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Phase Separation in the Ground State of the Model 2D Spin-Pseudospin System

100%
Chikov A., Panov Y., Moskvin A., Budrin K.
Acta Physica Polonica A
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2018
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vol. 133
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issue 3
432-434
EN
We addressed a simplified static 2D spin-pseudospin model which takes into account both conventional Heisenberg spin exchange coupling and the on-site and inter-site charge correlations. Classical Monte-Carlo calculations for large square lattices show that homogeneous ground state solutions found in a mean-field approximation are unstable with respect to phase separation with the charge and spin subsystems behaving like immiscible quantum liquids. For instance, with lowering the temperature one can observe two sequential phase transitions: first, antiferromagnetic ordering in the spin subsystem diluted by randomly distributed charges, then, the charge condensation in the charge droplets. Thermodynamic properties and phase diagram of the 2D spin-pseudospin system are studied by Monte-Carlo simulation with a special attention given to the role played by the on-site correlation.
2
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Superparamagnetic-Like Behaviour in RE_2WO_6 Tungstates (Where RE = Nd, Sm, Eu, Gd, Dy, Ho and Er)

80%
Groń T., Tomaszewicz E., Urbanowicz P., Duda H., Mydlarz T.
Acta Physica Polonica A
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2011
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vol. 119
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issue 5
708-710
EN
The magnetization isotherms were used to study the superparamagnetic-like behaviour in polycrystalline (powder) RE_2WO_6 tungstates (where RE = Nd, Sm, Eu, Gd, Dy, Ho and Er). The magnetization isotherms of the majority tungstates under study revealed both the spontaneous magnetic moments and hysteresis characteristic for the superparamagnetic-like behaviour with blocking temperature T_{B} ≈ 30 K except the Sm_2WO_6 and Eu_2WO_6 compounds.
3
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The Green Function Variational Approximation: Significance of Physical Constraints

80%
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.
4
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Frustration and Entanglement in Compass and Spin-Orbital Models

80%
Oleś A.
Acta Physica Polonica A
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2015
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vol. 127
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issue 2
163-169
EN
We review the consequences of intrinsic frustration of the orbital superexchange and of spin-orbital entanglement. While the Heisenberg perturbing interactions remove frustration in the compass model, the lowest columnar excitations are robust in the nanoscopic compass clusters and might be used for quantum computations. Entangled spin-orbital states determine the ground states in some cases, while in others concern excited states and lead to measurable consequences, as in the RVO₃ perovskites. On-site entanglement for strong spin-orbit coupling generates the frustrated Kitaev-Heisenberg model with a rich magnetic phase diagram on the honeycomb lattice. Frustration is here reflected in hole propagation which changes from coherent in an antiferromagnet via hidden quasiparticles in zigzag and stripe phases to entirely incoherent one in the Kitaev spin liquid.
5
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Interplay Between Spin-Orbit Coupling and Structural Deformations in Heavy Transition-Metal Oxides with Tetrahedral Coordination

80%
Forte F., Guerra D., Avella A., Autieri C., Romano A., Noce C.
Acta Physica Polonica A
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2018
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vol. 133
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issue 3
394-397
EN
We analyze the effects of a large spin-orbit coupling on the magnetic state of a d¹ transition-metal ion located in a tetrahedral environment. While in the ideal tetrahedral symmetry the spin-orbit coupling acts only as a perturbation on the atomic energy levels set by the crystal-field splitting, we demonstrate that its effects are strongly enhanced in the case of distorted geometries. In particular, we consider the specific case in which the tetrahedron is compressed along the z direction, and show that, by increasing the degree of flattening, a large spin-orbit interaction (i) can induce a substantial anisotropic, unquenched orbital momentum and (ii) can affect the hierarchy of the lowest energy levels that are involved in the magnetic exchange.
6
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On the Neglect of Local Coulomb Interaction on Oxygens in Perovskites Described by the Multiband d-p Model

80%
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).
7
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Order in Quantum Compass and Orbital e_{g} Models

80%
Czarnik P., Dziarmaga J., Oleś A.
Acta Physica Polonica A
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2018
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vol. 133
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issue 3
359-361
EN
We investigate thermodynamic phase transitions in the compass model and in e_{g} orbital model on an infinite square lattice by variational tensor network renormalization (VTNR) in imaginary time. The onset of nematic order in the quantum compass model is estimated at 𝓣_{c}/J=0.0606(4). For the e_{g} orbital model one finds: (i) a very accurate estimate of 𝓣_{c}/J=0.3566± 0.0001 and (ii) the critical exponents in the Ising universality class. Remarkably large difference in frustration results in so distinct values of 𝓣_{c}, while entanglement influences the quality of 𝓣_{c} estimation.
8
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Quantum Phase Transitions in the Kitaev-Heisenberg Model on a Single Hexagon

80%
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.
9
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Multiband d-p Model for the Description of Sr₂RuO₄

80%
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.
10
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Hole Propagation in the Orbital Compass Models

80%
Brzezicki W., Daghofer M., Oleś A.
Acta Physica Polonica A
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2015
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vol. 127
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issue 2
263-265
EN
We explore the propagation of a single hole in the generalized quantum compass model which interpolates between fully isotropic antiferromagnetic phase in the Ising model and nematic order of decoupled antiferromagnetic chains for frustrated compass interactions. We observe coherent hole motion due to either interorbital hopping or due to the three-site effective hopping, while quantum spin fluctuations in the ordered background do not play any role.
11
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Stripe Phases in Layered Nickelates

80%
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.
12
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Entangled Spin-Orbital Phases in the d^{9} Model

80%
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.
13
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Exotic Spin Order due to Orbital Fluctuations

80%
Brzezicki W., Dziarmaga J., Oleś A.
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vol. 126
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issue 4a
A-40-A-49
EN
We investigate the phase diagrams of the spin-orbital d^9 Kugel-Khomskii model for increasing system dimensionality: from the square lattice monolayer, via the bilayer to the cubic lattice. In each case we find strong competition between different types of spin and orbital order, with entangled spin-orbital phases at the crossover from antiferromagnetic to ferromagnetic correlations in the intermediate regime of Hund's exchange. These phases have various types of exotic spin order and are stabilized by effective interactions of longer range which follow from enhanced spin-orbital fluctuations. We find that orbital order is in general more robust while spin order melts first under increasing temperature, as observed in several experiments for spin-orbital systems.
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