2,231 research outputs found
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
Theory of optically forbidden d-d transitions in strongly correlated crystals
A general multiband formulation of linear and non-linear optical response
functions for realistic models of correlated crystals is presented. Dipole
forbidden d-d optical transitions originate from the vertex functions, which we
consider assuming locality of irreducible four-leg vertex. The unified
formulation for second- and third-order response functions in terms of the
three-leg vertex is suitable for practical calculations in solids. We
illustrate the general approach by consideration of intraatomic spin-flip
contributions, with the energy of 2J, where J is a Hund exchange, in the
simplest two-orbital model.Comment: 9 pages, 4 figures, to appear in J. Phys. Cond. Matte
Nonlocal correlations in the vicinity of the - phase transition in iron within a DMFT plus spin-fermion model approach
We consider nonlocal correlations in iron in the vicinity of the
- phase transition within the spin-rotationally-invariant
dynamical mean-field theory (DMFT) approach, combined with the recently
proposed spin-fermion model of iron. The obtained nonlocal corrections to DMFT
yield a decrease of the Curie temperature of the phase, leading to an
agreement with its experimental value. We show that the corresponding nonlocal
corrections to the energy of the phase are crucially important to
obtain the proximity of energies of and phases in the
vicinity of the iron - transformation.Comment: 5 pages, 2 figure
Effect of density of states peculiarities on Hund's metal behavior
We investigate a possibility of Hund's metal behavior in the Hubbard model
with asymmetric density of states having peak(s). Specifically, we consider the
degenerate two-band model and compare its results to the five-band model with
realistic density of states of iron and nickel, showing that the obtained
results are more general, provided that the hybridization between states of
different symmetry is sufficiently small. We find that quasiparticle damping
and the formation of local magnetic moments due to Hund's exchange interaction
are enhanced by both, the density of states asymmetry, which yields stronger
correlated electron or hole excitations, and the larger density of states at
the Fermi level, increasing the number of virtual electron-hole excitations.
For realistic densities of states these two factors are often interrelated
because the Fermi level is attracted towards peaks of the density of states. We
discuss the implication of the obtained results to various substances and
compounds, such as transition metals, iron pnictides, and cuprates.Comment: 7 pages, 7 figure
Momentum-dependent susceptibilities and magnetic exchange in bcc iron from supercell DMFT calculations
We analyze the momentum- and temperature dependences of the magnetic
susceptibilities and magnetic exchange interaction in paramagnetic bcc iron by
a combination of density functional theory and dynamical mean-field theory
(DFT+DMFT). By considering a general derivation of the orbital-resolved
effective model for spin degrees of freedom for Hund's metals, we relate
momentum-dependent susceptibilities in the paramagnetic phase to the magnetic
exchange. We then calculate non-uniform orbital-resolved susceptibilities at
high-symmetry wave vectors by constructing appropriate supercells in the DMFT
approach. Extracting the irreducible parts of susceptibilities with respect to
Hund's exchange interaction, we determine the corresponding orbital-resolved
exchange interactions, which are then interpolated to the whole Brillouin zone.
Using the spherical model we estimate the temperature dependence of the
resulting exchange between local moments.Comment: 18 pages, 6 figure
Hubbard U and Hund's Exchange J in Transition Metal Oxides: Screening vs. Localization Trends from Constrained Random Phase Approximation
In this work, we address the question of calculating the local effective
Coulomb interaction matrix in materials with strong electronic Coulomb
interactions from first principles. To this purpose, we implement the
constrained random phase approximation (cRPA) into a density functional code
within the linearized augmented plane wave (LAPW) framework.
We apply our approach to the 3d and 4d early transition metal oxides SrMO3
(M=V, Cr, Mn) and (M=Nb, Mo, Tc) in their paramagnetic phases. For these
systems, we explicitly assess the differences between two physically motivated
low-energy Hamiltonians: The first is the three-orbital model comprising the
t2g states only, that is often used for early transition metal oxides. The
second choice is a model where both, metal d- and oxygen p-states are retained
in the construction of Wannier functions, but the Hubbard interactions are
applied to the d-states only ("d-dp Hamiltonian"). Interestingly, since -- for
a given compound -- both U and J depend on the choice of the model, so do their
trends within a family of these compounds. In the 3d perovskite series SrMO3
the effective Coulomb interactions in the t2g Hamiltonian decrease along the
series, due to the more efficient screening. The inverse -- generally expected
-- trend, increasing interactions with increasing atomic number, is however
recovered within the more localized "d-dp Hamiltonian". Similar conclusions are
established in the layered 4d perovskites series Sr2MO4 (M=Mo, Tc, Ru, Rh).
Compared to their isoelectronic and isostructural 3d analogues, the 4d 113
perovskite oxides SrMO3 (M=Nb, Mo, Tc) exhibit weaker screening effects.
Interestingly, this leads to an effectively larger U on 4d shells than on 3d
when a t2g model is constructed.Comment: 21 pages, 7 figure
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