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Strona główna » Instytut » Pracownicy » Karol Izydor Wysokiński

prof. dr hab. Karol Izydor Wysokiński

Zakład

Zakład Teorii Fazy Skondensowanej

Stanowisko:

profesor zwyczajny

Kontakt:

pokój: 308
telefon: (081) 537-62-36
e-mail: karol.wysokinski@umcs.pl
strona www: http://serwisy.umcs.lublin.pl/karol.wysokinski

Specjalność naukowa:

fizyka teoretyczna

Funkcje pełnione w IF UMCS:

Kierownik Zakładu

Publikacje: Ukryj abstrakty

  1. K. I. Wysokiński,A. Ciechan,J. Krzyszczak, Theoretical analysis of the short coherence length superconductors, Acta Physica Polonica A, 118(), 2010, 232-237

  2. K. I. Wysokiński, Thermal transport of molecular junctions in the pair tunneling regime, Physical Review B, 82(), 2010, 115423(1-6)

  3. A. Ciechan,K. I. Wysokiński, Inter-orbital effects in the impure pnictide superconductors, Acta Physica Polonica A, 118(), 2010, 356-359

  4. J. Krzyszczak,T. Domański,K. I. Wysokiński, Inhomogeneities in superconductors described by the two-component model, Acta Physica Polonica A, 118(), 2010, 360-363

  5. J. Krzyszczak,T. Domański,K. I. Wysokiński,R. Micnas,S. Robaszkiewicz, Real space inhomogeneities in high temperature superconductors: the perspective of the two-component model, Journal of Physics: Condensed Matter, 22(), 2010, 255702(1-11)

  6. K. I. Wysokiński, J. F. Annett, B. L. Gyorffy, Orbital-dependent pairing effects in the nuclear spin-lattice relaxation rate of Sr2Ru04, Superconductor Science and Technology, 21(), 2009, 014009 (1-7)

  7. A. Ciechan, K. I. Wysokiński, Interorbital pair scattering in two-band superconductors, Physical Review B, 80(), 2009, 224523(1-9)

  8. J. F. Annett, B. L. Gyorffy, K. I. Wysokiński, Orbital magnetic moment of a chiral p-wave superconductor, New Journal of Physics, 11(), 2009, 055063(1-12)

  9. Ciechan, A. Wysokiński, K., Pairing correlations around negative-U centre in the two-band model, Acta Physica Polonica A, 114(), 2008, 123-127

  10. Annett, J. Gorffy, B. Litak, G. Wysokiński, K., Magnetic field induced rotation of the d-vector in the spin-triplet superconductor Sr2RuO4, Physical Review B, 78(), 2008, 054511(1-6)

  11. Krzyszczak, J. Domański, T. Wysokiński, K., Real space Bogoliubov-de Gennes equations study of the boson-fermion model, Acta Physica Polonica A, 114(), 2008, 165-169

  12. Domański, T. Donabidowicz, A. Wysokiński, K., Meservey-Tedrow-Fulde effect in quantum dot embedded between metallic and superconducting electrodes, Physical Review B, 78(), 2008, 144515(1-6)

  13. Domanski, T. Donabidowicz, A. Wysokinski, K.I., Influence of the correlation effects on charge transport through quantum dots, Physica Status Solidi B-Basic Solid State Physics, 244(7), 2007, 2437-2442

    Abstract:
    We study the effect of the Coulomb interactions on a charge transport through a single level quantum dot. The excitation spectrum is at low temperatures characterized by a narrow Kondo resonance (the low energy feature) formed on top of the line broadening (high energy part of the spectrum). We investigate mutual interplay between these low and high energy parts and discuss their influence on the transport phenomena for a semi-equilibrium situation. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  14. Wysokinski, K.I. Litak, G. Annett, J.F. Gyorffy, B.L., Superconducting phases in strontium ruthenate, Physica Status Solidi B-Basic Solid State Physics, 244(7), 2007, 2415-2420

    Abstract:
    We review our recent work concerning the search for the superconducting state realised in Sr2RuO4. The orbital symmetry of the order parameter in this spin triplet superconductor is widely believed to be chiral one with d-vector along the crystal c-axis. The three band, three dimensional model with two different phenomenological interaction parameters is able to explain most of the existing data in quantitative way. This is also true for the recently observed "rotation" of the d-vector in magnetic field directed along c-axis. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  15. Litak, G. Wysokinski, K.I., Magnetic field orientation effect on specific heat in a p-wave superconductor, Physica Status Solidi B-Basic Solid State Physics, 244(7), 2007, 2492-2496

    Abstract:
    The effect of magnetic field on specific heat of a p-wave spin triplet superconductor has been analyzed. To describe superconductivity we used a single band model with various realizations of p-wave order parameter and analyzed the effect of a Doppler shift in different temperatures. (c) 2007 WILEY-NCH Verlag GmbH & Co. KGaA, Weinheim.

