Search results

1

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

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.

3

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.

4

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.

5

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.

6

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.

7

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.

8

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.

9

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.

10

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.

11

Ferromagnetic Systems with Hopping Interaction

63%

Górski G., Mizia J.

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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.

12

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.

13

Bound States of Fermions on 2d Lattice in a Dilute Limit

51%

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.

14

Anomalous Self-Energy Features in the 2D Hubbard Model

51%

Avella A., Mancini F.

Acta Physica Polonica A

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2008

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vol. 113

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issue 1

395-398

EN

The electronic self-energy of the two-dimensional Hubbard model, computed in the non-crossing approximation within the composite operator method, is studied as a function of frequency as well as temperature at the Fermi surface, for a value of doping at which the pseudogap is already well developed. For values of momentum belonging to the phantom arc of the Fermi surface, both dependences show anomalous power law behavior in contrast to that expected from the Fermi liquid theory.

15

Non-Fermi Liquid Behavior of Correlated Fermions in Applied Field

51%

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}.

16

Lieb-Wu Solution, Gutzwiller-Wave-Function, and Gutzwiller-Ansatz Approximations with Adjustable Single-Particle Wave Function for the Hubbard Chain

51%

Kurzyk J., Spałek J., Wójcik W.

Acta Physica Polonica A

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2007

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vol. 111

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issue 4

603-618

EN

The optimized single-particle wave functions contained in the parameters of the Hubbard model (t and U) were determined for an infinite atomic chain. In effect, the electronic properties of the chain as a function of interatomic distance R were obtained and compared for the Lieb-Wu exact solution, the Gutzwiller-wave-function approximation, and the Gutzwiller-ansatz case. The ground state energy and other characteristics for the infinite chain were also compared with those obtained earlier for a nanoscopic chain within the exact diagonalization combined with an ab initio adjustment of the single-particle wave functions in the correlated state (exact diagonalization combined with an ab initio method). For the sake of completeness, we briefly characterize also each of the solutions. Our approach completes the Lieb-Wu solution, as it provides the system electronic properties evolution as a function of physically controlable parameter - the interatomic distance.

17

Surprises in Correlated Electron Physics

51%

Byczuk K., Hofstetter W., Kollar M., Vollhardt D.

Acta Physica Polonica A

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2007

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vol. 111

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issue 4

549-561

EN

Strong electronic correlations and especially the interplay between correlations and disorder lead to many interesting and quite unexpected phenomena. A short summary of our recent investigations into the properties of strongly correlated electron systems with and without disorder using the dynamical mean-field theory is presented.

18

Nonfrustrated Antiferromagnet in a Frustrated Lattice Due to Charge Ordering: A Scenario for Pairing in Layered Cobaltates

51%

Wróbel P., Suleja W.

Acta Physica Polonica A

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2007

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vol. 111

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issue 4

563-572

EN

The mechanism of pairing-symmetry selection in the weakly electron doped t-J model on the honeycomb lattice has been analyzed. The discussion of that problem has been motivated by some recent suggestions that due to charge ordering which may take place in the unconventional superconductor Na_xCoO_2· yH_2O at doping levels near x=1/3 the physics of CoO_2 planes may be effectively described in terms of a model for a weakly electron doped antiferromagnet on the honeycomb lattice. In the current publication the main emphasis has been put on reviewing experimental and theoretical work, the results of which support the scenario of charge order. In the calculation, the so-called string picture has been used. It has been demonstrated that spin fluctuations may induce in the honeycomb lattice the formation of an unconventional two-particle bound state. Upon the condensation of bound particles this mechanism may give rise to unconventional pairing. The critical value of the ratio J/t which is sufficient to induce binding has not been evaluated. It has been assumed instead that in the case of cobaltates some additional isotropic attractive interaction, for example phonon mediated, is active. Despite that the exchange of spin fluctuations is not a dominating interaction, it selects the symmetry of the paired state because it is anisotropic. C_{3v} is the relevant point group for the t-J model on the honeycomb lattice. It has been shown that the bound state of two additional electrons doped to the half-filled antiferromagnetically ordered system has zero total momentum and p-wave symmetry of the irreducible representation E. The expected paired state is a mixture of a singlet and a triplet because the honeycomb lattice does not possess the inversion symmetry.

19

Study of the Mott Transition in the Three-Dimensional Hubbard Model

51%

Mancini F., Turkowski V.

Acta Physica Polonica A

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2002

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vol. 101

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issue 4

505-523

EN

The Mott-Hubbard transition in the three-dimensional Hubbard model is studied by means of the composite operator method. Analytical and numerical calculations show the existence of a critical value U_c of the Coulomb interaction which separates paramagnetic insulating and metallic phases. In the composite operator method the ratio U_c/W, where W is the band width, has the same value as in the two-dimensional case. The ground state of the Mott insulator is characterized by a local antiferromagnetic order where the electrons keep some mobility, but this latter must be compatible with the local ordering. By analyzing the intersite hopping it is possible to establish that some correlation functions behave as order parameters in the insulating phase.

20

Extended Hubbard Model in the Dimer Representation. II. Lattice Hamiltonian in the Large U Limit

51%

Grabiec B., Matlak M.

Acta Physica Polonica A

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2002

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vol. 101

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issue 4

549-557

EN

Using the exact decomposition of the sc lattice into a set of interacting dimers (each dimer is described by the extended Hubbard Hamiltonian) and exact solution of the dimer problem (preceding paper) we exactly find the form of the extended Hubbard model in the case of a crystal in the large U limit. We apply a new, nonperturbative approach based on the exact projection procedure onto a dimer subspace occupied by electrons in this limit (it is the only assumption). The resulting Hamiltonian is very complicated and contains a variety of multiple magnetic and nonmagnetic interactions deeply hidden in its original form (site representation). We also present a simplified version of the model to better visualize a mixture of different interactions resulting from this approach.

Refine search results

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

Magnetic Properties of Metallic Impurities with Strongly Correlated Electrons

Next-Nearest-Neighbor Hopping in the Falicov-Kimball Model

Mott Transition, Ferromagnetism and Conductivity in the Generalized Hubbard Model

Checkerboard-Like Charge Ordering in Underdoped Cuprates

Unconventional Superconductivity, Spontaneous Time-Reversal Symmetry Breaking and Lowering of Kinetic Energy in Doped, Short-Range Antiferromagnets

Antiferromagnetic Order in the Hubbard Model: Spin-Charge Rotating Reference Frame Approach

Dynamical Mean Field within Iterative Perturbation Theory

Ground-State Spin of Hubbard Ladder Model with Infinite Electron Repulsion

The Energy Spectrum of Single-Hole States in Transition Metal Oxides

Ferromagnetic Systems with Hopping Interaction

Ferromagnetism in Hubbard Chains

Bound States of Fermions on 2d Lattice in a Dilute Limit

Anomalous Self-Energy Features in the 2D Hubbard Model

Non-Fermi Liquid Behavior of Correlated Fermions in Applied Field

Lieb-Wu Solution, Gutzwiller-Wave-Function, and Gutzwiller-Ansatz Approximations with Adjustable Single-Particle Wave Function for the Hubbard Chain

Surprises in Correlated Electron Physics

Nonfrustrated Antiferromagnet in a Frustrated Lattice Due to Charge Ordering: A Scenario for Pairing in Layered Cobaltates

Study of the Mott Transition in the Three-Dimensional Hubbard Model

Extended Hubbard Model in the Dimer Representation. II. Lattice Hamiltonian in the Large U Limit