114 research outputs found
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
Exact spectral function for hole-magnon coupling in the ferromagnetic CuO-like chain
We present the exact spectral function for a single oxygen hole with spin
opposite to ferromagnetic order within a one-dimensional CuO-like spin
chain. We find that local Kondo-like exchange interaction generates five
different states in the strong coupling regime. It stabilizes a spin polaron
which is a bound state of a moving charge dressed by magnon excitations, with
essentially the same dispersion as predicted by mean field theory. We then
examine in detail the evolution of the spectral function for increasing
strength of the hole-magnon interaction. We also demonstrate that the and
symmetry of orbital states in the conduction band are essentially
equivalent to each other and find that the simplified models do not suffice to
reproduce subtle aspects of hole-magnon coupling in the charge-transfer model.Comment: 9 pages, 5 figure
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
Polaron states in a CuO chain
We introduce a one-dimensional model for a CuO chain, with holes and
spins localized in orbitals, and oxygen orbitals
without holes in the ground state. We consider a single hole doped at an oxygen
site and study its propagation by spin-flip processes. We develop the Green's
function method and treat the hole-spin coupling in the self-consistent Born
approximation, similar to that successfully used to study polarons in the
regular - model. We present an analytical solution of the problem and
investigate whether the numerical integration is a good approximation to this
solution.Comment: 3 pages, 2 figures, Physics of Magnetism, Pozna\'n 201
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
Orbiton-magnon interplay in the spin-orbital polarons of KCuF3 and LaMnO3
We present a quasi-analytical solution of a spin-orbital model of KCuF,
using the variational method for Green's functions. By analyzing the spectra
for different partial bosonic compositions as well as the full solution, we
show that hole propagation needs both orbiton and magnon excitations to
develop, but the orbitons dominate the picture. We further elucidate the role
of the different bosons by analyzing the self-energies for simplified models,
establishing that because of the nature of the spin-orbital ground state,
magnons alone do not produce a full quasiparticle band, in contrast to
orbitons. Finally, using the electron-hole transformation between the
states of KCuF and LaMnO we suggest the qualitative scenario for
photoemission experiments in LaMnO.Comment: 8 pages, 4 figures, accepted by Physical Review
Orbital Symmetry and Orbital Excitations in High- Superconductors
We discuss a few possibilities of high- superconductivity with more than
one orbital symmetry contributing to the pairing. First, we show that the high
energies of orbital excitations in various cuprates suggest a simplified model
with a single orbital of symmetry doped by holes. Next, several
routes towards involving both orbital symmetries for doped holes are
discussed: (i) some give superconductivity in a CuO monolayer on Bi2212
superconductors, SrCuO, BaCuO, while (ii)
others as nickelate heterostructures or EuSrNiO, could in
principle realize it as well. At low electron filling of Ru ions, spin-orbital
entangled states of symmetry contribute in SrRuO. Finally,
electrons with both and orbital symmetries contribute to the
superconducting properties and nematicity of Fe-based superconductors,
pnictides or FeSe. Some of them provide examples of orbital-selective Cooper
pairing.Comment: 12 pages, 3 figures; in: Special Issue "From Cuprates to Room
Temperature Superconductors", dedicated to the anniversary of Professor K.
Alex M\"ulle
The Green function variational approximation: Significance of physical constraints
We present a calculation of the spectral properties of a single charge doped
at a Cu() site of the Cu-F plane in KCuF. The problem is treated by
generating the equations of motion for the Green's function by means of
subsequent Dyson expansions and solving the resulting set of equations. This
method, dubbed the variational approximation, is both very dependable and
flexible, since it is a systematic expansion with precise control over
elementary physical processes. It allows for deep insight into the underlying
physics of polaron formation as well as for inclusion of many physical
constraints, such as excluding crossing diagrams and double occupation
constraint, which are not included in the Self-Consistent Born Approximation.
Here we examine the role and importance of such constraints by analyzing
various spectral functions obtained in second order VA.Comment: 5 pages, 1 figur
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