831 research outputs found
Orbital ordering in LaSrMnO studied by model Hartree-Fock calculation
We have investigated orbital ordering in the half-doped manganite
LaSrMnO, which displays spin, charge and orbital ordering,
by means of unrestricted Hartree-Fock calculations on the multiband -
model. From recent experiment, it has become clear that
LaSrMnO exhibits a cross-type orbital
ordering rather than the widely believed rod-type orbital
ordering. The calculation reveals that cross-type orbital
ordering results from an effect of in-plane distortion as well as from the
relatively long out-of-plane Mn-O distance. For the "Mn" site, it is
shown that the elongation along the c-axis of the MnO octahedra leads to an
anisotropic charge distribution rather than the isotropic one.Comment: 4 pages, 5 figure
Spin and charge ordering in self-doped Mott insulators
We have investigated possible spin and charge ordered states in 3d
transition-metal oxides with small or negative charge-transfer energy, which
can be regarded as self-doped Mott insulators, using Hartree-Fock calculations
on d-p-type lattice models. It was found that an antiferromagnetic state with
charge ordering in oxygen 2p orbitals is favored for relatively large
charge-transfer energy and may be relevant for PrNiO and NdNiO. On the
other hand, an antiferromagnetic state with charge ordering in transition-metal
3 orbitals tends to be stable for highly negative charge-transfer energy and
can be stabilized by the breathing-type lattice distortion; this is probably
realized in YNiO.Comment: 4 pages, 4 figure
Interplay between orbital ordering and lattice distortions in LaMnO3, YVO3, and YTiO3
We have studied the interplay between orbital ordering, Jahn-Teller and
GdFeO3-type lattice distortions in perovskite-type transition-metal oxides
using model Hartree-Fock calculations. It has been found that the covalency
between A-site cations and oxygens causes interaction between the Jahn-Teller
and GdFeO3-type distortions. The present calculations explain why the d-type
Jahn-Teller distortion and orbital ordering compatible with it are realized in
LaMnO3, YVO3 and YTiO3Comment: 5 pages, 8 figure
Theory of Mott insulator/band insulator heterostructure
A theory of heterostructures comprised of LaTiO (a Mott insulator) and
SrTiO (a band insulator) is presented. The band structure of the Ti %
-electrons is treated with a nearest neighbor tight-binding approximation; the
electric fields arising from the La/Sr charge difference and the
carriers are treated within a Hartree approximation; and the on-site
interactions are treated by unrestricted Hartree-Fock. The phase diagram as a
function of interaction strength and layer number is determined and predictions
are made for optical conductivity experiments. A note worthy finding is that
the edges of the heterostructure are generally metallic.Comment: 11 pages, 9 figure
Orbital polarons and ferromagnetic insulators in manganites
We argue that in lightly hole doped perovskite-type Mn oxides the holes
(Mn sites) are surrounded by nearest neighbor Mn sites in which
the occupied orbitals have their lobes directed towards the central hole
(Mn) site and with spins coupled ferromagnetically to the central spin.
This composite object, which can be viewed as a combined orbital-spin-lattice
polaron, is accompanied by the breathing type (Mn) and Jahn-Teller type
(Mn) local lattice distortions. We present calculations which indicate
that for certain doping levels these orbital polarons may crystallize into a
charge and orbitally ordered ferromagnetic insulating state.Comment: 5 pages, 4 figures, to be published in PR
Charge and orbital ordering in underdoped La1-xSrxMnO3
We have explored spin, charge and orbitally ordered states in La1-xSrxMnO3 (0
< x < 1/2) using model Hartree-Fock calculations on d-p-type lattice models. At
x=1/8, several charge and orbitally modulated states are found to be stable and
almost degenerate in energy with a homogeneous ferromagnetic state. The present
calculation indicates that a ferromagnetic state with a charge modulation along
the c-axis which is consistent with the experiment by Yamada et al. might be
responsible for the anomalous behavior around x = 1/8.Comment: 5 pages, 5 figure
Double exchange model in cubic vanadates
We investigate the role of the double exchange mechanism for stability of the
metallic C-type antiferromagnetic (C-AF) phase, which was observed
experimentally in hole doped LaSrVO. The double exchange model
treats localized electrons in xy orbitals as classical S=1/2 spins, which
interact by Hund's exchange with yz/zx electrons in partly filled
orbitals. Including strong on-site Coulomb repulsion between
electrons, and using slave boson method we demonstrate that C-AF and
metallic phase can be stabilized due to the specific features of the hopping in
degenerate and partly filled orbitals.Comment: 6 pages, 2 figure
Origin of G-type Antiferromagnetism and Orbital-Spin Structures in
The possibility of the distortion of octahedra is
examined theoretically in order to understand the origin of the G-type
antiferromagnetism (AFM(G)) and experimentally observed puzzling properties of
. By utilizing an effective spin and pseudospin Hamiltonian with
the strong Coulomb repulsion, it is shown that AFM(G) state is stabilized
through the lift of the -orbital degeneracy accompanied by a tiny
-distortion . The estimated spin-exchange interaction is in agreement
with that obtained by the neutron scattering. Moreover, the level-splitting
energy due to the distortion can be considerably larger than the spin-orbit
interaction even when the distortion becomes smaller than the detectable limit
under the available experimental resolution. This suggests that the orbital
momentum is fully quenched and the relativistic spin-orbit interaction is not
effective in this system, in agreement with recent neutron-scattering
experiment.Comment: 9 pages, 6 figure
- …