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1
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Remarks on the First Hundred Years of Superconductivity

100%
Wysokiński K.
Acta Physica Polonica A
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2012
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vol. 121
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issue 4
721-725
EN
On the occasion of centenary of superconductivity discovery I recall some facts from the first period and attempts to understand the phenomenon. It turns out that most famous physicists of the first half of XX century have tried to solve the puzzle. Bardeen, Cooper and Schrieffer succeeded in 1957. The BCS theory successfully described all known facts and offered new predictions, which soon have been confirmed experimentally contributing to the widespread acceptance of the theory. It have found applications in nuclear physics, theory of neutron stars and cold atomic gases. The discoveries of new superconductors in the last thirty years show that simple BCS model is not enough to understand new unconventional superconductors. The studies of superconductors develop vividly and still fascinate new generations of physicists working in such diverse fields as material science and string theory.
2
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Introduction to Quantum Hall Effects

100%
Wysokiński K.
Acta Physica Polonica A
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2001
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vol. 100
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issue 3
287-299
EN
The quantum Hall effect is a set of phenomena observed at low temperature in a two-dimensional electron gas subject to a strong perpendicular magnetic field. It manifests itself as a quantization of the nondiagonal elements (ρ_{xy}) of the resistivity tensor accompanied by simultaneous vanishingρ_{xx} for ranges of the magnetic field. For the integer quantum Hall effectρ_{xy}= h/νe^2, where h is the Planck constant, e - charge of an electron andν is an integer, while for the fractional quantum Hall effectν is a simple fraction. In spite of similar phenomenology deep and profound differences exist between these two effects. In the lecture the precision of the Hall quantization in the integer quantum Hall effect and briefly new types of quantum fluids observed in the fractional quantum Hall effect are discussed. Some recent theoretical and experimental discoveries connected with quantum Hall liquids are also mentioned.
3
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Pairing Correlations around Negative-U Centre in the Two-Band Model

64%
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.
4
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Finite Size Corrections in the Two-Level BCS Model

64%
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.
5
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CPA Theory of Superconducting Alloys with Diagonal and Off-Diagonal Disorder

64%
Michalik M., Wysokiński K.
Acta Physica Polonica A
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2004
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vol. 106
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issue 5
653-658
EN
The general theory of disorder in superconducting alloys was discussed. We use standard Wannier function representation and allow for diagonal and off-diagonal disorder. We generalized the coherent potential approximation in the version able to deal with off-diagonal disorder in normal systems to treat superconducting state. As an illustration we calculate the quasiparticle density of states and gap function of a d-wave superconductor. We show inter alia that the rate at which superconductivity disappears depends on the kind of disorder and for off-diagonal disorder it depends on the details of its realization.
6
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The Isotope Effect of Disordered Superconductors

64%
Mierzyńska M., Wysokiński K.
Acta Physica Polonica A
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2004
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vol. 106
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issue 5
647-652
EN
The isotope effects in disordered weak and strong coupling superconductors of different symmetries of the order parameter have been studied. We have found that even though weak localisation corrections to Eliashberg equations describe a strong degradation of transition temperature with disorder of s-wave superconductors, their influence on isotope coefficient is relatively weak. On the other hand the calculated change of relative isotope coefficient with T_c/T_{c0} in homogeneous thin film materials is similar to that observed in high temperature superconductors.
7
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On the Current Flow in Superconductors: Universal Trends and Holographic Analysis

64%
Rogatko M., Wysokiński K.
Acta Physica Polonica A
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2018
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vol. 133
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issue 3
376-379
EN
The superconducting state can be destroyed by the increase of temperature, magnetic field or current flow beyond their critical values. The critical current I_{c} is of special interest as most of the practical applications of superconductors crucially depend on its limiting value. Recent analysis of experimental data in many families of type I and type II superconductors have discovered an interesting universal relation between critical current density j_{c}, the critical magnetic field H_{c} and the penetration depth λ. For type II superconductors the role of the thermodynamic critical field H_{c} is played by the lower critical field H_{c1} and ratio between the relevant dimension of the system d with respect to the penetration depth matters. Thus the effective dimensionality of the system is important and rules the system behaviour. It turns out that the holographic analogy provides an interesting justification of the above findings. We have calculated the temperature dependence of the critical current in the strongly coupled holographic superconductors with the current flow. It has been found that, independently of the symmetry of the order parameter, the critical current depends on temperature in 2d systems as I_{c} ∝ (T_{c}-T)^{3/2} and agrees with that observed in thin films (d < λ). Similar calculations for 3d systems (d > λ) reveal linear T-dependence I_{c} ∝ (T_{c}-T).
8
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Holographic Superconductivity in the Presence of Dark Matter: Basic Issues

64%
Rogatko M., Wysokiński K.
Acta Physica Polonica A
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2016
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vol. 130
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issue 2
558-563
EN
The holographic approach to study strongly coupled superconductors in the presence of dark matter is reviewed. We discuss the influence of dark matter on the superconducting transition temperature of both s-wave and p-wave holographic superconductors. The upper critical field, coherence length, penetration depth of holographic superconductors as well as the metal-insulator transitions have also been analysed. Issues related to the validity of anti-de Sitter/conformal field theory correspondence for the description of superconductors studied in the laboratory and possible experiments directed towards the detection of dark matter are discussed. In doing so we shall compare our assumptions and assertions with those generally accepted in the elementary particle experiments aimed at the detection of dark matter particles.
9
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Thermodynamics of the Two-Level Richardson Model

