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1
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Exchange Interaction as a Source of Real Space Pairing in High Temperature Superconductors and Heavy Fermions: a Brief Overview

100%
Spałek J.
Acta Physica Polonica A
|
1994
|
vol. 85
|
issue 1
39-52
EN
We characterize the principal difference between the pairing induced by the kinetic exchange interactions for the Fermi-liquid and spin-liquid phases. In the case of hybridized electrons the Kondo-exchange induced pairing is invoked, whereas for a single narrow-band case kinetic exchange is responsible for the spin singlet pairing. The former interaction is applied to heavy-fermion systems, whereas the latter is employed to the high temperature superconductors. The current situation concerning the applicability of our theoretical models is briefly and critically assessed.
2
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Theory of Unconventional Superconductivity in Strongly Correlated Systems: Real Space Pairing and Statistically Consistent Mean-Field Theory - in Perspective

100%
Spałek J.
Acta Physica Polonica A
|
2012
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vol. 121
|
issue 4
764-784
EN
In this brief overview we discuss the principal features of real space pairing as expressed via corresponding low-energy (t-J or periodic Anderson-Kondo) effective Hamiltonian, as well as consider concrete properties of those unconventional superconductors. We also rise the basic question of statistical consistency within the so-called renormalized mean-field theory. In particular, we provide the phase diagrams encompassing the stable magnetic and superconducting states. We interpret real space pairing as correlated motion of fermion pair coupled by short-range exchange interaction of magnitude J comparable to the particle renormalized band energy ≈ tx, where x is the carrier number per site. We also discuss briefly the difference between the real-space and the paramagnon-mediated sources of superconductivity. The paper concentrates both on recent novel results obtained in our research group, as well as puts the theoretical concepts in a conceptual as well as historical perspective. No slave-bosons are required to formulate the present approach.
3
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t-J Model Then and Now: a Personal Perspective from the Pioneering Times

100%
Spałek J.
Acta Physica Polonica A
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2007
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vol. 111
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issue 4
409-424
EN
In this overview we sketch briefly the path to the so-called t-J model derived for the first time 30 years ago and provide its original meaning within the theory of strongly correlated magnetic metals with a non-Fermi (non-Landau) liquid ground state. An emergence of the concept of real space pairing is discussed in a historical prospective. A generalization of this model to the many-orbital situation is briefly discussed. The emphasis is put on didactical exposition of ideas, as they were transformed into mathematical language. The concept of hybrid pairing is introduced in the same context at the end.
4
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Kondo-Lattice Metallic and Semiconducting States and Their Instabilities

100%
Spałek J.
Acta Physica Polonica A
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2000
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vol. 97
|
issue 1
71-79
EN
We review briefly recent results concerning the stability of para-, ferro-, and antiferromagnetic states in the Kondo-lattice limit. Both the macroscopic quantities such as the specific heat and the electrical resistivity, as well as the magnitude of the magnetic moment and the mass enhancement (together with its spin dependence) are discussed.
5
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Luttinger Liquid as a New Metallic State of Correlated Fermions

64%
Byczuk K., Spałek J.
Acta Physica Polonica A
|
1996
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vol. 90
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issue 4
595-602
EN
This article provides an elementary introduction to the physics of the Luttinger liquid and summarizes its basic properties, as well as discusses examples, where such quantum liquid would be or has been observed experimentally. A comparison with the Fermi-liquid state is made
6
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Josephson Network as a Model for Inhomogeneous Superconductor: a Microwave Power Absorption

64%
Rycerz A., Spałek J.
Acta Physica Polonica A
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2007
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vol. 111
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issue 4
581-594
EN
We discuss the applied magnetic field dependence of the microwave absorption by a three-dimensional array of up to 30×30 ×30 Josephson junctions with random parameters including the resistivity, capacity, and inductance of each junction. The numerical simulation results for the networks show characteristic microwave absorption anomalies observed in the ceramic samples of high temperature superconductor YBa_2Cu_3O_{7-x}. We also provide a discussion of the absorption in simple analytical terms of the Josephson loop instabilities.
7
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Cooper Pair with Nonzero Momentum in System with Spin Dependent Mass of Quasiparticles