  16. Ciechan, A. Wysokinski, K.I., Thermodynamics of the two-level Richardson model, Acta Physica Polonica A, 111(4), 2007, 467-473

    Abstract:
    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 x 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.

  17. Donabidowicz, A. Domański, T. Wysokiński, K.I., Tunneling through the quantum dot coupled between normal and superconducting leads, Acta Physica Polonica A, 112(), 2007, 157-160

  18. Wysokiński, K.I., Transport in nanostructures. Recent developments, Materials Science-Poland, 25(), 2007, 459-465

  19. Annett, J.F. Litak, G. Gyorffy, B.L. Wysokiński, K.I., Magnetic field induced rotation of the d-vector in Sr2RuO4, Physica C, 460-462(), 2007, 995-996

  20. Krawiec, M. Wysokiński, K.I., Thermoelectric phenomena in a quantum dot asymmetrically coupled to external leads, Physical Review B, 75(), 2007, 155330 (1-6)

  21. Domański, T. Donabidowicz, A. Wysokiński, K.I., Influence of pair coherence on charge tunneling through a quantum dot connected to a superconducting lead, Physical Review B, 76(), 2007, 104514 (1-9)

  22. Krawiec, M. Wysokinski, K. I., Thermoelectric effects in strongly interacting quantum dot coupled to, Physica B-Condensed Matter, 378-80(), 2006, 933-934

    Abstract:
    We study thermoelectric effects in Kondo correlated quantum dot coupled to ferromagnetic electrodes by calculating thermopower S in the Kondo regime as function of on-dot energy level and temperature. The system is represented by the Anderson model and the results agree well with those recently measured for a quantum dot coupled to nonmagnetic leads. For magnetic electrodes, one observes marked dependence of S on the degree of their polarisation. (c) 2006 Elsevier B.V. All rights reserved.

  23. Krawiec, M. Wysokinski, K. I., Thermoelectric effects in strongly interacting quantum dot coupled to ferromagnetic leads, Physical Review B, 73(7), 2006, art. no.-075307

    Abstract:
    We study thermoelectric effects in a Kondo correlated quantum dot coupled to ferromagnetic electrodes by calculating conductance, thermopower, and thermal conductance in the Kondo regime. We also study the effect of the asymmetry in the coupling to the leads, which has important consequences for antiparallel magnetization configuration. We discuss the thermoelectric figure of merit, tunnel magnetoresistance, and violation of the Wiedemann-Franz law in this system. The results agree with recently measured thermopower of the quantum dot defined in a two-dimensional electron gas.

  24. Ciechan, A. Wysokinski, K. I., Finite size corrections in the two-level BCS model, Acta Physica Polonica A, 109(4-5), 2006, 569-572

    Abstract:
    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 proportional to N-1/3 seems to be the expansion parameter. The obtained results have been compared with those of other authors.

  25. Michalik, M. Wysokinski, K. I., Off-diagonal nonmagnetic disorder in s- and d-wave superconductors, Acta Physica Polonica A, 109(4-5), 2006, 623-626

    Abstract:
    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 approximate to 0.4. For x > x(cr) the order parameter continuously increases to its maximal value at x = 1.

  26. Annett, J. F. Litak, G. Gyorffy, B. L. Wysokinski, K. I., Spin-orbit coupling and symmetry of the order parameter in strontium ruthenate, Physical Review B, 73(13), 2006, art. no.-134501

    Abstract:
    Determination of the orbital symmetry of the pairing state in the spin triplet Sr2RuO4 superconductor is a challenge of considerable importance. Most of the experiments show that the chiral state of the z(k(x)+/- ik(y)) type is realized and remains stable on lowering the temperature. Here we have studied the stability of various superconducting states of Sr2RuO4 in the presence of spin-orbit coupling. Numerically we found that the chiral state is never the minimum energy. Alone among the five states studied it has < L center dot S >=0 and is therefore not affected to linear order in the coupling parameter lambda. We found that the stability of the chiral state requires spin-dependent pairing interactions. This imposes strong constraints on the pairing mechanism.

