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1
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On the Crossover from BCS Superconductivity to Bose Condensation

100%
Micnas R.
Acta Physica Polonica A
|
2001
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vol. 100
|
issue Supplement
177-194
EN
We outline a microscopic approach to the superconducting fluctuations and pairing correlations in the attractive Hubbard model above T_c, using the functional integral method. A crossover from BCS superconductivity to Bose condensation of preformed pairs is studied by constructing the appropriate Ginzburg-Landau functionals. The differences between the lattice and the continuum models are discussed. The case of quasi-two-dimensional superconductors as well as the model with non-local pairing interaction are also examined. The effects of Gaussian fluctuations of the order parameter are analyzed in the T-matrix approach, the self-consistent Hartree approach and finally within the Ginzburg-Landau theory. Similarities with a paramagnon theory of itinerant-electron magnetism are pointed out.
2
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Bound States of Fermions on 2d Lattice in a Dilute Limit

64%
Bąk M., Micnas R.
Acta Physica Polonica A
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2000
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vol. 97
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issue 1
209-212
EN
We examine extended bound states in a dilute limit of the extended Hubbard model on the two-dimensional square lattice. By solving exactly the two-body problem we have determined the binding energies, mobilities, and dispersion curves across the Brillouin zone for bound states of various symmetries. It turns out that the d-wave pairing is strongly favored by the nnn hopping and the intersite local pairs can have small effective masses, even in the case of strong binding.We have also found a possibility of extended s-d_{x^{2}-y^{2}} mixing of the bound states.
3
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Some Properties of the Spin-Polarized Attractive Hubbard Model

64%
Kujawa A., Micnas R.
Acta Physica Polonica A
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2009
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vol. 115
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issue 1
138-140
EN
The influence of a Zeeman magnetic field on the superconducting characteristics of the attractive Hubbard model was investigated. The ground state and temperature phase diagrams were obtained for a fixed number of particles. Two critical magnetic fields were found for the first order phase transition from the superconducting to the normal state for n ≠ 1. For some range of parameters a reentrant transition was found and gapless superconductivity was obtained. The significance of the Hartree term was also analyzed.
4
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From Weak to Strong Coupling Superconductivity: Collective Modes in a Model with Local Attraction

64%
Kostyrko T., Micnas R.
Acta Physica Polonica A
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1993
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vol. 83
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issue 6
837-848
EN
We discuss the density-density response functions and the collective excitation spectrum for the extended Hubbard model with on-site attraction and arbitrary electron density, in the superconducting ground state. For the short-range intersite interaction we find the sound wave-like modes with the velocity interpolating smoothly between the weak (v ∝ 2Zt, |U| ≪ 2Zt) and strong coupling (v ∝ Zt^{2}/|U|,|U| ≫ 2Zt) limits. The latter agrees with the results obtained from an effective pseudospin Hamiltonian valid in the strong coupling limit. In the weak coupling regime, we obtain that apart from a commensurate charge density wave instability, an increase in the intersite Coulomb repulsion can give rise to a charge density wave incommensurate with the lattice period, away from half-filling. The ground state phase diagram for 2D square lattice, including singlet superconducting ground state, electronic droplet formation, and charge density waves is determined. It is also shown that the energy of collective excitations evolves smoothly from weak to strong coupling limit for a 2D lattice, in the case of a long-range Coulomb interaction. Finally, the mode coupling effects in the frequency behavior of the density-density response functions are analyzed.
5
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Superconductivity and Charge-Density Wave Phase in the Holstein Model: a Weak Coupling Limit

64%
Grzybowski P., Micnas R.
Acta Physica Polonica A
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2007
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vol. 111
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issue 4
453-463
EN
The variational canonical transformation method has been applied to the Holstein model to obtain an effective polaronic Hamiltonian, which is subsequently analyzed in the limit of a weak effective electron-electron interaction. A competition between the superconducting and charge-density wave phases has been studied in the light of strong polaronic effects. The phase diagrams illustrating the system evolution from adiabatic to anti-adiabatic limit are presented.
6
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On the Phase Diagram of Spin-Polarized Attractive Hubbard Model: Weak Coupling Limit

64%
Kujawa A., Micnas R.
Acta Physica Polonica A
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2008
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vol. 114
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issue 1
43-50
EN
The superfluid properties of the attractive Hubbard model in a Zeeman magnetic field, and in the weak coupling regime were investigated. The temperature and magnetic field dependences of the order parameter were analyzed. Furthermore, the temperature vs. magnetic field and temperature vs. spin polarization phase diagrams for the 2D and 3D lattices were obtained. For some parameters a reentrant transition was found.
7
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Phase Fluctuations and BCS-LP Crossover in 2D Short Coherence Length Superconductors

64%
Tobijaszewska B., Micnas R.
Acta Physica Polonica A
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2000
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vol. 97
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issue 3
393-397
EN
We study the effects of phase fluctuations in 2D short-coherence length superconductors on the basis of the extended Hubbard model with intersite attraction. In the Kosterlitz-Thouless scenario the critical temperatures for anisotropic pairings were determined and compared with the ones from BCS Hartree-Fock approximation. The Uemura-type plots, i.e. the critical temperature vs. zero temperature phase stiffness, were obtained for extended s-wave and d_{x^{2}-y^{2}}-wave pairings. We also discuss the crossover from BCS to local pair superconductivity for d-wave pairing.
8
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Superconductivity versus Diagonal Disorder in the (Hard-Core) Boson-Fermion Model

