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Frustration and Entanglement in Compass and Spin-Orbital Models

100%
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.
2
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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.
3
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Hole Propagation in the Orbital Compass Models

100%
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.
4
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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.
5
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Exotic Spin Order due to Orbital Fluctuations

100%
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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