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Effects of Coulomb Interactions Between Fermions on Superconductivity in Boson-Fermion Model

100%
Kostyrko T.
Acta Physica Polonica A
|
1997
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vol. 91
|
issue 2
399-403
EN
Influence of Coulomb repulsion between fermions on stability of the superconducting state in the model of boson-like local electron pairs hybridized with wide-band fermions is studied. It is shown rigorously that the Bose condensation of the local pairs implies a divergence of the pairing susceptibility in the fermion subsystem. Charge carrier concentration dependence of the critical temperature T_{c} is evaluated with the help of random phase approximation and spectral properties of the bosons at T_{c} are analyzed.
2
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From Weak to Strong Coupling Superconductivity: Collective Modes in a Model with Local Attraction

63%
Kostyrko T., Micnas R.
Acta Physica Polonica A
|
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.
3
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Ab Initio Analysis of a Quantum Dot Induced by a Local External Potential in a Semiconducting Carbon Nanotube

63%
Kostyrko T., Krompiewski S.
Acta Physica Polonica A
|
2009
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vol. 115
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issue 1
387-389
EN
Using the density functional theory we study the influence of external charge probes on the electronic structure of semiconducting carbon nanotubes in the vicinity of the Fermi level. We show that the spatially limited potential due to the probe can create localized electronic states in the energy gap and at the edges of the conductance band. By filling these localized states with additional electrons one obtains a quantum dot, which can be tuned by modifying the properties of the external charge probe. We analyze dependence of the electronic structure of the dot on the spatial extension of the potential as well as on the nanotube radius.
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