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1
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Quasiparticles in Calcium Doped Yttrium-Iron Garnets

100%
Lehmann-Szweykowska A., Wojciechowski R. J., Gehring G. A., Tobijaszewski I.
Acta Physica Polonica A
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1997
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vol. 91
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issue 2
423-426
EN
In yttrium-iron garnets an electrical charge is carried by quasiparticles based on compensating holes which move either at oxygen or iron ions of the tetragonal sublattice. The quasiparticles can also get localized because of their coupling to optical phonons on the iron sites. A multi-band model of holes coupled to phonons is applied to find additional energy levels in the gap between the lower of the narrow 3d bands and the wide 2p band, whose occurrence plays a crucial role in a conductivity mechanism. The density of states is considerably modified by the interaction between the holes and local distortion of the lattice.
2
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Analysis of Magnetic Ground State in Thulium Iron Garnets

100%
Koper A., Lehmann-Szweykowska A., Wigen P. E., Bornfreund R. E.
Acta Physica Polonica A
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1997
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vol. 92
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issue 2
391-394
EN
Magnetic phase diagrams of thulium iron garnets at Τ = 0 K are obtained by means of the so-called Monte Carlo relaxation method. The diagrams are given in terms of the respective strength of the superexchange couplings. There are five different phases and one spin-flop line on the diagram obtained. Two of these phases are of a semi-spin glass type.
3
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Spin and Charge Current in a Short Semiconducting Chain of Paramagnetic Ionic Blocks

100%
Lehmann-Szweykowska A., Wojciechowski R., Micnas R.
Acta Physica Polonica A
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2009
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vol. 115
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issue 1
260-262
EN
We deal with the electric current flowing through a short chain of paramagnetic ionic blocks, coupled to metallic electrodes in the serial configuration. An original three-band Hubbard-Anderson Hamiltonian is diagonalised at the level of the single ionic block. A minimal but sufficient set of the latter's four hybridised eigenstates serves as a basis for the determination of the time-ordered temperature-dependent matrix Green functions, in terms of which all the current-voltage (I-V) characteristics can be expressed provided the coupling to the electrodes is weak. The separation of the opposite-spin contributions to the electric current and, consequently, the spin current from the left to right electrode can result from the on-site Coulomb repulsion term of Hubbard-Anderson Hamiltonian, with no spin polarisation at the electrodes, but with the Zeeman-like coupling of the centre to either a molecular or an external magnetic field.
4
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The Energy Spectrum of Single-Hole States in Transition Metal Oxides

100%
Lehmann-Szweykowska A., Wojciechowski R., Gehring G.
Acta Physica Polonica A
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2000
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vol. 97
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issue 3
563-566
EN
The p-d hybridised single-hole states of the transition-metal-oxygen tetrahedron (TMO_{4}) are collectivised due to the direct p-p hopping between oxygens of different clusters. The lowest-lying energy band is always narrow and fully occupied. The first excited band gets occupied as an effect of valence-uncompensated doping, so it can be almost localised. The possible hole excitations to the two higher energy bands, which are wider, may imply the Mott-like hopping form of charge transport in these systems.
5
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Structural and Magnetic Properties of Co Thin Films on Au(111) Substrates

100%
Walczak A., Ślusarski T., Lehmann-Szweykowska A., Kamieniarz G.
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EN
The structural and magnetic properties of thin Co films grown on Au(111) substrates are investigated using ab initio local-spin-density calculations in the generalized gradient approximation. It is shown that a large lattice intermismatch between Co and Au(111) leads to a strong contraction of the interlayer distances within the Co layer, which adopts either the hexagonal close-packed (α-phase) or the face-centered cubic (β-phase) structure of bulk Co. Magnetic moments in the Co layer are increased beyond their value in bulk Co, the enhancement being strongest at the free surface and at the Co/Au interface, and more modest in the central part of the adlayer. Moderating influence of capping Ag layers on the structure and the magnetic properties of the films has also been considered. It is shown that Ag capping reduces the magnetic moments in comparison to uncovered Co/Au systems.
6
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Properties of the Semi Spin Glass Ground State of Garnets

100%
Koper A., Lehmann-Szweykowska A., Thomas M., Wojciechowski R.
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EN
A strong magnetic ordering response to valence-uncompensated doping observed experimentally in yttrium iron garnet, indicates the importance of a thorough analysis of the problem. Within the framework of the classical two-sublattice Heisenberg model the position on the ground state phase diagram, corresponding to the experimentally found values of the superexchange parameters of yttrium iron garnet, turns out to lie close to the semi spin glass region. Thus, in this paper, the magnetic structure factor, the spectrum of magnetic excitations, the density of states and a magnetic contribution to the specific heat, are calculated and discussed.
7
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Anomalous Phase Transitions in LiCsSO_4 in the Compressible hcp Ising Model

88%
Lehmann-Szweykowska A., Kurzyński M., Wojciechowski R., Wiesner M., Mróz B.
Acta Physica Polonica A
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2012
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vol. 121
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issue 5-6
1108-1110
EN
Structural low-temperature phase transformations in LiCsSO_4 are analysed in a newly tailored simple model of the static soliton in the periodically modulated phases of the one-dimensional compressible Ising model. It is shown that the original hcp symmetry is crucial for understanding of the anomalous temperature-dependence of the Brillouin shift.
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