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1
Content available remote

Magnetic Exchange Interactions and Estimation of T_N in CsNiF_3 from First Principles

100%
Legut D., Rusz J.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
503-506
EN
The CsNiF_3 has been for long time studied as a prototype of quasi-1D planar ferromagnetic system. At very low temperature the studied system is insulator and therefore the magnetic exchange interactions should decay very rapidly. We treated the magnetic exchange coupling within the Heisenberg model for the nearest neighbor interaction between the anti-ferromagnetically coupled Ni-chains. The influence of up to the second-nearest neighbors on the ferromagnetic exchange coupling along the Ni-chains was determined. The exchange interactions were calculated for the experimental volume by the density functional theory, within the all-electron approach using the local density approximation for the exchange and correlation. The Néel temperature was calculated by means of the mean-field theory and by the random-phase approximation method.
2
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Thermodynamics of the Exactly Solvable Spin-Electron Tetrahedral Chain

100%
Gálisová L.
Acta Physica Polonica A
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2015
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vol. 128
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issue 2
156-158
EN
An exactly solvable spin-electron tetrahedral chain, where the Ising spins localized at nodal lattice sites regularly alternate with three equivalent lattice sites available for one mobile electron is considered. The system with ferromagnetic interaction between the Ising spins and electrons exhibits an enhanced magnetocaloric effect in the limit H/|J| → 0 when the entropy is very small, whereas the system with antiferromagnetic interaction between the Ising spins and electrons exhibits an enhanced magnetocaloric effect around the field H/J=1 when the entropy is sufficiently close to the value S/2N = ln[(1+√5)/2]. We study the thermodynamics of the system in these field regions.
3
Content available remote

Spin-1/2 XXZ Diamond Chain within the Jordan-Wigner Fermionization Approach

100%
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.
4
Content available remote

Ground State, Magnetization Process, and Magnetocaloric Effect of the Exactly Tractable Spin-Electron Tetrahedral Chain

80%
Gálisová L., Strečka J.
Acta Physica Polonica A
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2015
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vol. 127
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issue 2
216-218
EN
A hybrid spin-electron system on one-dimensional tetrahedral chain, in which the localized Ising spin regularly alternates with the mobile electron delocalized over three lattice sites, is exactly investigated using the generalized decoration-iteration transformation. The system exhibits either the ferromagnetic or antiferromagnetic ground state depending on whether the ferromagnetic or antiferromagnetic interaction between the Ising spins and mobile electrons is considered. The enhanced magnetocaloric effect during the adiabatic demagnetization suggests a potential use of the investigated system for low-temperature magnetic refrigeration.
5
Content available remote

Spin Chain in Magnetic Field: Limitations of the Large-N Mean-Field Theory

80%
Wohlfeld K., Chen C.-C., van Veenendaal M., Devereaux T.
Acta Physica Polonica A
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2015
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vol. 127
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issue 2
201-203
EN
Motivated by the recent success in describing the spin and orbital spectrum of a spin-orbital chain using a large-N mean-field approximation, we apply the same formalism to the case of a spin chain in the external magnetic field. It occurs that in this case, which corresponds to N=2 in the approximation, the large-N mean-field theory cannot qualitatively reproduce the spin excitation spectra at high magnetic fields, which polarize more than 50% of the spins in the magnetic ground state. This, rather counterintuitively, shows that the physics of a spin chain can under some circumstances be regarded as more complex than the physics of a spin-orbital chain.
6
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Thermal Entanglement and Quantum Non-Locality along the Stepwise Magnetization Curve of the Spin-1/2 Ising-Heisenberg Trimerized Chain

80%
Strečka J., Pavličko J.
Acta Physica Polonica A
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2017
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vol. 132
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issue 1
167-169
EN
The spin-1/2 Ising-Heisenberg trimerized chain in a magnetic field is revisited with the aim to explore the quantum entanglement and non-locality within the exactly solved spin system, which exhibits in a low-temperature magnetization curve two intermediate plateaux at zero and one-third of the saturation magnetization. The ground-state phase diagram involves two quantum (antiferromagnetic, ferrimagnetic I) and two classical (ferrimagnetic II, saturated paramagnetic) phases. We have rigorously calculated the concurrence and Bell function in order to quantify the quantum entanglement and non-locality at zero as well as non-zero temperatures. It is demonstrated that the entanglement can be thermally induced also above the classical ground states unlike the quantum non-locality, which means that the thermal entanglement is indispensable for a violation of the locality principle.
7
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The Hilbert Space and the Exact Solution of the Spin-Electron Double-Tetrahedral Chain