  27. Mierzynska, M. Wysokinski, K. I., The isotope effect in disordered strongly coupled superconductors, Physica Status Solidi B-Basic Solid State Physics, 243(3), 2006, 666-676

    Abstract:
    The isotope effect in disordered strongly coupled superconductors has been studied. Weakly disordered bulk s- and d-wave superconductors as well as strongly disordered homogeneous and granular thin films have been considered. To treat strong coupling s-wave superconductors Eliashberg theory has been used and the weak localization effects on isotope coefficient (IC) calculated within this approach. The interesting point is that weak localization effects known to properly describe degradation of transition temperature of s- wave superconductors have been found to decrease the isotope coefficient of impure material with respect to pure samples. This is in contrast to the strong increase of IC induced by magnetic impurities in s-wave or nonmagnetic impurities in d- wave spin singlet superconductors. We have also calculated the change of isotope coefficient for both homogeneous and granular thin films and found that the effect of disorder in granular materials on the isotope coefficient is similar to that observed in high-temperature superconductors. (c) 2006 WILEY- VCH Verlag GmbH & Co. KGaA, Weinheim.

  28. Krawiec, M. Wysokinski, K. I., Electron transport through a strongly interacting quantum dot coupled to a normal metal and BCS superconductor, Superconductor Science & Technology, 17(1), 2004, 103-112

    Abstract:
    We study electron transport through a quantum dot coupled to a normal metal and BCS-like superconductor (N-QD-S) in the presence of the Kondo effect and Andreev scattering. The system is described by the single impurity Anderson model in the limit of strong on-dot interaction. To study electron transport, we use the recently proposed equation of motion technique for the Keldysh non-equilibrium Green's function together with the modified slave boson approach. We derive the formula for the current which contains various tunnelling processes and we apply it to a study of the transport through the system. We find that the Andreev conductance is strongly suppressed and there is no zero-bias (Kondo) anomaly in the differential conductance. We discuss the effects of the particle-hole asymmetry in the electrodes as well as the asymmetry in the couplings.

  29. Mierzynska, M. Wysokinski, K. I., The isotope effect of disordered superconductors, Acta Physica Polonica A, 106(5), 2004, 647-652

    Abstract:
    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 Eliash-berg 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.

  30. Michalik, M. Wysokinski, K. I., CPA theory of superconducting alloys with diagonal and off- diagonal disorder, Acta Physica Polonica A, 106(5), 2004, 653-658

    Abstract:
    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.

  31. Ciechan, A. Donabidowicz, A. Wysokinski, K. I., Relativistic theory of superconductivity: BCS vs. Bogolyubov-de Gennes approaches, Acta Physica Polonica A, 106(5), 2004, 699-702

    Abstract:
    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.

  32. Litak, G. Annett, J. F. Gyorffy, B. L. Wysokinski, K. I., Horizontal line nodes in superconducting Sr2RuO4, Physica Status Solidi B-Basic Research, 241(5), 2004, 983-989

    Abstract:
    We analyze the possibilities of triplet pairing in Sr2RuO4 based upon an idea of interlayer coupling. We have considered two models differing by the effective interactions. In one model the quasi-particle spectra have horizontal line nodes on all three Fermi surface sheets, while in the other the spectra have line or point nodes on the a and sheets and no nodes on the gamma sheet. Both models reproduce the experimental heat capacity and penetration depth results, but the calculated specific heat is sightly closer to experiment in the second solution with nodes only on the alpha and beta sheets. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  33. Domanski, T. Ranninger, J. Wysokinski, K. I., Disorder induced changes of d-wave pairing amplitude in the boson fermion model, Acta Physica Polonica B, 34(2), 2003, 493-496

    Abstract:
    We investigate a system composed of the itinerant electrons coexisting with the localized pairs (hard-core bosons) whose energies are assumed to be site dependent. The randomness of boson energies induces in turn fluctuations of the effective pairing potential between fermions. We analyze an effect of such randomness on the anisotropic c d-wave superconducting phase of this model. In particular, we determine transition temperature T-c and the amplitude of the order parameter parameter X-ij = as functions of the boson energies fluctuations.