64%
Kaczmarczyk J., Spałek J.
Acta Physica Polonica A
|
2007
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vol. 111
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issue 4
595-602
EN
We consider a single Cooper pair with the spin dependent quasiparticle masses, with and without applied magnetic field. Such situation takes place for the strongly correlated electron systems, where a relatively strong Hubbard interaction differentiates the quasiparticle states in the majority- and the minority-spin subbands. In that situation, the two spin subbands in an applied magnetic field are not only shifted one with respect to the other, but also distorted differently, which results from the electronic correlations. Under these circumstances, the fermionic particles composing the Cooper pair, are quantum mechanically distinguishable. In result, the Cooper pair has a nonzero momentum (i.e. produces a stationary current in a similar manner as electron in the Bloch state), and there exists a critical value of the attractive interaction, below which the bound Cooper-pair state is not formed. The presence of the applied field (included via the Zeeman term) does not alter the picture qualitatively. Importance of these results for the formation of the Fulde-Ferrell-Larkin-Ovchinnikov phase in strongly correlated systems is mentioned.
8
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Anderson-Kondo Lattice Hamiltonian from the Anderson-Lattice Model: A Modified Schrieffer-Wolff Transformation and the Effective Exchange Interactions

64%
Kądzielawa-Major E., Spałek J.
|
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vol. 126
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issue 4a
A-100-A-104
EN
We derive the Anderson-Kondo lattice model by applying canonical perturbation expansion for the Anderson-lattice model in direct space. The transformation is carried out up to the fourth order by a modified Schrieffer-Wolff transformation: we separate the part of hybridization term responsible for the high-energy processes (involving the largest in-the-system intraatomic Coulomb interaction between f electrons) and replace it with the virtual processes in higher orders. The higher-order processes lead to three separate exchange interactions. The obtained Hamiltonian contains both the Kondo (f-c) and the superexchange (f-f) interactions, as well as a residual hybridization responsible for the heavy-quasiparticle formation. This effective Hamiltonian can be used to analyze the magnetic or the paired states, as well their coexistence in heavy-fermion systems. The magnitudes of both the Kondo exchange and the superexchange integrals are estimated as a function of bare hybridization magnitude.
9
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Universal Scaling of Basic Properties of the Heavy-Fermion Superconductors

64%
Karbowski J., Spałek J.
Acta Physica Polonica A
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1994
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vol. 85
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issue 2
341-345
EN
We show that the properties of the heavy-electron superconducting state induced by the interorbital kinetic exchange scale with the effective mass renormalization m*/m_{0} ~ 1/T_{K}. Explicitly, the pairing potential J̃ ~ J(m_{0}/m*)ln^{2}(m_{0}/m*), where J is the magnitude of the bare Kondo coupling; the coherence length ξ ~ T_{K}/T_{c} where T_{c} is the transition temperature, whereas the penetration depth λ ~ (m*/m_{0})^{½} so that λ/ξ >> 1. We also determine the scaling of magnetic critical fields.
10
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Strongly Correlated Electrons in a Magnetic Field: Fermi Liquid versus Spin Liquid

64%
Spałek J., Wójcik W.
Acta Physica Polonica A
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1994
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vol. 85
|
issue 2
357-361
EN
We compare the Fermi liquid (FL) and statistical spin liquid (SSL) representations of the electronic properties for strongly correlated systems. In particular, we discuss the entropy, as well as the magnetization as a function of band filling, temperature and magnetic field. These properties are quite different in the two (FL, SSL) states. Antiferromagnetic state disappears for a small number of holes in the doped Mott insulator.
11
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Application of Statistical Spin Liquid Concept to High-Temperature Superconductivity

