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1
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Pairing Correlations around Negative-U Centre in the Two-Band Model

100%
Ciechan A., Wysokiński K.
Acta Physica Polonica A
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2008
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vol. 114
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issue 1
123-127
EN
The electron energy spectrum of many superconducting materials is characterised by the presence of few bands at the Fermi level. In some cases the superconducting properties seem to be dominated by single band but in others the many-band approach is essential ingredient of their description. In this paper we shall study the properties of superconducting impurity placed in a non-superconducting medium. We are interested in the evolution of superconducting correlations within non-superconducting 1- or 2-dimensional system, their dependence on the distance from impurity and changes induced by the presence of the second band and interband scattering. We use real space description of the material and Bogolyubov-de Gennes approach to superconductivity.
2
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Finite Size Corrections in the Two-Level BCS Model

100%
Ciechan A., Wysokiński K.
Acta Physica Polonica A
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2006
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vol. 109
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issue 4-5
569-572
EN
Superconducting properties of small metallic grains modelled by highly degenerate two-level spectrum have been studied. We have solved numerically Richardson's exact equations for the system of 2N electrons in two levels. Characterising the size of the grain by the level degeneracy we study the finite size corrections to the thermodynamic limit of the ground and lowest excited state energy. The interparticle distance∝ N^{-1/3} seems to be the expansion parameter. The obtained results have been compared with those of other authors.
3
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Thermodynamics of the Two-Level Richardson Model

100%
Ciechan A., Wysokiński K.
Acta Physica Polonica A
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2007
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vol. 111
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issue 4
467-473
EN
The two-level version of the Richardson model presents a unique possibility to calculate numerically exactly thermodynamic properties of the system it describes. The point is that all energies and the degeneracies of the many-body system can be easily calculated. The energies are given by the eigenvalues of the small (of order of N×N, where 2N is a number of electrons in the system) tridiagonal matrices. Here we numerically obtain a complete spectrum of the interacting two-level model and calculate the specific heat and the pairing energy of the small system at finite temperatures.
4
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Inter-Orbital Effects in the Impure Pnictide Superconductors

100%
Ciechan A., Wysokiński K.
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issue 2
356-359
EN
We study the local properties of the iron oxypnictides by means of the real space Bogolubov-de Gennes equations. Starting with the realistic energy spectrum and assuming small amount of impurities we calculate the energy dependence of the local density of states and spatial distribution of the local values of the superconducting energy gap. We pay particular attention to the role of the inter-orbital scattering of pairs and the effect of impurity scattering on the superconducting state. The effect of inter-orbital impurities depends on the relative signs of the order parameters related to two orbitals in question. For opposite signs impurities produce bound states inside the gap, while for the same sign of the order parameters they hardly affect the superconductor. The results obtained for impure systems have been shown as maps of the order parameters and local density of states. They well compare with the existing scanning tunnelling microscopy spectra of the iron pnictides.
5
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The Pressure Effects on Electronic Structure of Iron Chalcogenide Superconductors FeSe_{1-x}Te_x

81%
Ciechan A., Winiarski M., Samsel-Czekała M.
Acta Physica Polonica A
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2012
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vol. 121
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issue 4
820-823
EN
We study the electronic structure of iron-based superconductors FeSe_{1-x}Te_x within the density functional theory. We pay particular attention to the pressure effects on the Fermi surface (FS) topology, which seem to be correlated with a critical superconducting temperature T_C of iron chalcogenides and pnictides. A reduction of the FS nesting between hole and electron cylinders with increasing pressure is observed, which can lead to higher values of T_C. The tellurium substitution into selenium sites yields FS changes similar to the pressure effect.
6
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Relativistic Theory of Superconductivity: BCS vs. Bogolyubov-de Gennes Approaches

81%
Ciechan A., Donabidowicz A., Wysokiński K.
Acta Physica Polonica A
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2004
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vol. 106
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issue 5
699-702
EN
In this paper we shall present the results of numerical calculations of the superconducting characteristics (temperature dependence of the gap function and specific heat) for few different symmetries of the order parameter in relativistic theory of superconductivity. We shall discuss relevance of the results to describe newly discovered superconductors, particularly those containing heavy elements with strong spin-orbit coupling.
7
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Theoretical Analysis of the Short Coherence Length Superconductors

81%
Wysokiński K., Ciechan A., Krzyszczak J.
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issue 2
232-237
EN
Properties of the short coherence length ξ superconductors are markedly different from those with long ξ. The main difference is connected with their response to impurities. Even the s-wave superconductors have been predicted not to obey the so-called Anderson theorem and to show appreciable fluctuations of order parameter. This observation has been recently confirmed in a number of beautiful scanning tunnelling experiments on high temperature cuprate and pnictide superconductors. In this work we study clean two-band systems within BCS-type approach and impure superconductors within mean field Bogolubov-de Gennes approach. In the weak disorder limit both s-wave and d-wave superconductors only weakly react to impurities. Presence of the Van Hove singularities in the density of states of the two-band clean systems strongly increases their superconducting transition temperature if the intra-band pair scattering is the only pairing interaction. The effect of impurities on such superconductors does depend on the sign of the inter-orbital interaction.
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