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1
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Correlation Functions in the Spin-fermion Model

100%
Bała J., Oleś A.
Acta Physica Polonica A
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2000
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vol. 97
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issue 1
213-216
EN
We show that the quasiparticle states found in a self-consistent Born approximation for the spin-fermion model which describes CuO_{2} planes agree well with experimental data in insulating Sr_{2}CuO_{2}Cl_{2}. It is found that the antiferromagnetic correlations are reduced in the neighbourhood of a moving oxygen hole and the correlation functions can exhibit similar dipolar distortions as reported in the context of the t-J model.
2
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Antiferromagnetism and Superconductivity in Three-Band Hubbard Model

100%
Dutka J., Oleś A.
Acta Physica Polonica A
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1994
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vol. 85
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issue 2
323-327
EN
We determined the phase diagram of the three-band model for CuO_{2} planes of high temperature superconductors with an attractive interoxygen interaction U_{pp}. The found magnetic moments of 0.47µ_{B} in the undoped system, the disappearance of antiferromagnetic order at the doping of δ ≈ 0.06 hole per CuO_{2} unit, as well as the enhanced tendency to superconductivity with the decreasing value of charge-transfer gap, are in good agreement with the experimental findings.
3
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The Novel Spin-Charge Patterns in Correlated Electron Systems Described by Extended Hubbard Hamiltonian

100%
Rościszewski K., Oleś A.
Acta Physica Polonica A
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2004
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vol. 105
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issue 5
495-500
EN
Inhomogeneous spin and charge orderings in doped two-dimensional correlated electron systems described by the extended Hubbard Hamiltonian were investigated. At the crossover from stripe phases to charge order phases, the novel types of ordering connected with highly symmetric superlattice-type spin and charge patterns were identified. In particular, the emergence of local hexagonal-like symmetry is of interest as such patterns are generic in complex, nonlinear dynamic systems.
4
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Magnetic Phases and Generalized t-J Models in Doped Mott-Hubbard Insulators

100%
Oleś A., Zaanen J.
Acta Physica Polonica A
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1994
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vol. 85
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issue 1
145-163
EN
We review some recent results obtained for the dynamics of a single hole and for the ground states at finite hole doping in t-J model. Next, we address the role of orbital degeneracy in doped Mott-Hubbard insulators and show examples of effective strong coupling models which include the orbital degrees of freedom. These new t-J models have interesting phase diagrams, with the new magnetic phases stabilized by a competition between magnetic energy and excitonic excitations. It is argued that the doped holes always bind to the excitons and that the new phases identified on the mean-field level give rise to local distortions of the lattice. We conclude that realistic t-J models derived from the electronic structure of particular compounds may be successfully applied for understanding both the observed magnetic ground states, and the results of photoemission experiments, as we have demonstrated recently for NiO.
5
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Interrelation between the Optical Conductivity and Crystal Field Splitting in Monolayer Manganites

100%
Rościszewski K., Oleś A.
Acta Physica Polonica A
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2009
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vol. 115
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issue 1
107-109
EN
We introduce an effective model for correlated e_g electrons which reproduces qualitatively the evolution of magnetic order in monolayer manganites when correlated wave functions are used. Here we address recent optical conductivity measurements suggesting that the splitting between the occupied and empty e_g states is very large in LaSrMnO_4, contrary to what is expected for the effective model. We argue that no contradiction was found but several simple-minded one-atom-based expectations concerning crystal-field splitting and optical conductivity are in error.
6
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Spin-Orbital Liquid on a Triangular Lattice

100%
Oleś A., Chaloupka J.
Acta Physica Polonica A
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2012
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vol. 121
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issue 5-6
1026-1028
EN
Using Lanczos exact diagonalization of finite clusters we demonstrate that the spin-orbital d^1 model for triply degenerate t_{2g} orbitals on a triangular lattice provides an example of a spin-orbital liquid ground state. We also show that the spin-orbital liquid involves entangled valence bond states which violate the Goodenough-Kanamori rules, and modify effective spin exchange constants.
7
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Ordered Phases and Magnetic Excitations in Doped Manganites

