3,821 research outputs found
Lattice Distortion and Magnetism of 3d- Perovskite Oxides
Several puzzling aspects of interplay of the experimental lattice distortion
and the the magnetic properties of four narrow -band perovskite oxides
(YTiO, LaTiO, YVO, and LaVO) are clarified using results of
first-principles electronic structure calculations. First, we derive parameters
of the effective Hubbard-type Hamiltonian for the isolated bands using
newly developed downfolding method for the kinetic-energy part and a hybrid
approach, based on the combination of the random-phase approximation and the
constraint local-density approximation, for the screened Coulomb interaction
part. Then, we solve the obtained Hamiltonian using a number of techniques,
including the mean-field Hartree-Fock (HF) approximation, the second-order
perturbation theory for the correlation energy, and a variational superexchange
theory. Even though the crystal-field splitting is not particularly large to
quench the orbital degrees of freedom, the crystal distortion imposes a severe
constraint on the form of the possible orbital states, which favor the
formation of the experimentally observed magnetic structures in YTiO,
YVO_, and LaVO even at the HF level. Beyond the HF approximation, the
correlations effects systematically improve the agreement with the experimental
data. Using the same type of approximations we could not reproduce the correct
magnetic ground state of LaTiO. However, we expect that the situation may
change by systematically improving the level of approximations for dealing with
the correlation effects.Comment: 30 pages, 17 figures, 8 tables, high-quality figures are available
via e-mai
Modeling of complex oxide materials from the first principles: systematic applications to vanadates RVO3 with distorted perovskite structure
"Realistic modeling" is a new direction of electronic structure calculations,
where the main emphasis is made on the construction of some effective
low-energy model entirely within a first-principle framework. Ideally, it is a
model in form, but with all the parameters derived rigorously, on the basis of
first-principles electronic structure calculations. The method is especially
suit for transition-metal oxides and other strongly correlated systems, whose
electronic and magnetic properties are predetermined by the behavior of some
limited number of states located near the Fermi level. After reviewing general
ideas of realistic modeling, we will illustrate abilities of this approach on
the wide series of vanadates RVO3 (R= La, Ce, Pr, Nd, Sm, Gd, Tb, Yb, and Y)
with distorted perovskite structure. Particular attention will be paid to
computational tools, which can be used for microscopic analysis of different
spin and orbital states in the partially filled t2g-band. We will explicitly
show how the lifting of the orbital degeneracy by the monoclinic distortion
stabilizes C-type antiferromagnetic (AFM) state, which can be further
transformed to the G-type AFM state by changing the crystal distortion from
monoclinic to orthorhombic one. Two microscopic mechanisms of such a
stabilization, associated with the one-electron crystal field and electron
correlation interactions, are discussed. The flexibility of the orbital degrees
of freedom is analyzed in terms of the magnetic-state dependence of interatomic
magnetic interactions.Comment: 23 pages, 13 figure
Fingerprints of Spin-Orbital Physics in Crystalline O
The alkali hyperoxide KO is a molecular analog of strongly-correlated
systems, comprising of orbitally degenerate magnetic O ions. Using
first-principles electronic structure calculations, we set up an effective
spin-orbital model for the low-energy \textit{molecular} orbitals and argue
that many anomalous properties of KO replicate the status of its orbital
system in various temperature regimes.Comment: 4 pages, 2 figures, 1 tabl
Ferromagnetic zigzag chains and properties of the charge ordered perovskite manganites
The low-temperature properties of the so-called ''charge ordered'' state in
50% doped perovskite manganites are described from the viewpoint of the
magnetic spin ordering. In these systems, the zigzag antiferromagnetic
ordering, combined with the double-exchange physics, effectively divides the
whole sample into the one-dimensional ferromagnetic zigzag chains and results
in the anisotropy of electronic properties. The electronic structure of one
such chain is described by an effective 33 Hamiltonian in the basis of
Mn() orbitals. We treat this problem analytically and consider the
following properties: (i) the nearest-neighbor magnetic interactions; (ii) the
distribution of the Mn() and Mn() states near the Fermi level, and
