3,819 research outputs found
Double-exchange theory of ferroelectric polarization in orthorhombic manganites with twofold periodic magnetic texture
We argue that many aspects of improper ferroelectric activity in orthorhombic
manganites can be rationalized by considering the limit of infinite
intra-atomic splitting between the majority- and minority-spin states (or the
double exchange limit), which reduces the problem to the analysis of a spinless
double exchange (DE) Hamiltonian. We apply this strategy to the low-energy
model, derived from the first-principles calculations, and combine it with the
Berry-phase theory of electric polarization. We start with the simplest
two-orbital model, describing the behavior of the eg bands, and apply it to the
E-type antiferromagnetic (AFM) phase, which in the DE limit effectively breaks
up into one-dimensional zigzag chains. We derive an analytical expression for
the electronic polarization (Pel) and explain how it depends on the orbital
ordering and the energy splitting Delta between eg states. Then, we evaluate
parameters of this model, starting from a more general five-orbital model for
all Mn 3d bands and constructing a new downfolded model for the eg bands. From
the analysis of these parameters, we conclude that the behavior of Pel in
realistic manganites corresponds to the limit of large Delta. We further
utilize this property in order to derive an analytical expression for Pel in a
general two-fold periodic magnetic texture, based on the five-orbital model and
the perturbation-theory expansion for the Wannier functions in the first order
of 1/Delta. This expression explains the functional dependence of Pel on the
relative directions of spins. Furthermore, it suggests that Pel is related to
the asymmetry of the transfer integrals, which should simultaneously have
symmetric and antisymmetric components. Finally, we explain how the
polarization can be switched between orthorhombic directions a and c by
inverting the zigzag AFM texture in every second ab plane.Comment: 41 page, 10 figure
Spin dependence of ferroelectric polarization in the double exchange model for manganites
The double exchange (DE) model is systematically applied for studying the
coupling between ferroelectric (FE) and magnetic orders in several prototypical
types of multiferroic manganites. The model was constructed for the
magnetically active Mn bands in the basis of Wannier functions and include
the effect of screened on-site Coulomb interactions. The essence of our
approach for the FE polarization is to use the Berry phase theory, formulated
in terms of occupied Wannier functions, and to evaluate the asymmetric
spin-dependent change of these functions in the framework of the DE model. This
enables us to quantify the effect of the magnetic symmetry breaking and derive
several useful expressions for the electronic polarization , depending
on the relative directions of spins. The proposed theory is applied to the
solution of three major problems: (i) The magnetic-state dependence of in hexagonal manganites; (ii) The microscopic relationship between canted
ferromagnetism and in monoclinic BiMnO; (iii) The origin of FE
activity in orthorhombic manganites. We show that for an arbitrary noncollinear
magnetic structure, propagating along the orthorhombic axis
and antiferromagnetically coupled , can be obtained
by scaling the one of the E-phase with the prefactor depending only on the
relative directions of spins and being the measure of the spin inhomogeneity.
This picture works equally well for the twofold (HoMnO) and fourfold
(TbMnO) periodic manganites. The basic difference is that the twofold
periodic magnetic structure is strongly inhomogeneous, that leads to large
. On the contrary, the fourfold periodic magnetic structure can be
viewed as a moderately distorted homogeneous spin spiral, which corresponds to
weaker .Comment: 32 pages, 7 figure
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
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
Realization of anisotropic compass model on the diamond lattice of Cu in CuAlO
Spin-orbit (SO) Mott insulators are regarded as a new paradigm of magnetic
materials, whose properties are largely influenced by SO coupling and featured
by highly anisotropic bond-dependent exchange interactions between the
spin-orbital entangled Kramers doublets, as typically manifested in
iridates. Here, we propose that a very similar situation can be realized in
cuprates when the Cu ions reside in a tetrahedral environment, like in
spinel compounds. Using first-principles electronic structure calculations, we
construct a realistic model for the diamond lattice of the Cu ions in
CuAlO and show that the magnetic properties of this compound are
largely controlled by anisotropic compass-type exchange interactions that
dramatically modify the magnetic ground state by lifting the spiral spin-liquid
degeneracy and stabilizing a commensurate single- spiral
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
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
Hybridization and spin-orbit coupling effects in quasi-one-dimensional spin-1/2 magnet Ba3Cu3Sc4O12
We study electronic and magnetic properties of the quasi-one-dimensional
spin-1/2 magnet Ba3Cu3Sc4O12 with a distinct orthogonal connectivity of CuO4
plaquettes. An effective low-energy model taking into account spin-orbit
coupling was constructed by means of first-principles calculations. On this
basis a complete microscopic magnetic model of Ba3Cu3Sc4O12, including
symmetric and antisymmetric anisotropic exchange interactions, is derived. The
anisotropic exchanges are obtained from a distinct first-principles numerical
scheme combining, on one hand, the local density approximation taking into
account spin-orbit coupling, and, on the other hand, projection procedure along
with the microscopic theory by Toru Moriya. The resulting tensors of the
symmetric anisotropy favor collinear magnetic order along the structural chains
with the leading ferromagnetic coupling J1 = -9.88 meV. The interchain
interactions J8 = 0.21 meV and J5 = 0.093 meV are antiferromagnetic. Quantum
Monte Carlo simulations demonstrated that the proposed model reproduces the
experimental Neel temperature, magnetization and magnetic susceptibility data.
The modeling of neutron diffraction data reveals an important role of the
covalent Cu-O bonding in Ba3Cu3Sc4O12.Comment: 11 pages, 12 figure
Superexchange Interactions in Orthorhombically Distorted Titanates RTiO3 (R= Y, Gd, Sm, and La)
Starting from the multiorbital Hubbard model for the t2g-bands of RTiO3 (R=
Y, Gd, Sm, and La), where all parameters have been derived from the
first-principles calculations, we construct an effective superexchange (SE)
spin model, by treating transfer integrals as a perturbation. We consider four
approximations for the SE interactions: (i) the canonical crystal-field (CF)
theory, where the form of the the occupied t2g-orbitals is dictated by the CF
splitting, and three extensions, namely (ii) the relativistic one, where
occupied orbitals are confined within the lowest Kramers doublet obtained from
the diagonalization of the crystal field and relativistic spin-orbit (SO)
interactions; (iii) the finite-temperature extension, which consider the effect
of thermal orbital fluctuations near the CF configuration; (iv) the
many-electron extension, which is based on the diagonalization of the full
Hamiltonian constructed in the basis of two-electron states separately for each
bond of the system. The main results are summarized as follows. (i) Thermal
fluctuations of the orbital degrees of freedom can substantially reduce the
value of the magnetic transition temperature. (ii) The anisotropic and
antisymmetric Dzyaloshinsky-Moriya interactions are rigorously derived and
their implications to the magnetic properties are discussed. (iii) The CF
theory, although applicable for YTiO3 and high-temperature structures of GdTiO3
and SmTiO3, breaks down in LaTiO3. Instead, the combination of the
many-electron effects and SO interaction can be responsible for the AFM
character of interatomic correlations in LaTiO3. (iv) The SE interactions in
YTiO3 strongly depend on the details of the crystal structure. Distortions in
the low-temperature structure tend to weaken the ferromagnetic interactions.Comment: 23 pages, 9 tables, 4 figure
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