100%
Sikora O., Oleś A.
Acta Physica Polonica A
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2004
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vol. 106
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issue 5
659-664
EN
We present theoretical models concerning magnetic and charge ordering in half-doped manganites. The new approach to microscopic description of this group of compounds, taking into consideration the oxygen degrees of freedom, is discussed. The spin wave dispersion relations are shown both in the model of effective spins on Mn ions and in the new approach when two Mn ions form a large effective spin S=7/2.
8
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On the Neglect of Local Coulomb Interaction on Oxygens in Perovskites Described by the Multiband d-p Model

100%
Rościszewski K., Oleś A.
Acta Physica Polonica A
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2018
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vol. 133
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issue 3
356-358
EN
On the example of TiO₄ layer (such as realized in Sr₂TiO₄) we study electronic structure of multiband d-p models describing transition metal perovskites. As suggested by experiment, the studied system is predicted to be a robust nonmagnetic insulator. A realistic treatment of electronic structure requires one to introduce non-zero Coulomb local interactions at 2p oxygen orbitals. However, up till now majority of papers based upon multiband models made an approximation of neglecting such interactions. We show that this simplification does not lead to serious problems in predictions of the electronic structure provided the Coulomb interactions at titanium ions and charge transfer gap are suitably renormalized (so they become entirely different with respect to the true microscopic d-p model parameters).
9
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Orbital Correlations in Monolayer Manganites - From Spin t-J Model to Orbital t-J Model

100%
Daghofer M., Oleś A.
Acta Physica Polonica A
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2007
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vol. 111
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issue 4
497-512
EN
On the example of monolayer manganites we show that the theoretical ideas developed long ago along the derivation of the spin t-J model from the Hubbard model are nowadays very helpful in strongly correlated oxides with partly filled degenerate orbitals. We analyze a realistic orbital t-J model for e_g electrons in La_{1-x}Sr_{1+x}MnO_4 monolayer manganites, and discuss the evolution of spin and orbital correlations under increasing doping by performing exact diagonalization of finite clusters, with electronic kinetic energy determined self-consistently at a finite temperature by classical Monte Carlo for t_{2g} spins. Several experimental results are reproduced.
10
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Quantum Phase Transitions in the Kitaev-Heisenberg Model on a Single Hexagon

100%
Gotfryd D., Oleś A.
Acta Physica Polonica A
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2015
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vol. 127
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issue 2
318-320
EN
We present a detailed analysis of the Kitaev-Heisenberg model on a single hexagon. The energy spectra and spin-spin correlations obtained using exact diagonalisation indicate quantum phase transitions between antiferromagnetic and anisotropic spin correlations when the Kitaev interactions increase. In cluster mean-field approach frustrated nearest neighbor exchange stabilizes the stripe phase in between the Néel phase and frustrated one which evolves towards the Kitaev spin liquid.
11
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Spin Excitations in Doped Manganites

100%
Oleś A., Feiner L.
Acta Physica Polonica A
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2000
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vol. 97
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issue 1
193-196
EN
We generalize the double exchange to describe holes doped in degenerate e_{g} orbitals in La_{1-x}A_{x}MnO_{3} manganites, where A are divalent alkaline earth ions. Assuming an orbital liquid of disordered e_{g} orbitals we find isotropic ferromagnetic exchange interactions which increase with hole doping. The magnon dispersion agrees very well with the experimental data in ferromagnetic metallic manganites.
12
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Multiband d-p Model for the Description of Sr₂RuO₄

100%
Rościszewski K., Oleś A.
Acta Physica Polonica A
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2015
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vol. 127
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issue 2
260-262
EN
We study electronic structure of multiband d-p model describing RuO₄ layer such as realized in Sr₂RuO₄. 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 RuO₄ 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 CoO₂ layers.
13
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Stripe Phases in Layered Nickelates

100%
Rościszewski K., Oleś A.
Acta Physica Polonica A
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2012
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vol. 121
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issue 5-6
1048-1050
EN
To describe quasi two-dimensional nickelates we introduce an effective Hamiltonian for e_{g} 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.
14
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Entangled Spin-Orbital Phases in the d^{9} Model

