-shell in high-spin state. Calculated spectral functions
are in a good agreement with experimental PES and IPES data. Experimentally
observed metal-insulator transition at high pressure is successfully reproduced
in calculations. In contrast to MnO and FeO ( configuration) where
metal-insulator transition is accompanied by high-spin to low-spin transition,
in FeO ( configuration) average value of magnetic moment
is nearly the same in the insulating phase at AP and
metallic phase at HP in agreement with X-Ray spectroscopy data (Phys. Rev.
Lett. {\bf83}, 4101 (1999)). The metal-insulator transition is orbital
selective with only orbitals demonstrating spectral function typical
for strongly correlated metal (well pronounced Hubbard bands and narrow
quasiparticle peak) while states remain insulating.Comment: 4 pages, 4 figure
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