194 research outputs found
Correlated electronic structure, orbital-dependent correlations, and Lifshitz transition in tetragonal FeS
Using density functional plus dynamical mean-field theory method (DFT+DMFT)
with full self-consistency over the charge density, we study the effect of
electronic correlations on the electronic structure, magnetic properties,
orbital-dependent band renormalizations, and Fermi surface of the tetragonal
phase of bulk FeS. We perform a direct structural optimization of the
crystal structure of paramagnetic FeS, with respect to the lattice constant
and the internal coordinate of atom S. Our results show an
anomalous sensitivity of the electronic structure and magnetic properties of
FeS to fine details of its crystals structure. Upon expansion of the lattice
volume, we observe a remarkable change of the electronic structure of FeS which
is associated with a complete reconstruction of the Fermi surface topology
(Lifshitz transition). This behavior is ascribed to a correlation-induced shift
of the Van Hove singularity associated with the Fe orbitals at the
point across the Fermi level. The Lifshitz phase transition is accompanied by a
significant growth of local magnetic moments and emergence of strong
orbital-selective correlations. It is seen as a pronounced anomaly (`kink') in
the total energies upon expansion of the lattice, associated with a remarkable
enhancement of compressibility. This behavior is accompanied by an
orbital-dependent formation of local moments, a crossover from itinerant to
localized orbital-selective moment behavior of the Fe electrons. While
exhibiting weak effective mass enhancement of the Fe states , correlation effects reveal a strong impact on a position of the Van
Hove singularity at the point, implying a complex interplay between
electronic correlations and band structure effects in FeS
Correlation strength, Lifshitz transition and the emergence of a two- to three-dimensional crossover in FeSe under pressure
We report a detailed theoretical study of the electronic structure, spectral
properties, and lattice parameters of bulk FeSe under pressure using a fully
charge self-consistent implementation of the density functional theory plus
dynamical mean-field theory method (DFT+DMFT). In particular, we perform a
structural optimization and compute the evolution of the lattice parameters
(volume, ratio, and the internal position of Se) and the electronic
structure of the tetragonal (space group ) paramagnetic FeSe. Our
results for the lattice parameters are in good quantitative agreement with
experiment. The ratio is slightly overestimated by about ~\%,
presumably due to the absence of the van der Waals interactions between the
FeSe layers in our calculations. The lattice parameters determined within DFT
are off the experimental values by a remarkable -~\%, implying a
crucial importance of electron correlations. Upon compression to ~GPa, the
ratio and the lattice volume show a decrease by and ~\%,
respectively, while the Se coordinate weakly increases by ~\%.
Most importantly, our results reveal a topological change of the Fermi surface
(Lifshitz transition) which is accompanied by a two- to three-dimensional
crossover. Our results indicate a small reduction of the quasiparticle mass
renormalization by about ~\% for the and less than ~\% for
the states, as compared to ambient pressure. The behavior of the
momentum-resolved magnetic susceptibility shows no topological
changes of magnetic correlations under pressure, but demonstrates a reduction
of the degree of the in-plane stripe-type nesting. Our results for
the electronic structure and lattice parameters of FeSe are in good qualitative
agreement with recent experiments on its isoelectronic counterpart
FeSeS.Comment: 10 pages, 6 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
Differential Form of the Skornyakov--Ter-Martirosyan Equations
The Skornyakov--Ter-Martirosyan three-boson integral equations in momentum
space are transformed into differential equations. This allows us to take into
account quite directly the Danilov condition providing self-adjointness of the
underlying three-body Hamiltonian with zero-range pair interactions. For the
helium trimer the numerical solutions of the resulting differential equations
are compared with those of the Faddeev-type AGS equations.Comment: 4 pages, 2 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
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
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