  34. Annett, J. F. Gyorffy, B. L. Litak, G. Wysokinski, K. I., Gap nodes and time reversal symmetry breaking in strontium ruthenate, European Physical Journal B, 36(3), 2003, 301-312

    Abstract:
    We study the superconducting state of Sr2RuO4 on the bases of a phenomenological but orbital specific description of the electron-electron attraction and a realistic quantitative account of the electronic structure in the normal state. We found that a simple model which features both 'in plane' and 'out of plane' coupling with strengths U-parallel to = 40 meV and U-perpendicular to = 48 meV respectively reproduced the experimentally observed power law behaviour of the low temperature specific heat C-v(T), super fluid density n(s)(T) and thermal conductivity in quantitative detail. Moreover, it predicts that the quasi-particle spectrum on the gamma-sheet is fully gaped and the corresponding order parameter breaks the time reversal symmetry. We have also investigated the stability of this model to inclusion of further interaction constants in particular 'proximity coupling' between orbitals contributing to the gamma sheet of the Fermi surface and the alpha and beta sheets. We found that the predictions of the model are robust under such changes. Finally, we have incorporated a description of weak disorder into the model and explored some of its consequences. For example we demonstrated that the disorder has a more significant effect on the f-wave component of the order parameter than on the p-wave one.

  35. Mierzynska, M. Wysokinski, K. I., Isotope effect in impure high-Tc superconductors, Physical Review B, 68(1), 2003, art. no.-014504

    Abstract:
    The influence of various kinds of impurities on the isotope shift exponent alpha of high-temperature superconductors has been studied. In these materials the dopant impurities, like Sr in La2-xSrxCuO4, play different roles and usually occupy different sites than impurities like Zn, Fe, Ni, etc., intentionally introduced into this system to study its superconducting properties. In this paper the in-plane and out- of-plane impurities present in layered superconductors have been considered. They differently affect the superconducting transition temperature T-c. The relative change of isotope shift coefficient, however, is a universal function of T-c/T-c0 (T-c0 refers to an impurity-free system), i.e., for angle independent scattering rate and density of states function it does not depend on whether the change of T-c is due to in- or out-of-plane impurities. The role of the anisotropic impurity scattering in changing oxygen isotope coefficient of superconductors with various symmetries of the order parameter is elucidated. The comparison of the calculated and experimental dependence of alpha/alpha(0), where alpha(0) is the clean system isotope shift coefficient, on T-c/T-c0 is presented for a number of cases studied. The changes of alpha calculated within the stripe model of superconductivity in copper oxides reasonably well describe the data on La1.8Sr0.2Cu1-x(Fe,Ni)(x)O-4, without any fitting parameters.

  36. Wysokinski, K. I. Litak, G. Annett, J. F. Gyorffy, B. L., Spin triplet superconductivity in Sr2RuO4, Physica Status Solidi B-Basic Research, 236(2), 2003, 325-331

    Abstract:
    Sr2RuO4 is at present the best candidate for being a superconducting analogue of the triplet super-fluidity in He-3. This material is a good (albeit correlated) Fermi liquid in the normal state and an exotic superconductor below T-c. The mechanism of superconductivity and symmetry of the order parameter are the main puzzling issues of on-going research. Here we present the results of our search for a viable description of the superconducting state realised in this material. Our calculations are based on a three-dimensional effective three-band model with a realistic band structure. We have found a state with non-zero order parameter on each of the three sheets of the Fermi surface. The corresponding gap in, the quasi-particle spectrum has line or point nodes on the alpha and beta sheets and is complex with no nodes on the gamma sheet. This state describes remarkably well a number of existing experiments including power low temperature dependence of the specific heat, penetration depth, thermal conductivity etc. The stability of the state with respect to disorder and different interaction parameters are also analyzed briefly.

  37. Michalik, M. Wysokinski, K. I., Charge transport and metal-insulator transition in correlated and disordered systems, Physica Status Solidi B-Basic Research, 236(2), 2003, 372-375

    Abstract:
    The metal-insulator transition induced by sufficiently strong short range repulsive electron-electron interaction U is called Mott-Hubbard transition. In this work we study the effect of disorder on the critical value of the repulsion U-c. The Hubbard model with random site energies to describe the correlated and disordered system has been used. The Kotliar- Ruckenstein slave-boson technique to treat correlations and coherent potential approximation (CPA) to deal with disorder have been applied. We have found that the low temperature conductivity is an increasing or decreasing function of U depending on the carrier concentration. The critical value U-c for metal-insulator transition is decreasing function of the disorder strength.