100%
Brzezicki W., Oleś A.
Acta Physica Polonica A
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2012
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vol. 121
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issue 5-6
1045-1047
EN
We investigate the phase diagrams of the spin-orbital d^9 Kugel-Khomskii model for a bilayer and a monolayer square lattice using the Bethe-Peierls-Weiss method. For a bilayer we obtain valence bond phases with interlayer singlets, with alternating planar singlets, and two entangled spin-orbital phases, in addition to the antiferromagnetic and ferromagnetic order. Possibility of such entangled phases in a monolayer is under investigation at present.
15
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Polaron States in a CuO Chain

100%
Bieniasz K., Oleś A.
Acta Physica Polonica A
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2015
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vol. 127
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issue 2
269-271
EN
We introduce a one-dimensional model for a CuO chain, with holes and S=1/2 spins localized in 3d_{x²-y²} orbitals, and p_σ 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 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 t-J model. We present an analytical solution of the problem and investigate whether the numerical integration is a good approximation to this solution.
16
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Quantum versus Classical Polarons in a Ferromagnetic CuO_3-Like Chain

100%
Bieniasz K., Oleś A.
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vol. 126
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issue 4a
A-80-A-84
EN
We present an exact solution for an itinerant hole added into the oxygen orbitals of a CuO_{3}-like ferromagnetic chain. Using the Green function method, the quantum polarons obtained for the Heisenberg SU(2) interaction between localized Cu spins are compared with the polarons in the Ising chain. We find that magnons with large energy are favorable towards quasiparticle existence, even in the case of relatively modest electron-magnon coupling. We observe two quasiparticle states with dispersion ~2t each, which emerge from the incoherent continuum when the exchange coupling J increases. Quantum fluctuations in the spin system modify the incoherent part of the spectrum and change the spectral function qualitatively, beyond the bands derived from the perturbation theory.
17
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Quantum Phase Transition in the One-Dimensional XZ Model

100%
Brzezicki W., Oleś A.
Acta Physica Polonica A
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2009
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vol. 115
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issue 1
162-164
EN
We introduce a one-dimensional XZ model with alternating σ_i^zσ_{i+1}^z and σ_i^xσ_{i+1}^x interactions on even/odd bonds, interpolating between the Ising model and the quantum compass model. We present two ways of its exact solution by: (i) mapping to the quantum Ising models, and (ii) using fermions with spin 1/2. In certain cases the nearest neighbor pseudospin correlations change discontinuously at the quantum phase transition, where one finds highly degenerate ground state of the 1D compass model.
18
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Stripe Phases in the Hubbard Model

81%
Góra D., Rościszewski K., Oleś A.
Acta Physica Polonica A
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2000
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vol. 97
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issue 1
225-228
EN
We investigate the magnetic and charge ordering in the stripe phases obtained while using correlated wave functions for finite 8×8 clusters described by the Hubbard model with extended hopping. Nonmagnetic vertical (01) site-centered domain walls and (11) bond-centered stripes are found for the parameters of La_{2-x}Sr_{x}CuO_{4} and YBa_{2}Cu_{3}O_{6+x}, respectively, at doping δ=1/8. The obtained half-filled domain nonmagnetic walls reproduce the maxima observed in neutron scattering for La_{2-x}Sr_{x}CuO_{4}.
19
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The Green Function Variational Approximation: Significance of Physical Constraints

81%
Bieniasz K., Berciu M., Oleś A.
Acta Physica Polonica A
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2016
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vol. 130
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issue 2
659-663
EN
We present a calculation of the spectral properties of a single charge doped at a Cu(3d) site of the Cu-F plane in KCuF₃. The problem is treated by generating the equations of motion for the Green 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 variational approximation.
20
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Reiter's Polaron Wave Function Applied to a t rm_{2g} Orbital t-J Model

81%
Wohlfeld K., Oleś A., Daghofer M., Horsch P.
Acta Physica Polonica A
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2009
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vol. 115
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issue 1
110-113
EN
Using the self-consistent Born approximation we calculate Reiter's wave function for a single hole introduced into the undoped and orbitally ordered ground state of the t-J model with t_{2g} orbital degrees of freedom. While the number of excitations is similar to the spin t-J model for a given J/t, a distinct structure of the calculated wave function and its momentum dependence is identified suggesting the formation of a novel type of mobile polarons.
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