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  1. 104 Acta Physica Polonica A

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1
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The Role of the Exchange Interaction in the One-Dimensional n-Component Hubbard Model

100%
Szirmai E., Legeza Ö., Sólyom J.
Acta Physica Polonica A
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2009
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vol. 115
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issue 1
98-100
EN
The commensurate p/q-filled n-component Hubbard chain was investigated by bosonization and high-precision density-matrix renormalization-group analysis. It was found that depending on the relation between the number of components n, and the filling parameter q, the system shows metallic or insulating behavior, and for special fillings bond-ordered (dimerized, trimerized, tetramerized etc.) ground state develops in the insulating phase. A mean-field analysis shows that this bond ordering is a direct consequence of the spin-exchange interaction, which plays a crucial role in the one-parameter Hubbard model - not only for infinite Coulomb repulsion, but for intermediate values as well.
2
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The Energy Spectrum of Single-Hole States in Transition Metal Oxides

63%
Lehmann-Szweykowska A., Wojciechowski R., Gehring G.
Acta Physica Polonica A
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2000
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vol. 97
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issue 3
563-566
EN
The p-d hybridised single-hole states of the transition-metal-oxygen tetrahedron (TMO_{4}) are collectivised due to the direct p-p hopping between oxygens of different clusters. The lowest-lying energy band is always narrow and fully occupied. The first excited band gets occupied as an effect of valence-uncompensated doping, so it can be almost localised. The possible hole excitations to the two higher energy bands, which are wider, may imply the Mott-like hopping form of charge transport in these systems.
3
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Ferromagnetic Systems with Hopping Interaction

63%
Górski G., Mizia J.
Acta Physica Polonica A
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2010
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vol. 118
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issue 2
336-339
EN
We analyze the influence of hopping interaction on magnetic ordering. Scattering scheme of the Hubbard III approximation with included inter-site kinetic electron-electron correlation is used. The hopping interaction and inter-site correlation lead to two spin dependent effects: the band width correction and the band-shift correction. The band-shift correction factor causes an exchange splitting between the spin-up and spin-down spectrum, and its role is similar to the exchange interaction in the classic Stoner model. The spin dependent band width correction enhanced strongly by the inter-site kinetic correlation lowers the kinetic energy of electrons by decreasing the majority spin band width for some electron occupations with respect to the minority spin band width. The results show that in the case of the symmetrical density of states there is only ferromagnetic enhancement. For the strongly asymmetrical density of states there is a ferromagnetic transition.
4
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Ferromagnetism in Hubbard Chains

63%
Cuoco M., Noce C., Romano A.
Acta Physica Polonica A
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1997
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vol. 91
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issue 2
387-390
EN
We analyze the possible occurrence of ferromagnetism in the Hubbard model, by means of an exact diagonalization study performed on 3- to 8-site chains with periodic boundary conditions. In the case of one hole in the half-filled configuration, we find that the Nagaoka state is reached only in the 3- and in the 4-site case. Ground states characterized by unsaturated ferromagnetism are found when the case of more than one hole is considered.
5
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Next-Nearest-Neighbor Hopping in the Falicov-Kimball Model

63%
Czajka K., Maśka M.
Acta Physica Polonica A
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2006
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vol. 109
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issue 4-5
465-469
EN
Results of Monte Carlo simulations for the spinless Falicov-Kimball model with the next-nearest-neighbor hopping are presented. We find the critical value of the next-nearest-neighbor hopping integral, below which the low temperature configuration of the localized particles is the same as in the presence of only the nearest-neighbor hopping. Beyond this critical value the localized particles form horizontal or vertical stripes.
6
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Mott Transition, Ferromagnetism and Conductivity in the Generalized Hubbard Model

63%
Skorenkyy Y., Didukh L., Kramar O., Dovhopyaty Y.
Acta Physica Polonica A
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2007
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vol. 111
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issue 4
635-644
EN
The electron correlations in narrow energy bands are examined within the framework of the Hubbard model, generalized by taking into account the correlated hopping of electrons. Electronic conductivity and ferromagnetic ordering stabilization in the system with various forms of electronic density of states are studied. The influence of magnetic field, temperature and the form of density of states on concentration dependence of conductivity and magnetization is investigated. The correlated hopping is shown to cause the electron-hole asymmetry of transport and ferromagnetic properties of narrow band materials.
7
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Unconventional Superconductivity, Spontaneous Time-Reversal Symmetry Breaking and Lowering of Kinetic Energy in Doped, Short-Range Antiferromagnets