  38. Domanski, T. Wysokinski, K. I., Effect of disorder on superconductivity in the boson-fermion model, Physical Review B, 66(6), 2002, art. no.-064517

    Abstract:
    We study how a randomness of either boson or fermion site energies affects the superconducting phase of the boson fermion model. We find that, contrary to what is expected for s-wave superconductors, the nonmagnetic disorder is detrimental to the s-wave superconductivity. However, depending on which subsystem the disorder is located, we can observe different channels being affected. Weak disorder of the fermion subsystem is mainly responsible for the renormalization of the single- particle density of states, while disorder in the boson subsystem leads directly to fluctuations of the strength of the effective pairing between fermions.

  39. Krawiec, M. Wysokinski, K. I., Nonequilibrium Kondo effect in asymmetrically coupled quantum dots, Physical Review B, 66(16), 2002, art. no.-165408

    Abstract:
    A quantum dot asymmetrically coupled to external leads has been analyzed theoretically by means of the equation-of-motion technique and the noncrossing approximation. The system has been described by the single-impurity Anderson model. To calculate the conductance across the device the nonequilibrium Green's function technique has been used. The obtained results show the importance of the asymmetry of the coupling for the appearance of the Kondo peak at nonzero voltages and qualitatively explain recent experiments.

  40. Annett, J. F. Litak, G. Gyorffy, B. L. Wysokinski, K. I., Interlayer coupling and p-wave pairing in strontium ruthenate, Physical Review B, 66(13), 2002, art. no.-134514

    Abstract:
    On the basis of a three-orbital model and an effective attractive interaction between electrons, we investigate the possible superconducting states, with p- and f-wave internal symmetries, of Sr2RuO4. For an orbital-dependent interaction which acts between in-plane and out-of-plane nearest-neighbor ruthenium atoms, we find a state for which the gap in the quasiparticle spectra has nodes on the alpha and beta sheets of the Fermi surface, but which is complex with no nodes on the gamma sheet. We show that this state is consistent with the available experimental data. In particular, we present the results of our calculations of the specific heat and penetration depth as functions of temperature.

  41. Wysokinski, K. I., Introduction to quantum Hall effects, Acta Physica Polonica A, 100(3), 2001, 287-299

    Abstract:
    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 (rho (xy)) of the resistivity tensor accompanied by simultaneous vanishing rho (xy) for ranges of the magnetic field. For the integer quantum Hall effect rho (xy) = h/ve(2), where h is the Planck constant, e - charge of an electron and v is an integer, while for the fractional quantum Hall effect v 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.

  42. Krawiec, M. Wysokinski, K. I., Charge on the quantum dot in the presence of tunneling current, Solid State Communications, 115(3), 2000, 141-144

    Abstract:
    The calculation of the charge present in central region of the double barrier structure at nonequilibrium conditions is discussed. We propose here a simple method to calculate nonequilibrium Green's functions which allows consistent calculations of retarded and distribution functions. To illustrate the approach, we calculate the charge on the quantum dot coupled via tunnel barriers to two external leads having different chemical potentials mu(L) and mu(R). The obtained results have been compared with other approaches existing in the literature. They all agree in the equilibrium situation and the departures grow with increasing the difference mu(L) - mu(R). (C) 2000 Published by Elsevier Science Ltd. All rights reserved.

  43. Krawiec, M. Wysokinski, K. I., Spectral functions of the quantum dot coupled to normal and/or superconducting leads, Acta Physica Polonica A, 97(1), 2000, 197-200

    Abstract:
    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.

  44. Domanski, T. Wysokinski, K. I., Superconducting phases in the presence of Coulomb interaction: From weak to strong correlations, Physical Review B, 59(1), 1999, 173-176

    Abstract:
    We study the evolution of the superconducting phases in a model with competing short-range attractive (W) and on-site repulsive (U) interactions. The influence of the correlations on the phase diagram of s- and d-wave superconductors is studied by means of various approximations. The knowledge of the normal state correlation effects allows the tracking of the influence of U on the superconductivity in correlated systems. We have found the qualitatively different response of the superconducting phases in systems with WU-cr, when the system remains superconducting for arbitrarily large U. [S0163-1829(99)07701-2].