51%
Pawłowski G., Robaszkiewicz S., Micnas R.
Acta Physica Polonica A
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2004
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vol. 106
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issue 5
745-749
EN
The influence of randomness of local pair site energies on superconducting properties of a system of coexisting local pairs and itinerant electrons described by the (hard-core) boson-fermion model is analyzed within variational BCS Hartree-Fock approach. Assuming the rectangular distribution function of the random potential the phase diagrams, critical temperatures, and the chemical potential behaviours are determined.
9
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Conductivity of Strongly Correlated Bosons in the Simple Cubic Lattice - Magnetic Field and Hopping Anisotropy Effects

51%
Sajna A., Polak T., Micnas R.
Acta Physica Polonica A
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2015
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vol. 127
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issue 2
448-450
EN
We investigate optical conductivity in three dimensional system of bosons under strong magnetic field. In particular, we consider Bose Hubbard model in the strongly correlated limit, where Mott insulator phase emerges. For chosen rational number of magnetic flux per cell we show that response of the system gains complex peaks behavior on the order of frequency corresponding to on-site boson repulsive interaction. Moreover, when anisotropy in hopping energy for the direction parallel to magnetic field is tuned up, the non-monotonous behavior of the optical conductivity could appear. The obtained results can be experimentally probed in the system of ultracold atoms loaded on an optical lattice.
10
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Spin and Charge Current in a Short Semiconducting Chain of Paramagnetic Ionic Blocks

51%
Lehmann-Szweykowska A., Wojciechowski R., Micnas R.
Acta Physica Polonica A
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2009
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vol. 115
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issue 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.
11
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Superconductivity in Anderson Lattice Model

51%
Romano A., Noce C., Micnas R.
Acta Physica Polonica A
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1997
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vol. 91
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issue 2
381-385
EN
We study the superconducting instabilities generated by the inclusion in the Anderson lattice model of a density-density attractive potential between correlated electrons on nearest-neighbouring sites. Using a description of the normal phase based on a perturbative expansion around the atomic limit, we treat the attractive potential in the broken-symmetry Hartree-Fock scheme and analyze which of the possible symmetries of the superconducting order parameter leads to the highest possible transition temperature in the case of a two-dimensional square lattice. For values of the on-site f-repulsion large compared to the hopping amplitude, a suppression of any possible superconducting phase occurs, regardless of the symmetry of the order parameter.
12
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Unconventional Superconductivity, Spontaneous Time-Reversal Symmetry Breaking and Lowering of Kinetic Energy in Doped, Short-Range Antiferromagnets

45%
Wróbel P., Maciąg A., Eder R., Fulde P., Micnas R.
Acta Physica Polonica A
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2004
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vol. 106
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issue 5
575-581
EN
In this paper we focus on the anomalous temperature dependence of the in-plane conductivity and symmetry mixing of the superconducting order parameter observed in various experiments on cuprates. We show that the one-band Hubbard model is not capable of describing the physics of cuprates because the kinetic energy is lowered in this model in the superconducting state, which contradicts experimental observations. The proper model to investigate doped, short-range antiferromagnets is the t-J model, for which our results agree with experiments. We analyze a spin polaron model, that is an effective model for a doped antiferromagnet. In the framework of this model we also study the superconducting order-parameter symmetry-mixing phenomenon. We show that the expected mixing of d-wave symmetry with p-wave symmetry takes place in the superconducting order-parameter at a finite value of the doping parameter. This symmetry mixing brakes the time-reversal symmetry.
13
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Spin Polaron-Bipolaron Scenario of Three Gap-Like Energy Scales in Superconducting Cuprates

45%
Wróbel P., Maciąg A., Suleja W., Eder R., Micnas R.
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issue 2
292-298
EN
We argue that three gaps observed in underdoped cuprates can be attributed to the formation of antiferromagnetic spin polarons and bipolarons. Within the spin polaron scenario the antinodal pseudogap at he high energy scale originates from the change of the Fermi surface topology, induced by antiferromagnetic correlations. That change gives rise to the diminishing of the spectral weight at the antinodal region near the Brillouin zone boundary. We demonstrate that effect by analyzing effective models of doped antiferromagnets. The second type of pseudogap appearing at the intermediate energy scale originates from the phenomena which are precursory to superconductivity and predominantly concern the portion of the Fermi surface near the nodal region. In order to analyze the latter phenomenon we use the negative U Hubbard model, in which many details typical to spin polaron physics are neglected, but which contains the essential ingredient of it, that is the strong short range attraction. The lowest energy scale is related to the true superconducting gap which develops with doping, although both types of pseudogap diminish with doping. This behavior can be explained by the fact that the spin polaron band is empty in the undoped system and therefore the formation of the superconducting state in the system is forbidden. Due to a pedagogical character of this report, we present in the introduction a short overview of mostly recent experimental results which are related to the gap-pseudogap physics.
14
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Proceedings of the European Conference Physics of Magnetism 96. Preface.

39%
Krompiewski S., Micnas R., Fechner B., Kowalewski L., Morkowski J., Szajek A., Thomas M.
Acta Physica Polonica A
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1997
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vol. 91
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issue 1
3-6
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