63%
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.
8
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Checkerboard-Like Charge Ordering in Underdoped Cuprates

63%
Wróbel P.
Acta Physica Polonica A
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2006
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vol. 109
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issue 4-5
533-539
EN
Recent scanning tunneling microscopy measurements which indicate the formation of two-dimensional density modulations at some doping levels in cuprates were reviewed. A model of hard-core bosons which represent bound hole pairs in the two-dimensional doped antiferromagnet was discussed in the context of these experimental results. By means of an exact numerical diagonalization it was shown that the Coulomb repulsion between bosons brings about the formation of charge modulations.
9
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Antiferromagnetic Order in the Hubbard Model: Spin-Charge Rotating Reference Frame Approach

63%
Zaleski T., Kopeć T.
Acta Physica Polonica A
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2008
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vol. 114
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issue 1
247-251
EN
We study the antiferromagnetic phase of three-dimensional Hubbard model with nearest neighbors hopping on a bipartite cubic lattice. We use the quantum SU(2)×U(1) rotor approach that yields a fully self-consistent treatment of the antiferromagnetic state that respects the symmetry properties of the model and satisfies the Mermin-Wagner theorem. As our theory describes the evolution from a Slater (U ≪ t) to a Mott-Heisenberg (U ≫ t) antiferromagnet, we present the phase diagram of the antiferromagnetic Hubbard model as a function of the crossover parameter U/t.
10
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Dynamical Mean Field within Iterative Perturbation Theory

63%
Radzimirski B., Wojciechowski R.
Acta Physica Polonica A
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2007
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vol. 111
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issue 5
753-761
EN
We consider the iterative-perturbation theory and its generalization to multi-orbital Hubbard models. We discuss in detail all aspects of numerical implementations of the method.
11
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Ground-State Spin of Hubbard Ladder Model with Infinite Electron Repulsion

63%
Cheranovskii V., Ezerskaya E., Klein D., Tokarev V.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
916-918
EN
We apply perturbation theory and cyclic spin permutation formalism to study the lowest energy states of the infinite-repulsion Hubbard model on n-leg ladders with alternating values of one-site energies α_{i} for neighboring rungs. We establish the "ferromagnetic" character of ladder ground-state at electron densities in the interval 1 - (2n)¯¹ ≤ ρ ≤ 1 and sufficiently large alternation of one-site energies of neighbor rungs of the ladder. We also show the stability of this state against the small deviations of the values of α_{i} in contrast to the case of two-leg ladder formed by weakly interacting neighbor rungs with equal one-site energies.
12
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Magnetic Properties of Metallic Impurities with Strongly Correlated Electrons

63%
Janiš V., Ringel M.
Acta Physica Polonica A
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2009
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vol. 115
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issue 1
30-35
EN
We study the single impurity Anderson model in an external magnetic field. There are no exact results for the spectral function in this situation. Using a resummation of the diagrammatic expansion we demonstrate that the strong coupling regime in a weak magnetic field is Kondo-like with a quasiparticle resonant peak split into two. We find two exponentially small Kondo scales (temperatures), one for transverse and one for longitudinal spin fluctuations. We show that the salient features of the spectral function in the Kondo regime can be seen already within an extended random phase approximation. To reveal the dependence of the Kondo scales on the bare electron interaction, however, one has to employ a two-particle self-consistency with renormalized vertices. We use the parquet approach to derive the dependence of the Kondo scales on magnetic field.
13
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Critical Exponents for Metal-Insulator Transition in Two-Dimensional Systems

51%
Wojtkiewicz J.
Acta Physica Polonica A
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2009
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vol. 115
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issue 5
931-934
EN
Three years ago, a new universality class, associated with the metal-insulator transition in quasi-two-dimensional compounds, was discovered. Imada has given explanation of the observed critical behaviour. Considerations in this work are based on the assumption that the one-particle dispersion is quartic one instead of the standard quadratic behaviour. In this paper, it is shown that other possible natural non-standard dispersions lead to other possible critical behaviour and critical exponents.
14
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On the Phase Diagram of Spin-Polarized Attractive Hubbard Model: Weak Coupling Limit