64%
Byczuk K., Spałek J.
Acta Physica Polonica A
|
1994
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vol. 85
|
issue 2
337-340
EN
We apply the concept of statistical spin liquid, in which the doubly occupied quasimomentum configurations {|k⇅⟩} for quasiparticles are excluded from the Fock space, to a planar superconductor with real space pairing. The results compare very well with experimental data for the cuprates, namely (i) the condensed state appears only for the number of holes 0 < δ < 0.15-0.25; (ii) the temperature dependence of the gap is close to the BCS result, but the gap has systematically lower value and is of an extended s-wave form.
12
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Application of Pair Approximation to Bound-Magnetic-Polaron State in Diluted Magnetic Semiconductors

64%
Bednarski H., Spałek J.
Acta Physica Polonica A
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2012
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vol. 122
|
issue 6
1052-1055
EN
We present a fully quantum-mechanical model of the bound magnetic polaron state in a diluted magnetic semiconductor based on the pair approximation to the exchange interaction among localized magnetic moments (spins). The model is applicable also to the limiting situation of the bound magnetic polaron magnetization due to the spins approaching saturation. On its basis, a simple extension of the Dietl-Spałek bound magnetic polaron theory is proposed and compared with the data for p-Cd_{0.95}Mn_{0.05}Te.
13
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Kondo-Lattice Metals and Semiconductors with Compensated Magnetic Moments: a Brief Overview

64%
Spałek J., Doradziński R.
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vol. 96
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issue 5
677-688
EN
We provide a brief overview of recent theoretical results concerning the metallic and insulating states with very heavy quasiparticle masses and almost compensated magnetic moments, primarily in the antiferromagnetic state. The temperature dependence of the Kondo-insulator band gap is also discussed.
14
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Coexistence of Spin-Triplet Superconductivity with Antiferromagnetism in Orbitally Degenerate System: Hartree-Fock Approximation

64%
Zegrodnik M., Spałek J.
Acta Physica Polonica A
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2012
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vol. 121
|
issue 4
801-804
EN
We consider the coexistence of the Hund's-rule-exchange induced spin-triplet paired state with the antiferromagnetic ordering by starting from the extended Hubbard model for a doubly degenerate band. We use the density of states appropriate for the square lattice and treat the problem in the Hartree-Fock approximation. The temperature dependences of the superconducting gaps, the magnetic moment, and the chemical potential are presented. The free energy in the considered phase is evaluated, as well as the corresponding free energies in four additional phases: paramagnetic, ferromagnetic, superconducting of type A and superconducting of type A1 coexisting with ferromagnetism; they occur in the proper range of parameters: band filling n and the interaction parameters U/W and J/W. The low temperature values of the superconducting gaps and staggered magnetic moment are also analyzed as a function of band filling.
15
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Bound-Magnetic-Polaron Molecule in Diluted Magnetic Semiconductors within Heitler-London Approximation

64%
Bednarski H., Spałek J.
Acta Physica Polonica A
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2011
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vol. 120
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issue 5
967-969
EN
We determine the states of a pair of bound magnetic polarons in a diluted magnetic semiconductor by taking into account both the molecular nature of the two-electron wave function in the Heitler-London approximation, as well as the thermodynamic fluctuations of magnetization due to localized 3d spins. We extend the theory of a single bound magnetic polaron to molecular case.
16
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Kondo Insulating State in CeRhSb_{1-x}Sn_x as a Function of Carrier Number and Its Subsequent Metallization

64%
Ślebarski A., Spałek J.
Acta Physica Polonica A
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2007
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vol. 111
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issue 4
487-496
EN
A quantum critical point represents an essential singularity in the phase diagram of the electron-correlated compounds. In the CeRhSb_{1-x}Sn_x system we have observed a quantum criticality at the boundary Kondo insulator-non-Fermi liquid state. In this series of compounds, CeRhSb has a Kondo insulating ground state, whereas CeRhSn exhibits a non-Landau Fermi liquid behavior. In view of different types of behavior of CeRhSb and CeRhSn, we review the results of the solution in CeRhSb_{1-x}Sn_x and discuss the effect of a quasi-continuous change of the number of conduction electrons on the gap formation in the Kondo-insulating regime, as well as on the critical behavior appearance near the quantum critical point.
17
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Stabilization of Kondo Semiconductor State by Sb Doping of CeNi_{1-δ}Sn_{1+δ} and the General Criterion of Its Appearance

