20 research outputs found
A possibility of high spin hole states in doped CoO layered systems
We introduce and investigate an effective five-band model for and
electrons to describe doped cobalt oxides with Co and Co
ions in two-dimensional CoO triangular lattice layers, as in
NaCoO. The effective Hamiltonian includes anisotropic kinetic
energy (due to both direct Co-Co and indirect Co-O-Co hoppings), on-site
Coulomb interactions parameterized by intraorbital Hubbard repulsion and
full Hund's exchange tensor, crystal-field terms and Jahn-Teller static
distortions. We study it using correlated wave functions on
clusters with periodic boundary conditions. The computations indicate low S=0
spin to high S=2 spin abrupt transition in the undoped systems when increasing
strength of the crystal field, while intermediate S=1 spins are not found.
Surprisingly, for the investigated realistic Hamiltonian parameters describing
low spin states in CoO planes, doping generates high spins
at Co ions that are pairwise bound into singlets, seen here as pairs of
up and down spins. It is found that such singlet pairs self-organize at higher
doping into lines of spins with coexisting antiferromagnetic and ferromagnetic
bonds, forming stripe-like structures. The ground states are insulating within
the investigated range of doping because computed HOMO-LUMO gaps are never
small enough.Comment: 20 pages, 5 figure
model and spin-orbital order in the vanadium perovskites
Using the multi-band model and unrestricted Hartree-Fock approximation
we investigate the electronic structure and spin-orbital order in
three-dimensional VO lattice. The main aim of this investigation is testing
if simple model, with partly filled orbitals (at vanadium ions) and
orbitals (at oxygen ions), is capable of reproducing correctly nontrivial
coexisting spin-orbital order observed in the vanadium perovskites. We point
out that the multi-band model has to include partly filled orbitals
at vanadium ions. The results suggest weak self-doping as an important
correction beyond the ionic model and reproduce the possible ground states with
broken spin-orbital symmetry on vanadium ions: either -type alternating
orbital order accompanied by -type antiferromagnetic spin order, or -type
alternating orbital order accompanied by -type antiferromagnetic spin order.
Both states are experimentally observed and compete with each other in YVO
while only the latter was observed in LaVO. Orbital order is induced and
stabilized by particular patterns of oxygen distortions arising from the
Jahn-Teller effect. In contrast to time-consuming \textit{ab-initio}
calculations, the computations using model are very quick and should be
regarded as very useful in solid state physics, provided the parameters are
selected carefully.Comment: 10 pages, 3 figures, accepted by Physical Review
Multiband model and self-doping in the electronic structure of BaIrO
We introduce and investigate the multiband model describing a IrO
layer (such as realized in BaIrO) where all orbitals per unit cell
are partly occupied, i.e., and orbitals at iridium and
orbitals at oxygen ions. The model takes into account anisotropic
iridium-oxygen and oxygen-oxygen hopping processes, crystal-field
splittings, spin-orbit coupling, and the on-site Coulomb interactions, both at
iridium and at oxygen ions. We show that the predictions based on assumed
idealized ionic configuration (with electrons per IrO
unit) do not explain well the independent \textit{ab initio} data and the
experimental data for BaIrO. Instead we find that the total electron
density in the states is smaller, ). When we fix
, the predictions for the model become more realistic and weakly
insulating antiferromagnetic ground state with the moments lying within IrO
planes along (110) direction is found, in agreement with experiment and
\textit{ab initio} data. We also show that: (i) holes delocalize over the
oxygen orbitals and the electron density at iridium ions is enhanced, hence
(ii) their orbitals are occupied by more than one electron and have to be
included in the multiband model describing iridates.Comment: 12 pages, 4 figure
Charge transfer model for the electronic structure of layered ruthenates
Motivated by the earlier experimental results and \textit{ab initio} studies
on the electronic structure of layered ruthenates (SrRuO and
CaRuO) we introduce and investigate the multiband charge transfer
model describing a single RuO layer, similar to the charge transfer model
for a single CuO plane including apical oxygen orbitals in high
cuprates. The present model takes into account nearest-neighbor anisotropic
ruthenium-oxygen and oxygen-oxygen hopping elements, crystal-field
splittings and spin-orbit coupling. The intraorbital Coulomb repulsion and
Hund's exchange are defined not only at ruthenium but also at oxygen ions. Our
results demonstrate that the RuO layer cannot be regarded to be a pure
ruthenium system. We examine a different scenario in which ruthenium
orbitals are partly occupied and highlight the significance of oxygen
orbitals. We point out that the predictions of an idealized model based on
ionic configuration (with electrons per RuO unit) do
not agree with the experimental facts for SrRuO which support our
finding that the electron number in the states is significantly smaller.
In fact, we find the electron occupation of and orbitals for a single
RuO unit , being smaller by at least 1--1.5 electrons from that in
the ionic model and corresponding to self-doping with .Comment: 12 pages, 3 figure
Multiband d-p model for the description of Sr_{2}RuO_{4}
We study electronic structure of multiband d-p model describing RuO_{4} layer such as realized in Sr_{2}RuO_{4}. The model takes into account nearest-neighbor anisotropic ruthenium-oxygen and oxygen-oxygen hoppings, intra-atomic Coulomb interaction, Hund's exchange and in addition spin-orbit coupling on ruthenium. The RuO_{4} is universally considered as a pure t_{2g} system (with e_{g} orbitals empty) due to sizable gap between t_{2g} and e_{g} levels. We show that ruthenium e_{g} orbitals are in fact occupied, similarly like showed earlier for CoO_{2} layers
Stripe Phases in Layered Nickelates
To describe quasi two-dimensional nickelates we introduce an effective
Hamiltonian for electrons which includes the kinetic energy, on-site
Coulomb interactions, spin-spin and Jahn-Teller (static) terms. The
experimental stripe phases are correctly reproduced by the model. The
mechanisms responsible for stripe formation are different than those reported
in cuprates and manganites.Comment: 3 pages, 3 figures, presented at Euroconference Physics of Magnetism
201
Ground-state properties of rutile: electron-correlation effects
Electron-correlation effects on cohesive energy, lattice constant and bulk
compressibility of rutile are calculated using an ab-initio scheme. A
competition between the two groups of partially covalent Ti-O bonds is the
reason that the correlation energy does not change linearly with deviations
from the equilibrium geometry, but is dominated by quadratic terms instead. As
a consequence, the Hartree-Fock lattice constants are close to the experimental
ones, while the compressibility is strongly renormalized by electronic
correlations.Comment: 1 figure to appear in Phys. Rev.
Spin-orbital order in : d−p model study
Using the multiband model and unrestricted Hartree-Fock approximation
we investigate the electronic structure and spin-orbital order in
three-dimensional MnO lattice such as realized in LaMnO. The orbital
order is induced and stabilized by particular checkerboard pattern of oxygen
distortions arising from the Jahn-Teller effect in the presence of strong
Coulomb interactions on orbitals of Mn ions. We show that the
spin-orbital order can be modeled using a simple \textit{Ansatz} for local
crystal fields alternating between two sublattices on Mn ions, which have
non-equivalent neighboring oxygen distortions in planes. The simple and
computationally very inexpensive model reproduces correctly nontrivial
spin-orbital order observed in undoped LaMnO. Orbital order is very robust
and is reduced by \% for large self-doping in the metallic regime.Comment: 10 pages, 2 figures, 3 tables, accepted by Physical Review