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1
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Analysis of the Finite-Temperature Phase Diagram of the Spinless Falicov-Kimball Model

100%
Lemański R.
Acta Physica Polonica A
|
2016
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vol. 130
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issue 2
577-580
EN
The finite-temperature phase diagram of the spinless Falicov-Kimball model on the Bethe lattice is analyzed using the dynamic mean field theory. Comparing the temperature-dependent density of states at various phases we detected a difference between two phases of the ordered insulator (OI-X and OI-Y) whose stability areas in the phase diagram are separated by the stability area of the ordered conductor. It appears that the difference between OI-X and OI-Y phases is due to the band inversion, consisting in a reversal of those subbands, that lie just above and below the Fermi level and are derived only from one or the other sublattice (+ or -).
2
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Comparison of the Multi-Band Hubbard and Heisenberg Models on Small Finite Clusters

63%
Matysiak J., Lemański R.
Acta Physica Polonica A
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2018
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vol. 133
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issue 3
405-407
EN
We perform exact numeric calculations for a few versions of the two-orbital Hubbard model on the four site cluster. We show that the conventional spin 1 Heisenberg Hamiltonian can be obtained in the strong coupling limit, but only for some versions of the two-orbital Hubbard model. Then we propose a modified version of the Heisenberg Hamiltonian, that we call the two-orbital Heisenberg model, which is relevant in those cases, where the conventional spin 1 Heisenberg model fails.
3
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Peculiar Thermodynamic Properties of the Falicov-Kimball Model for small U Couplings

63%
Krawczyk J., Lemański R.
Acta Physica Polonica A
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2018
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vol. 133
|
issue 3
369-371
EN
We analyse a behaviour of the order parameter and specific heat of the Falicov-Kimball model (FKM) on the Bethe lattice using the Dynamical Mean Field Theory (DMFT) formalism, which provides the exact solution in the limit of large spatial dimensions. In the large U limit, the FKM maps onto the effective Ising model, with the order parameter of the Curie-Weiss form. However, in the small U limit the order parameter takes on unusual shape, with a sharp reduction near T ≈ T_{c}/2. We focus our investigation on a crossover between these two limits, thus we perform our calculations for a set of intermediate and small values of U. We find the overall behaviour of the order parameter and specific heat as a function of temperature to be quite anomalous.
4
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One-Electron Excitations vs. Collective Excitations in the 1D Falicov-Kimball Model with Hund Coupling at Half Filling

63%
Wrzodak J., Lemański R.
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|
issue 2
366-368
EN
The Falicov-Kimball model enriched with Ising-type Hund coupling between spins of itinerant electrons and magnetic ions is studied exactly on finite 1D rings. At half filling (one electron per site) and for the density of magnetic ions equal to 1/2 it is shown that many-electron collective excitations coupled to spin reorientations of the magnetic ions have much lower energy than single electron excitations. This property is caused by energy gaps formed at the Fermi levels in one-electron energy spectra of all relevant magnetic configurations of the ions. Consequently, low temperature properties of the system are not driven by one-particle, elementary electron excitations but many-particle collective excitations, where both electrons and spins of magnetic ions are involved. In addition, it is shown that many-body excitation spectra derived both from spin fluctuations and from charge fluctuations have a regular structure.
5
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Mapping of the DFT Spin Configuration Energies of Cr8Cd Molecular Ring onto the Energy Structure of Falicov-Kimball Model

51%
Wojciechowski M., Brzostowski B., Lemański R., Kamieniarz G.
Acta Physica Polonica A
|
2015
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vol. 127
|
issue 2
410-412
EN
A comprehensive study of electronic and magnetic properties of a recently synthesized nonametalic chromium-based heteronuclear molecule Cr₈CdF₉(O₂C-C(CH₃)₃)₁₈ is presented, using DFT and Falicov-Kimball (FK) model approach. The magnetic moments are calculated and the spin charge density map is discussed. The exchange coupling parameters between transition metals ions are extracted, taking into account all the nonequivalent spin configurations. It is demonstrated that the energies of the spin configurations can be reproduced by the FK model with a set of parameters consistent with that for the parent Cr8 molecule. For molecular ring considered, the ground state corresponds to the antiferromagnetic configuration and the ferromagnetic configuration yields the highest energy.
6
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DFT and Falicov-Kimball Model Approach to Cr_{9} Molecular Ring

39%
Brzostowski B., Wojciechowski M., Lemański R., Kamieniarz G., Timco G., Tuna F., Winpenny R.
|
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vol. 126
|
issue 1
270-271
EN
Based on density functional theory (DFT) calculations, we present electronic and magnetic properties of nanometallic homo-nuclear chromium-based molecular rings Cr_{9}F_{9}Cl_{2}(O_{2}C-C(CH_{3})_{3})_{17} recently synthesized. The magnetic moments are calculated, the spin density maps are discussed and the exchange interaction parameter is extracted. The complementary studies are carried out using the Falicov-Kimball model which reproduces very well the energy levels determined by different magnetic broken symmetry configurations obtained by DFT.
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