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1
Content available remote

Marshall-Peierls Sign Rule in Frustrated Heisenberg Chains

100%
Voigt A., Richter J.
Acta Physica Polonica A
|
2000
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vol. 97
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issue 5
979-982
EN
We consider the frustrated antiferromagnetic s=1 Heisenberg quantum spin chain with regard to the Marshall-Peierls sign rule. Using exact diagonalization data we investigate the breakdown of the Marshall- Peierls sign rule in dependence on frustration and compare our findings with data for s=1/2. We calculate a critical value of frustration J_{2}^{crit}, where the Marshall-Peierls sign rule is violated. The extrapolation of this value to the infinite chain limit holds J_{2}^{crit} ≈ 0.016, lower than in the case of s=1/2 (J_{2}^{crit} ≈ 0.027). T his points to a stronger influence of frustration in the case of s=1. Nevertheless, the calculation of the weight of the Ising-states violating the Marshall- Peierls sign rule shows that the latter holds approximately even for a quite large frustration and may be used for numerical techniques.
2
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J_{1}-J_{2}-J_{3} Model with Long-Range Lieb-Mattis Interaction

100%
Krüger S. E., Richter J.
Acta Physica Polonica A
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1997
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vol. 91
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issue 2
377-380
EN
Stimulated by the two-dimensional frustrated Heisenberg antiferromagnet with first-, second-, and third-neighbor couplings (J_{1}-J_{2}-J_{3} model) we consider a corresponding three-parameter model with a long-range antiferromagnetic Lieb-Mattis interaction. This model can be solved exactly and leads to a better understanding of the role of frustration in the J_{1}-J_{2}-J_{3} model. We calculate the correlations in the groundstate and consider their finite size behavior. Furthermore we present the full thermodynamic phase diagram. We find the possibility of a disordered phase at T=0.
3
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Numerical Study of Weak Ferromagnetism in 2D J_{1}-J_{2} Heisenberg Model with Dzyaloshinskii-Moriya Interaction

100%
Voigt A., Richter J.
Acta Physica Polonica A
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1997
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vol. 91
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issue 2
371-375
EN
By means of exact diagonalization we consider the groundstate of the J_{1}-J_{2} spin 1/2 Heisenberg model with an anisotropic interaction term of Dzyaloshinskii-Moriya type. We find that a Dzyaloshinskii-Moriya interaction may create a weak ferromagnetic moment. The interplay between the quantum nature of the groundstate of the pure J_{1}-J_{2} model and the anisotropic Dzyaloshinskii-Moriya interaction favour energetically a well-defined direction of spin alignment creating anisotropic correlations.
4
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Ferromagnetic Ground State of Two-Band Hubbard Model with One Electron per Site

100%
Schulenburg J., Richter J.
Acta Physica Polonica A
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1997
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vol. 91
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issue 2
367-370
EN
We study the multiband mechanism to favour a ferromagnetic insulating ground state within a two-band Hubbard model. Besides perturbation theory we use exact diagonalization studies to examine the ground state of chains and 2D-lattices. According to second order perturbation theory the exact diagonalization yields a fully polarized ferromagnetic ground state, if the hopping between two ground state orbitals of neighbouring atoms t_{gg} is small and the hopping between a ground state orbital and an excited orbital t_{ge} dominates. However, in contrast to the suggestion from the second order perturbation theory this ferromagnetic state is stable only for very small hopping integrals U≫ t_{ge}>t_{gg}. For larger t _{ge} quantum interference effects lead to complex magnetic structures.
5
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One-Dimensional Tasaki-Hubbard Model in Paramagnetic Limit

81%
Baliha V., Richter J., Derzhko O.
Acta Physica Polonica A
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2017
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vol. 132
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issue 4
1256-1260
EN
The one-dimensional Tasaki-Hubbard model belongs to the class of flat-band ferromagnets. By introducing an infinitesimally small external magnetic field, we examine the properties of the model in the paramagnetic regime. We compare and contrast them to the properties of the conventional Curie paramagnet.
6
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Localized-Magnon Chains and Interchain Interactions

81%
Krupnitska O., Richter J., Derzhko O.
Acta Physica Polonica A
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2017
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vol. 132
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issue 4
1234-1237
EN
We consider the spin-1/2 antiferromagnetic XXZ Heisenberg model on a two-dimensional network of weakly coupled frustrated diamond chains, i.e., a spin model related to the celebrated azurite compound. Using the strong-coupling approach we construct an effective Hamiltonian which yields the low-temperature high-field properties of the spin system. We examine effects of interchain interactions on localized-magnon physics.
7
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Spin-1/2 Periodic Nonuniform XX Chains and the Spin-Peierls Instability

81%
Derzhko O., Richter J., Zaburannyi O.
Acta Physica Polonica A
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2000
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vol. 97
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issue 5
931-934
EN
Using continued fractions we obtained the exact result for the density of magnon states of the regularly alternating spin-1/2 XX chain with the Dzyaloshinskii-Moriya interaction. We examined the stability of the magnetic chain with respect to the spin-Peierls dimerization.
8
Content available remote

Quantum Heisenberg Antiferromagnet on Frustrated Bilayer Lattice in Strong Magnetic Fields