51%
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.
15
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The Fock Term in the Charge Ordered Phase of the t-W Model

51%
Bak M.
Acta Physica Polonica A
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2012
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vol. 121
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issue 5-6
1085-1088
EN
The paper shows the importance of retaining Hartree and Fock terms, appearing after mean-field decoupling, in the treatment of charge ordered phase of the t-W model, with implications to extended Hubbard model.
16
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Band Magnetism with Inter-Site Correlations and Interactions

51%
Mizia J., Górski G., Kucab K.
Acta Physica Polonica A
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2014
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vol. 126
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issue 4a
A-30-A-32
EN
We introduce the Hamiltonian to describe narrow band electrons. The physics of driving forces towards ferromagnetism is re-examined. Using different approximations it has been shown that the magnetic moments created by inter-site interaction and inter-site kinetic correlation decrease quickly with temperature. As a result of these interactions and the realistic density of states the Curie temperatures calculated after fitting magnetic moments to their low temperature values are realistic. In the past the Curie temperatures calculated using only the on-site interaction were much higher than the experimental temperatures.
17
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Kondo-Lattice Metallic and Semiconducting States and Their Instabilities

51%
Spałek J.
Acta Physica Polonica A
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2000
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vol. 97
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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.
18
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Electron Spectral Functions in the Presence of the Antiferromagnetic Order in the Two-Dimensional Hubbard Model

51%
Zaleski T., Kopeć T.
Acta Physica Polonica A
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2010
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vol. 118
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issue 2
267-272
EN
We use a recently proposed quantum SU(2)×U(1) rotor approach for the Hubbard model to calculate electronic spectral functions in a presence of an antiferromagnetic state for any value of the Coulomb interaction. We isolate the collective variables for charge and spin in the form of the space-time fluctuating U(1) phase field and SU(2) rotating spin quantization axis, respectively. As a result, the fermion Green function in the space-time domain becomes a product of a CP^1 propagator resulting from the SU(2) gauge fields, U(1) phase propagator and the pseudo-fermion correlation function. In turn, the spectral lines are obtained by performing the convolution of spin, charge and pseudo-fermion Green's functions. We observe an emergence of a sharp peak in the electron spectral functions within the antiferromagnetic phase, whose spectral weight is equal to the antiferromagnetic order parameter.
19
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Spin Polarons in Weakly Doped Antiferromagnets: Experimental Evidence Obtained for Cuprates

51%
Wróbel P., Suleja W., Eder R.
Acta Physica Polonica A
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2008
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vol. 114
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issue 1
51-58
EN
The mechanism of spin polaron formation in moderately doped cuprates is discussed. These objects represent holes embedded into heavy clouds formed by spin fluctuations. Wave functions of spin polarons are spatially confined due to the increase in the exchange energy which is induced by hole motion giving rise to the creation of spin fluctuations. These wave functions are eigenstates of an "unperturbed" Hamiltonian which is defined by processes responsible for the tendency toward confinement. The eigenstates transform according to different irreducible representations of the point group reflecting the symmetry of the problem. Thus, the spin polarons being local wave functions resemble orbital states. The spectrum of optical conductivity in the mid-infrared range is determined by transitions between s-wave and p-wave spin polarons. The hybridization between different spin polarons which is induced by some high order processes gives rise to the formation of energy bands. The pronounced transfer of the spectral weight between different bands is induced by the coupling between spin fluctuations created by the hopping hole and local quantum fluctuations in the empty antiferromagnetic background from which an electron has been rejected, for example during the photoemission process. The form of the energy dispersion for spin polarons gives rise to the formation of a small Fermi surface at the low hole-doping range. These three above mentioned phenomena were observed in cuprates which seems to confirm the spin polaron scenario. The discussion of related experiments is the additional objective of this paper.
20
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Periodic Anderson Model with d-f Interaction

51%
Hagymási I., Itai K., Sólyom J.
Acta Physica Polonica A
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2012
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vol. 121
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issue 5-6
1070-1072
EN
We investigate an extended version of the periodic Anderson model where an interaction is switched on between the doubly occupied d- and f-sites. We perform variational calculations using the Gutzwiller trial wave function. We calculate the f-level occupancy as a function of the f-level energy with different interaction strengths. It is shown that the region of valence transition is sharpened due to the new interaction.
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