59 research outputs found
Ab initio investigation of the exchange interactions in BiFeO: The Cairo pentagonal lattice compound
We present the \emph{ab initio} calculation of the electronic structure and
magnetic properties of BiFeO. This compound crystallizes in the
orthorhombic crystal structure with the Fe ions forming the Cairo
pentagonal lattice implying strong geometric frustration. The neutron
diffraction measurements reveal nearly orthogonal magnetic configuration, which
at first sight is rather unexpected since it does not minimize the total energy
of the pair of magnetic ions coupled by the Heisenberg exchange interaction.
Here we calculate the electronic structure and exchange integrals of Bi2Fe4O9
within the LSDA+U method. We obtain three different in-plane (J3=36 K, J4=73 K,
J5=23 K) and two interplane (J1=10 K, J2=12 K) exchange parameters. The derived
set of exchange integrals shows that the realistic description of Bi2Fe4O9
needs a more complicated model than the ideal Cairo pentagonal lattice with
only two exchange parameters in the plane. However, if one takes into account
only two largest exchange integrals, then according to the ratio x\equiv
J3/J4=0.49<\sqrt{2} (a critical parameter for the ideal Cairo pentagonal
lattice, see. Ref.~1) the ground state should be the orthogonal magnetic
configuration in agreement with experiment. The microscopic origin of different
exchange interactions is also discussed.Comment: 6 pages, 4 figure
Magnetism of sodium superoxide
By combining first-principles electronic-structure calculations with the
model Hamiltonian approach, we systematically study the magnetic properties of
sodium superoxide (NaO2), originating from interacting superoxide molecules. We
show that NaO2 exhibits a rich variety of magnetic properties, which are
controlled by relative alignment of the superoxide molecules as well as the
state of partially filled antibonding molecular \pi_g-orbitals. The orbital
degeneracy and disorder in the high-temperature pyrite phase gives rise to weak
isotropic antiferromagnetic (AFM) interactions between the molecules. The
transition to the low-temperature marcasite phase lifts the degeneracy, leading
to the orbital order and formation of the quasi-one-dimensional AFM spin
chains. Both tendencies are consistent with the behavior of experimental
magnetic susceptibility data. Furthermore, we evaluate the magnetic transition
temperature and type of the long-range magnetic order in the marcasite phase.
We argue that this magnetic order depends on the behavior of weak isotropic as
well as anisotropic and Dzyaloshinskii-Moriya exchange interactions between the
molecules. Finally, we predict the existence of a multiferroic phase, where the
inversion symmetry is broken by the long-range magnetic order, giving rise to
substantial ferroelectric polarization.Comment: 10 pages, 7 figure
First principles investigation of exchange interactions in quasi-one-dimensional antiferromagnet CaV2O4
The effect of orbital degrees of freedom on the exchange interactions in the
spin-1 quasi-one-dimensional antiferromagnet CaV2O4 is systematically studied.
For this purpose a realistic low-energy model with the parameters derived from
the first-principles calculations is constructed. The exchange interactions are
calculated using both the theory of infinitesimal spin rotations near the
mean-field ground state and the superexchange model, which provide a consistent
description. The obtained behaviour of exchange interactions substantially
differs from the previously proposed phenomenological picture based on the
magnetic measurements and structural considerations, namely: (i) Despite
quasi-one-dimensional character of the crystal structure, consisting of the
zigzag chains of edge-sharing VO6 octahedra, the electronic structure is
essentially three-dimensional, that leads to finite interactions between the
chains; (ii) The exchange interactions along the legs of the chains appear to
dominate; and (iii) There is a substantial difference of exchange interactions
in two crystallographically inequivalent chains. The combination of these three
factors successfully reproduces the behaviour of experimental magnetic
susceptibility.Comment: 15 pages, 6 figures, supplementary materia
Theoretical Analysis of Electronic and Magnetic Properties of NaVO: Crucial Role of the Orbital Degrees of Freedom
Using realistic low-energy model with parameters derived from the
first-principles electronic structure calculation, we address the origin of the
quasi-one-dimensional behavior in orthorhombic NaVO, consisting of the
double chains of edge-sharing VO octahedra. We argue that the geometrical
aspect alone does not explain the experimentally observed anisotropy of
electronic and magnetic properties of NaVO. Instead, we attribute the
unique behavior of NaVO to one particular type of the orbital ordering,
which respects the orthorhombic symmetry. This orbital ordering acts to
divide all states into two types: the `localized' ones, which are
antisymmetric with respect to the mirror reflection , and
the symmetric `delocalized' ones. Thus, NaVO can be classified as the
double exchange system. The directional orientation of symmetric orbitals,
which form the metallic band, appears to be sufficient to explain both
quasi-one-dimensional character of interatomic magnetic interactions and the
anisotropy of electrical resistivity.Comment: 16 pages, 4 figure
First Principle Electronic Model for High-Temperature Superconductivity
Using the structural data of the La2CuO4 compound both in the low temperature
tetragonal phase and in the isotropic phase we have derived an effective t-J
model with hoppings t and superexchange interactions J extended up to fourth
and second neareast neighbors respectively. By numerically studying this
hamiltonian we have then reproduced the main experimental features of this HTc
compound: d-wave superconductivity is stabilized at small but finite doping
delta>6% away from the antiferromagnetic region and some evidence of dynamical
stripes is found at commensurate filling 1/8.Comment: 4 pages including 4 figures and 2 table
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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