64%
Spałek J., Ślebarski A.
Acta Physica Polonica A
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2008
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vol. 114
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issue 1
7-14
EN
Semimetallic off-stoichiometric CeNi_{1-δ}Sn_{1+δ-x}Sb_x system with δ≈0.06 is shown to transform into a Kondo semiconductor upon the substitution of few percent of Sb for Sn. The full-gap formation is associated with f-electron localization induced by the combined effect of the collective Kondo-singlet formation and the atomic disorder. Namely, the extra valence electrons introduced with the Sb doping (one per Sb atom) contribute additionally to the formation of the collective Kondo spin-singlet state at low temperatures, as seen by a substantial reduction of the magnetic susceptibility. The precise general definition of the Kondo semiconductor is provided and the difference with either the simple band or the Mott-Hubbard insulators is stressed.
18
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Spin-Triplet Pairing Induced by Hund's Rule Exchange in Orbitally Degenerate Systems: Hartree-Fock Approximation

64%
Zegrodnik M., Spałek J.
Acta Physica Polonica A
|
2012
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vol. 121
|
issue 5-6
1051-1055
EN
We discuss the spin-triplet pairing mechanism induced by the Hund rule ferromagnetic exchange. We include explicitly the effect of interband hybridization and treat the problem by starting from an extended Hubbard model for a doubly degenerate band, making the simplest Hartree-Fock approximation for the part involving the pairing and the Hubbard interaction. The conditions of stability of various phases are determined as a function of both band filling and microscopic parameters. The phase diagram contains regions of stability of the spin-triplet superconducting phase coexisting with either saturated or non-saturated ferromagnetism. Phase diagrams for the cases of constant density of states and that of square lattice have been provided. The influence of hybridization on the stability of considered phases, as well as the temperature dependences of magnetic moment and the superconducting gap are also discussed.
19
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Unconventional Superconducting States of an Almost Localized Fermionic Liquid with Nonstandard Quasiparticles: Generalized Gutzwiller Approach

52%
Kaczmarczyk J., Jędrak J., Spałek J.
|
|
issue 2
261-266
EN
We apply the concept of generalized (almost localized) Fermi liquid and associated with it unconventional superconductivity with Cooper pairs composed of quasiparticles with the spin dependent masses (in an applied field) and with the effective field, both induced by electron correlations. The pairing among quasiparticles takes place either in reciprocal space (Sect. 2) or in real space (Sect. 3) and is induced by the kinetic exchange of either superexchange (in the generic narrow-band situation) or Kondo-type interaction (in the Kondo-lattice limit of the periodic Anderson model). While the main features of this type of Fermi liquid have been introduced earlier, we present here a picture which is applicable to both heavy-fermion and high-T_{c} superconductivity within a single narrow-band representation of correlated states. Our approach introduces a set of additional concepts (spin-dependent masses, effective fields induced by electron correlations), for which the Landau concept of the Fermi liquid represents still a workable scheme. In the limit of the Kondo lattice, we present the phase diagram incorporating the Fulde-Ferrell-Larkin-Ovchinnikov phase within the BCS-type of pairing, whereas in that of the t-J model we show that the proper choice of renormalization factors and constraints is crucial for the mean-field description of the superconducting state.
20
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Non-Fermi Liquid Behavior of Correlated Fermions in Applied Field

52%
Korbel P., Spałek J., Wójcik W.
Acta Physica Polonica A
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1997
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vol. 92
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issue 2
323-326
EN
We have determined an instability of the Fermi-liquid state of almost localized fermions in an applied magnetic field. It is proposed that a transition to a strongly correlated fermions (statistical-spin-liquid) state takes place at that point. The resultant magnetization curve and the field dependence of the specific heat are calculated and compared with those for CeRu_{2}Si_{2}.
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