their contribution to the optical conductivity and the resonant x-ray
scattering near the Mn -absorption edge. We argue that the anisotropy of
magnetic interactions in the double-exchange limit, combined with the isotropic
superexchange interactions, readily explains both the local and the global
stability of the zigzag antiferromagnetic state. The two-fold degeneracy of
levels plays a very important role in the problem and explains the
insulating behavior of the zigzag chain, as well as the appearance of the
orbital ordering in the double-exchange model. Importantly, however, the charge
ordering itself is expected to play only a minor role and is incompatible with
the ferromagnetic coupling within the chain. We also discuss possible effects
of the Jahn-Teller distortion and compare the tight-binding picture with
results of band structure calculations in the local-spin-density approximation.Comment: 35 pages, 8 figure
Susceptibility of a single photon wave packet
The explicit compact expression for the susceptibility tensor of a single
photon wave packet on the photon mass-shell is derived. It is assumed that the
probe photon is hard, the test photon is soft, and their total energy is below
the electron-positron pair creation threshold. It turns out that a single
photon wave packet can be regarded as a birefringent gyrotropic dispersive
medium in the process of light-by-light scattering. The explicit expression for
the inclusive probability to record the probe photon in the process of
light-by-light scattering is obtained in the first nontrivial order of
perturbation theory where the interference effect of the free passed and
scattered parts of the photon wave function dominates. This effect is of order
in contrast to the standard contribution to the light-by-light
scattering cross-section which is of order . The possible nontrivial
shapes of the wave functions of probe and test photons are taken into account.
The evolution of the Stokes parameters of a probe photon is described. The
change of the Stokes parameters is rather large for hard probe photons and
sufficiently intense beams of soft test photons.Comment: 14 pp., 1 fig; some misprints correcte
First-Principles Computation of YVO3; Combining Path-Integral Renormalization Group with Density-Functional Approach
We investigate the electronic structure of the transition-metal oxide YVO3 by
a hybrid first-principles scheme. The density-functional theory with the
local-density-approximation by using the local muffin-tin orbital basis is
applied to derive the whole band structure. The electron degrees of freedom far
from the Fermi level are eliminated by a downfolding procedure leaving only the
V 3d t2g Wannier band as the low-energy degrees of freedom, for which a
low-energy effective model is constructed. This low-energy effective
Hamiltonian is solved exactly by the path-integral renormalization group
method. It is shown that the ground state has the G-type spin and the C-type
orbital ordering in agreement with experimental indications. The indirect
charge gap is estimated to be around 0.7 eV, which prominently improves the
previous estimates by other conventional methods
Construction of Wannier functions from localized atomic-like orbitals
The problem of construction of the Wannier functions (WFs) in a restricted
Hilbert space of eigenstates of the one-electron Hamiltonian (forming
the so-called low-energy part of the spectrum) can be formulated in several
different ways. One possibility is to use the projector-operator techniques,
which pick up a set of trial atomic orbitals and project them onto the given
Hilbert space. Another possibility is to employ the downfolding method, which
eliminates the high-energy part of the spectrum and incorporates all related to
it properties into the energy-dependence of an effective Hamiltonian. We show
that by modifying the high-energy part of the spectrum of the original
Hamiltonian , which is rather irrelevant to the construction of WFs in
the low-energy part of the spectrum, these two methods can be formulated in an
absolutely exact and identical form, so that the main difference between them
is reduced to the choice of the trial orbitals. Concerning the latter part of
the problem, we argue that an optimal choice for trial orbitals can be based on
the maximization of the site-diagonal part of the density matrix. The main idea
is illustrated for a simple toy model, consisting of only two bands, as well as
for a more realistic example of bands in VO. An analogy with
the search of the ground state of a many-electron system is also discussed.Comment: 13 pages, 6 figure
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