  45. Krawiec, M. Wysokinski, K. I., Superconductivity in correlated systems: Constraint quantization of slave bosons, Physical Review B, 59(14), 1999, 9500-9507

    Abstract:
    The quantization rules obtained by Le Guillou and Ragoucy in the slave particle theories of correlated systems have been applied to study the superconductivity in extended Hubbard and t-J models. The quantization scheme has the virtue of treating the local constraint exactly. We compare our numerical results with those obtained in the mean-field approximation for usual slave boson technique. We also discuss the relation of the present approach with method of equation of motion for Hubbard operators. Where possible the obtained results are also compared with experimental data on high-temperature superconductors, as in these materials the essential physics is believed to depend on the presence of strong correlations. [S0163-1829(99)11913-1].

  46. Wysokinski, K. I., Spectral and transport properties of strongly correlated disordered systems, Physical Review B, 60(24), 1999, 16376-16381

    Abstract:
    It is shown that strong electron-electron interactions in disordered systems lead to the enhancement of intrinsic disorder. The one-band Hubbard model with random on-site energies is studied in the infinite U limit. Electron correlations are treated in the mean-field slave boson technique. To average over disorder, both intrinsic (diagonal) and interaction induced (off-diagonal), we use a version of the coherent potential approximation. We illustrate the approach by calculating density of states, ac electrical conductivity, and effective carrier concentration. The obtained results are discussed in the context of recent experimental data on disordered strongly correlated materials. [S0163-1829(99)05243- 1].

  47. Wysokinski, K. I., On the interplay of disorder and correlations, Journal of Low Temperature Physics, 117(3-4), 1999, 235-239

    Abstract:
    I address here the question of the mutual interplay of strong correlations and disorder in the system. I consider random version of the Hubbard model. Diagonal randomness is introduced via random on-site energies and treated by the coherent potential approximation. Strong, short ranged, electron electron interactions are described by the slave boson technique and found to induce additional disorder in the system. As an example I calculate the density of states of the random interacting binary alloy and compare it with that for non-interacting system.

  48. Martin, A. M. Litak, G. Gyorffy, B. L. Annett, J. F. Wysokinski, K. I., Coherent potential approximation for d-wave superconductivity in disordered systems, Physical Review B, 60(10), 1999, 7523-7535

    Abstract:
    A coherent-potential approximation CPA is developed for s-wave and d-wave superconductivity in disordered systems. We show that the CPA formalism reproduces the standard pair breaking formula, the self-consistent Born approximation and the self- consistent T-matrix approximation in the appropriate limits. We implement the theory and compute T-c for s-wave and d-wave pairing using an attractive nearest-neighbor Hubbard model featuring both binary-alloy disorder and a uniform distribution of scattering site potentials. We determine the density of states and examine its consequences for low-temperature heat capacity. [S0163-1829(99)02034-2].

  49. Krawiec, M. Domanski, T. Wysokinski, K. I., Do Van Hove singularities in leads influence tunneling current through quantum dot?, Acta Physica Polonica A, 94(3), 1998, 411-414

    Abstract:
    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 Rove 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.

  50. Litak, G. Martin, A. M. Gyorffy, B. L. Annett, J. F. Wysokinski, K. I., Van Hove singularity and d-wave pairing in disordered superconductors, Physica C-Superconductivity and Its Applications, 309(3-4), 1998, 257-262

    Abstract:
    We apply the coherent potential approximation (CPA) to a simple model for disordered superconductors with d-wave pairing and demonstrate that whilst the effectiveness of an electronic Van Hove singularity to enhance the transition temperature T-c is reduced by disorder it is not eliminated. In fact we give a qualitative account of changes in the T-c vs. doping curve with increasing disorder and compare our results with experiments on the Y0.8Ca0.2Ba2(Cu(1-y)Zny)(3)O7-delta alloys. (C) 1998 Published by Elsevier Science B.V. All rights reserved.