81%
Derzhko O., Krokhmalskii T., Richter J.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
433-436
EN
In the present paper we consider the low-temperature thermodynamic properties of the quantum Heisenberg antiferromagnet on the frustrated bilayer lattice in strong magnetic fields.Under certain conditions the isolated localized magnon states (singlets on vertical bonds;simultaneous occupation of neighboring vertical bonds is forbidden) are the ground states which dominate thermodynamics and induce a behavior inherent in the well known model of hard squares on a square lattice.We focus on magnetic properties discussing the magnetization and the susceptibility of the spin model.
9
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Low-Temperature Properties of the Quantum Heisenberg Antiferromagnet on Some One-Dimensional Lattices Containing Equilateral Triangles

81%
Maksymenko M., Derzhko O., Richter J.
Acta Physica Polonica A
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2011
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vol. 119
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issue 6
860-862
EN
We consider the quantum Heisenberg antiferromagnet in a magnetic field on two one-dimensional lattices containing equilateral triangles (a chain of corner-sharing double tetrahedra and a frustrated three-leg ladder) which support localized-magnon states. By mapping of the localized-magnon degrees of freedom on a classical lattice gas we obtain high-field thermodynamic quantities of the models at low temperatures.
10
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Content available

The Ground-State of the Quantum Spin-1/2 Heisenberg Antiferromagnet on Square-Kagomé Lattice: Resonating Valence Bond Description

81%
Tomczak P., Molińska A., Richter J., Schulenburg J.
Acta Physica Polonica A
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2006
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vol. 109
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issue 6
781-787
EN
In this paper we report on some ground-state properties of the spin-1/2 Heisenberg antiferromagnet on the two-dimensional square-kagomé lattice. Finite N-spin systems were investigated with the use of the resonating valence bond method. Like in the case of spin system on kagomé lattice we find the almost flat dependence of mean singlet length on 1/N.
11
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Spin-1/2 XXZ Diamond Chain within the Jordan-Wigner Fermionization Approach

81%
Verkholyak T., Strečka J., Jaščur M., Richter J.
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issue 5
978-979
EN
The spin-1/2 XXZ diamond chain is considered within the Jordan-Wigner fermionization. The fermionized Hamiltonian contains the interacting terms which are treated within the Hartree-Fock approximation. We obtain the ground-state magnetization curve of the model for some particular cases and compare the results with the exact diagonalization data for finite chains of 30 spins and known exact results. We also analyze the validity of the suggested approximation.
12
Content available remote

Lieb-Mattis Model for Two Interpenetrating Antiferromagnets

81%
Schmalfuss D., Krüger S., Richter J.
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EN
We investigate a frustrated Lieb-Mattis-like spin-1/2 model that is a reference model for the corresponding square-lattice Heisenberg model describing the unusual magnetic properties of Ba_{2}Cu_{3}O_{4}Cl_{2}. Due to frustration we obtain a rich magnetic phase diagram. We find two critical temperatures in accordance with recent experiments on Ba_{2}Cu_{3}O_{4}Cl_{2}.
13
Content available remote

Diagonalization of a Bosonic Quadratic Form Using CCM: Application on a System with Two Interpenetrating Square Lattice Antiferromagnets

81%
Krüger S., Schmalfuss D., Richter J.
Acta Physica Polonica A
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2000
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vol. 97
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issue 5
951-954
EN
While the diagonalization of a quadratic bosonic form can always be done using a Bogolyubov transformation, the practical implementation for systems with a large number of different bosons is a tedious analytical task. Here we use the coupled cluster method to exactly diagonalise such complicated quadratic forms. This yields to a straightforward algorithm which can easily be implemented using computer algebra even for a large number of different bosons. We apply this method on a Heisenberg system with two interpenetrating square lattice antiferromagnets, which is a model for the quasi-2D antiferromagnet Ba_{2}Cu_{3}O_{4}Cl_{2}. Using a four-magnon spin wave approximation we get a complicated Hamiltonian with four different bosons, which is treated with coupled cluster method. Results are presented for magnetic ground state correlations.
14
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Heisenberg Antiferromagnet on a 1/7-Depleted Triangular Lattice

81%
Schulenburg J., Richter J., Betts D.
Acta Physica Polonica A
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2000
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vol. 97
|
issue 5
971-974
EN
Based on the triangular lattice and its depletions there are three simple frustrated antiferromagnetic Heisenberg models in two dimensions. The first two, the triangular and kagomé lattices, have been examined in the recent past. The triangular lattice seems to have a long range order whereas the kagomé does not show the long range order. But these results are still controversial. This work is concentrated on a third type of this lattice family in order to improve the understanding of the connection between the long range order and coordination number in low dimensional systems. Bets has described the geometric properties of this lattice. It has a coordination number 5, which lies precisely between coordination numbers 6 and 4 of the other two lattices. The low-lying spectra and the correlation functions of finite lattices have been examined to discuss the possibility of a long range ordered ground state in the 1/7-depleted triangular lattice. The low-lying spectrum is generated by an exact diagonalization, and the tower of states behavior points to a long range ordered ground state.
15
Content available remote

Magnetic Properties of the Hubbard Model on Kagome Stripes

71%
Derzhko O., Maksymenko M., Richter J., Honecker A., Moessner R.
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issue 5
736-737
EN
We consider the one-orbital N-site repulsive Hubbard model on two kagome like chains, both of which yield a completely dispersionless (flat) one-electron band. Using exact many-electron ground states in the subspaces with n ≤ n_{max} (n_{max} ∝ N) electrons, we calculate the square of the total spin in the ground state to discuss magnetic properties of the models. We have found that although for n < n_{max} the ground states contain fully polarized states, there is no finite region of electron densities n/{\cal N} < 1 ({\cal N} = N/3 or {\cal N} = N/5) where ground-state ferromagnetism survives for {\cal N} → ∞.
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