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1
Content available Dynamical Mean Field within Iterative Perturbation Theory
100%
Radzimirski B. , Wojciechowski R.
Acta Physica Polonica A
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2007
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tom 111
|
nr 5
753-761
EN
We consider the iterative-perturbation theory and its generalization to multi-orbital Hubbard models. We discuss in detail all aspects of numerical implementations of the method.
2
Content available remote Andreev Reflection and Creation of Spin-Entangled Charge Carriers in Double Barrier Junctions
100%
Wojciechowski R. , Kowalewski L.
Acta Physica Polonica A
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2010
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tom 118
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nr 2
249-253
EN
We discuss spin dependent single particle charge transport as well as the spin pairwise entanglement processes in a double barrier junction. The junction consists of a superconductor connected to two normal or spin-polarized conductors, or semiconductors. Special attention is focused on a non-local process known as a crossed Andreev reflection. The non-local Andreev reflection occurs when an incident electron (hole) from one lead, injected with a subgap energy onto a superconducting layer, transforms into an Andreev hole (electron) in the second lead. At the same time, a Cooper pair is created in the superconductor. Using the Bogolubov-de Gennes equation with appropriate boundary conditions we calculate and discuss probabilities of processes relevant for charge transport through the junction. The dependence of the tunneling charge transport on the strength and orientation of the exchange field in the ferromagnetic electrodes, and on the height of the tunnel barriers as well as on the distance between the electrodes are presented. We discuss briefly the physics of creation and charge transport of spin entangled electron-hole pairs in the junction.
3
Content available remote Josephson Effect in Graphene-Based Junctions
100%
Wojciechowski R. , Kowalewski L.
Acta Physica Polonica A
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2018
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tom 133
|
nr 3
632-634
EN
We study the Josephson effect in graphene based junctions where superconductivity in graphene is induced by the proximity effect from external substrate materials. The electronic properties of the junction is described by the Dirac-Bogoliubov-de-Gennes equations. We consider the junction consisting of two superconductors with different pairing potentials. Using appropriate boundary conditions imposed on the normal region-superconductors interfaces, we calculated the Andreev bound state energy, in the ballistic limit, taking into account two types of reflections namely the retro and specular Andreev reflections.
4
Content available remote Spin Dependent Electron Tunneling in a Ferromagnet Superconductor Ferromagnet Junction
100%
Kowalewski L. , Wojciechowski R.
Acta Physica Polonica A
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2009
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tom 115
|
nr 1
257-259
EN
The coherent spin polarized transport in ferromagnet/superconductor/ferromagnet double barrier junctions is analyzed. Using the Bogolubov-de Gennes equation with appropriate boundary conditions, we calculate probabilities of spin dependent transport processes in the ferromagnet/superconductor/ferromagnet junction. In particular, we discuss the nonlocal processes such as the crossed Andreev reflection and elastic co-tunneling. These processes contribute to tunneling a current when the distance between the two magnetic electrodes is comparable to the superconducting coherence length. The dependences of the tunneling transport processes on the strength of the exchange field in the ferromagnetic electrodes, and on the height of the tunnel barriers are presented.
5
Content available remote Properties of the Semi Spin Glass Ground State of Garnets
80%
Koper A. , Lehmann-Szweykowska A. , Thomas M. , Wojciechowski R.
Acta Physica Polonica A
|
1994
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tom 85
|
nr 1
233-236
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.
6
Content available remote Tunneling Current into the Vortex Lattice States of s- and p-Wave Superconductors
80%
Kowalewski L. , Nogala M. , Thomas M. , Wojciechowski R.
Acta Physica Polonica A
|
2000
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tom 97
|
nr 1
237-240
EN
The tunneling current between the metallic tip of a scanning microscope and s-wave and p-wave superconductors in a quantizing magnetic field is investigated. The differential conductance is calculated both as a function of bias voltage at the centre of the vortex line and for varying position of the scanning tunneling microscope tip at a stable voltage.
7
Content available Resonance Tunneling in Ferromagnet-Triplet Superconductors Junctions
80%
Kowalewski L. , Wojciechowski R. , Wojtuś P.
Acta Physica Polonica A
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2007
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tom 111
|
nr 5
683-691
EN
We discuss the tunneling conductance in a ferromagnet-insulator-triplet superconductor junction. We consider the superconducting order parameters with spin triplet pairing having nodes. The nodal structure of the order parameter has been recently confirmed experimentally in Sr_2RuO_4. In particular, we study how a mid-gap structure of the tunnelling conductance depends on the phase difference of the pairing potential as well as on the orientation of the interface.
8
Content available remote Tunneling Charge Transport in Graphene-Based Superconductor Junctions
80%
Wojciechowski R. , Kowalewski L. , Nogala M.
|
EN
We study the spin polarized electron and hole tunneling transport through a graphene-based ferromagnet(GF_1)-insulator(GI_1)-superconductor(G_{S})-insulator(GI_2)-ferromagnet(GF_2) junction. Proximity induced spin polarization and superconductivity in a graphene sheet are assumed to be created by superconducting and ferromagnetic electrodes placed on the top of the graphene. Using a four-dimensional version of the Dirac-Bogoliubov-de Gennes equation with appropriate boundary conditions we investigate the tunneling processes through the junctions. In particular, we present calculations of the amplitudes of normal and Andreev reflections as a function of the energy of the incident electron for a wide range of the model parameters, such as the strength and orientation of the exchange field, the barrier strength, and the distance between the two ferromagnetic layers. The tunneling transport processes in the graphene-based double junction GF/GI/G_{S}/GI/GF are compared with those in non-graphene-based junctions.
9
Content available remote Heavy Fermion Superconductor Order Parameter in High Magnetic Field
80%
Kowalewski L. , Wojciechowski R. , Thomas M. , Nogala M.
Acta Physica Polonica A
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1994
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tom 85
|
nr 2
347-350
EN
It is shown that, independently on either the nature and the strength of pairing, the orbital effects of a high magnetic field in heavy fermion superconductor change the form of both the effective pairing and the gap function.
10
Content available remote The Energy Spectrum of Single-Hole States in Transition Metal Oxides
80%
Lehmann-Szweykowska A. , Wojciechowski R. , Gehring G.
Acta Physica Polonica A
|
2000
|
tom 97
|
nr 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.
11
Content available remote Spin and Charge Current in a Short Semiconducting Chain of Paramagnetic Ionic Blocks
80%
Lehmann-Szweykowska A. , Wojciechowski R. , Micnas R.
Acta Physica Polonica A
|
2009
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tom 115
|
nr 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.
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