  51. Litak, G. Gyorffy, B. L. Wysokinski, K. I., Charge and order parameter fluctuations in disordered superconductors, Physica C, 308(1-2), 1998, 132-146

    Abstract:
    The impurity induced fluctuations in the superconducting system have been studied. We have used the single band negative U Hubbard model to find the response of the charge and superconducting order parameters to the local fluctuations of the on-site energies. The second central moment(i.e., the width) of the corresponding distribution of the order parameter and/or local charge have been related to the corresponding moment of the impurity distribution. This defines a kind of a 'response' function. We performed the explicit calculations of this response function assuming weak scattering limit and two- dimensional lattice with nearest and next-nearest neighbour hopping. Special attention has been paid to study of the half- filled (n = 1) particle-hole symmetric band where previous analyses have shown vanishing of the disorder induced fluctuations both in the weak scattering limit and near Anderson metal-insulator transition. The gap parameter starts to fluctuate even for n = 1 but with asymmetric band or/and in systems exhibiting both superconducting and charge density wave order. (C) 1998 Elsevier Science B.V. All rights reserved.

  52. Wysokinski, K. I., Symmetry of the order parameter in correlated electron-phonon superconductor, Physica C, 282(), 1997, 1701-1702

    Abstract:
    In this work we study the possible symmetries of the superconducting order parameter by analysing the kernels of the recently derived Eliashberg equations. It is the pecularity of correlated systems that Eliashberg equations become very anisotropic, with two different kernels. One of them describes the normal and other superconducting state properties. The numerical data obtained for optical phonons indicate the possibility of the d-wave super-coductivity at low (less than or equal to 0.3) hole doping. The influence of the band structure effects (e.g. next nearest neighbour hopping t') is also discussed. The effect of t' strongly depends on its sign. For sign opposite to that of nearest neighbour hopping t the possibility of d-wave order parameter is enhanced, while for the same it is reduced and the s-wave symmetry becomes favourable for all carrier concentrations.

  53. Wysokinski, K. I., On strong coupling theory of correlated superconductors, Acta Physica Polonica A, 91(2), 1997, 329-332

    Abstract:
    The strong coupling or Eliashberg equations of superconductivity have been generalised to describe strongly correlated systems. Calculations show that d-wave superconductivity is stable for small hole doping. It also follows that in correlated systems one should define three different Eliashberg functions and corresponding electron- phonon coupling constants lambda(N), lambda(S) and lambda(tr), (to describe normal(N), superconducting (S) and transport (tr) properties) each of which is measured in different kind of experiment.

  54. Domanski, T. Wysokinski, K. I. Ramakumar, R., Superconductivity in a strongly correlated one-band system, Physical Review B, 54(5), 1996, 3058-3061

    Abstract:
    We study superconductivity in the extended Hubbard model with a strong on-site repulsion U and weak attractive density-density interaction V-ij. The electron correlation effects, beyond the weak-coupling regime of U, are included approximately by employing Hubbard decoupling approximations. The formulas for superconducting transition temperature T-c of the present approach are compared to those obtained with the strong- coupling expansion and slave-boson techniques. The methods share some common features. We discuss behavior of T-c for superconducting phases with s- and d-wave symmetry of the superconducting order parameter. In contrast to the mean-field treatment of U, we find that the strong on-site repulsion does not destroy superconductivity induced by V-ij < 0 even when V much greater than \V-ij\.

  55. Wysokinski, K. I., Electron-phonon induced superconductivity in correlated systems, Czechoslovak Journal of Physics, 46(), 1996, 601-602

    Abstract:
    The issue of electron - phonon induced superconductivity in strongly correlated systems has been discussed. To treat correlation and strong coupling effects we use slave boson method in the U = infinity limit and Eliashberg type of theory. Short range Coulomb interactions severally modify Eliashberg equations making possible s, p and d - wave symmetry of the order parameter. At hole dopings delta less than or equal to 0.3 the d - wave component of the coupling constant is the largest one.

  56. Wysokinski, K. I., Eliashberg-type equations for correlated superconductors, Physical Review B, 54(5), 1996, 3553-3561

    Abstract:
    The derivation of the Eliashberg-type equations for a superconductor with strong electron correlations and electron- phonon interaction has been presented. The proper account of short range Coulomb interactions results in strongly anisotropic equations. Possible symmetries of the order parameter include a, p, and d wave. We found the carrier concentration dependence of the coupling constants corresponding to these symmetries. At low hole doping the d- wave component is the largest one.


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