64%
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.
10
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Spectral Functions of the Quantum Dot Coupled to Normal and/or Superconducting Leads

64%
Krawiec M., Wysokiński K.
Acta Physica Polonica A
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2000
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vol. 97
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issue 1
197-200
EN
We study the spectrum of the quantum dot coupled to two external leads, which may be normal and/or superconducting. The dot is described by the Anderson-Hubbard impurity model in the infinity U limit. The impurity spectral function shows complex behavior depending on the temperature and the state of the leads. For normal leads and at low temperature we observe the appearance of the Kondo resonance, while for BCS-like superconducting leads the bound states emerge due to absence of low energy excitations in the leads.
11
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Remarks on the Hall Conductivity in Chiral Superconductors: Weak vs. Strong Coupling Approach

64%
Rogatko M., Wysokiński K.
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vol. 126
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issue 4a
A-9-A-13
EN
We discuss the calculations of the ac Hall conductivity for superconductors with the time reversal symmetry breaking states. In the weak coupling theories these states show vanishing Hall response in one band models, even though one expects otherwise on symmetry grounds. On the other hand, the strong coupling approach based on the anti-de Sitter-conformal field theory correspondence leads to the non-vanishing Hall conductivity. We discuss the possible reasons of the discrepancy. The weak coupling many orbital theory leading to the Hall conductivity with correct temperature dependence is also briefly presented.
12
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Thermoelectric Transport through the Double Quantum Dot in the Sequential Tunneling Regime

64%
Szukiewicz B., Wysokiński K.
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vol. 126
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issue 5
1159-1162
EN
The charge and spin thermoelectric transport in the system composed of a central molecule in contact with two normal electrodes in an external magnetic field of the Zeeman type have been studied. Such system can support pure spin current for applications in spintronics. By appropriate gate tuning of each of the dots it is possible to electrically control the direction of the spin current or tune the device operating as a thermoelectric generator to optimal performance.It has been shown that the device is poor energy converter in the parameter region where its thermoelectric figure of merit attains very large values.
13
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Inter-Orbital Effects in the Impure Pnictide Superconductors

64%
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.
14
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Off-Diagonal Nonmagnetic Disorder in s- and d-Wave Superconductors

64%
Michalik M., Wysokiński K.
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EN
The previously developed approach to study off-diagonal disorder has been extended and applied to both s- and d-wave superconductors. The derived equations allow the self-consistent solution of the problem. For a special case of negative U centres embedded in non-superconducting host we have calculated their critical concentration for the appearance of superconductivity and found it equal to≈0.4. For x>x_{cr} the order parameter continuously increases to its maximal value at x=1.
15
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Wave Functions of Disordered Two-Dimensional Electron Gas in Strong Magnetic Field

64%
Wysokiński K., Brenig W.
|
EN
The wave functions of the disordered two-dimensional electron gas (2DEG) placed in strong perpendicular magnetic field are calculated numerically. Even though the purpose of this work is mainly pedagogical, we shall shortly discuss our motivation which is possible application of the results to the calculations of the conductivity tensor in transition region between consecutive plateaus of the Hall conductivity in the quantum Hall effect.
16
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Statistics of Tunneling Events in Three-Terminal Hybrid Devices with Quantum Dot

52%
Michałek G., Bułka B., Domański T., Wysokiński K.
Acta Physica Polonica A
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2018
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vol. 133
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issue 3
391-393
EN
We investigate statistics of the tunneling events in the short time limit in terms of the waiting time distribution (WTD), defined as the probability for a delay time between two subsequent transitions of particles, and consider it for a quantum dot (QD) strongly coupled to a superconducting and weakly coupled to two normal electrodes. Our study focuses on the WTD in the subgap transport, when coherent exchange of the Cooper pairs occurs between the QD and the superconductor. The dynamics can be described in terms of a Markovian generalized master equation for the reduced density matrix. We observe coherent oscillations between the Andreev bound states in the correlated jumps, both for the local and non-local WTDs. In addition the analysis of the transient currents give us some insight into dominant relaxation processes in short time scales.
17
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Relativistic Theory of Superconductivity: BCS vs. Bogolyubov-de Gennes Approaches

52%
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.
18
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Tunneling through the Quantum Dot Coupled between Normal and Superconducting Leads

52%
Donabidowicz A., Domański T., Wysokiński K.
Acta Physica Polonica A
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2007
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vol. 112
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issue 2
157-160
EN
We study the charge tunneling via the quantum dot coupled to normal and superconducting leads, where the superconducting electrode has either an isotropic or anisotropic (of d-wave symmetry) energy gap. We use the single impurity Anderson model and apply the nonequilibrium Green function formalism to determine the differential tunneling conductance. The influence of the proximity effect between the quantum dot and superconductor on the transport properties of the system manifests itself in the Andreev conductance.
19
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Theoretical Analysis of the Short Coherence Length Superconductors

52%
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.
20
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Do Van Hove Singularities in Leads Influence Tunneling Current through Quantum Dot?

52%
Krawiec M., Domański T., Wysokiński K.
Acta Physica Polonica A
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1998
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vol. 94
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issue 3
411-414
EN
Using the Landauer type formula for the current through quantum dot we have studied the influence of the singularities in the density of states in leads on the differential conductance of the system and answered affirmatively the above question. The presence of Van Hove singularities in the leads may induce an additional structure on the conductance vs. voltage curve. The dot itself has been described by the single impurity Anderson model with a very strong "on-dot" Coulomb repulsion. We also simulate the effect of electron-phonon interaction in the dot by allowing for the fluctuations of the "impurity" energy level.
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