80%
Jakubczyk D., Gálisová L.
Acta Physica Polonica A
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2015
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vol. 128
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issue 2
173-175
EN
We discuss an exactly solvable double-tetrahedral chain, in which the localized Ising spins regularly alternate with triangular clusters - each available for one mobile electron. We give an exact solution for the considered hybrid system by the construction of the N-th tensor power of the Fourier transformation and using the generalized decoration-iteration mapping tranformation.
8
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Some Exact Results in the One-Dimensional Attractive Hubbard Model

80%
Jakubczyk D.
Acta Physica Polonica A
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2015
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vol. 128
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issue 2
170-172
EN
The one-dimensional attractive Hubbard model (Ułl 0) is discussed for the chains of N nodes and the same number of electrons, where N-1 of them have the same spin projection, assuming periodic boundary conditions and the half-filling case. Based on the analysis of the eigenvalue problem we provided the general analytical expression for the eigenvalues, for any number N. This formula implies the existence of two elementary particles with mutually dependent momenta on the ring with N sites the same number of electrons including N-1 of the same spin projection.
9
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Kinetically-Driven Frustration in Hybrid Spin Ladders

80%
Carvalho R., Pereira M., Lyra M., Rojas O., Strecka J.
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vol. 126
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issue 1
12-13
EN
We introduce a spin ladder model incorporating localized and delocalized spins, which is exactly solvable using the decoration-iteration and transfer-matrix techniques. Quantum correlations between delocalized spins induce an antiferromagnetic coupling between the spin chains that competes with a direct ferromagnetic exchange coupling. A resulting kinetically-driven frustration is predominant in the regime of low temperatures and large hopping amplitudes. We provide the full ground-state phase diagram, as well as the frustration diagram on distinct coupling regimes.
10
Content available remote

Magnetocaloric Effect in NiCl₂(bipy) at Low Temperatures

80%
Ráczová K., Čižmár E., Feher A.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
922-924
EN
Magnetothermal properties of the compound NiCl₂(bipy), where bipy = 4,4'-bipyridine, were studied. The single-ion anisotropy of Ni(II) ions was estimated and its influence on magnetocaloric effect in NiCl₂(bipy) was investigated. Above the metamagnetic transition a maximum in the isothermal change of the magnetic entropy is reached near 11 K at field change from 0T to 7T with peak value -ΔS = 6J K¯¹ kg¯¹. The temperature dependence of ΔS above 7K agrees with the assumed easy-axis type of single-ion anisotropy. The inverse magnetocaloric effect, typical for ordered antiferromagnets, was observed in the ordered state of NiCl₂(bipy) only for isothermal magnetization in small fields up to 2T.
11
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Comparison of the Multi-Band Hubbard and Heisenberg Models on Small Finite Clusters

80%
Matysiak J., Lemański R.
Acta Physica Polonica A
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2018
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vol. 133
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issue 3
405-407
EN
We perform exact numeric calculations for a few versions of the two-orbital Hubbard model on the four site cluster. We show that the conventional spin 1 Heisenberg Hamiltonian can be obtained in the strong coupling limit, but only for some versions of the two-orbital Hubbard model. Then we propose a modified version of the Heisenberg Hamiltonian, that we call the two-orbital Heisenberg model, which is relevant in those cases, where the conventional spin 1 Heisenberg model fails.
12
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The Magnetocaloric Effect in {[Cu(bapa)]₃[Cr(CN)₆]₂}ₙ · 6n H₂O at Low Temperatures

80%
Ráczová K., Vavra M., Hegedüs M., Čižmár E.
Acta Physica Polonica A
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2018
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vol. 133
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issue 3
429-431
EN
The crystals of {[Cu(bapa)]₃[Cr(CN)₆]₂}ₙ·6nH₂O (bapa = bis(3-aminopropyl)amine) are formed by infinite Cu(II)-Cr(III) antiparallel chains, which are connected into the third direction by additive [Cu(bapa)] moieties. The onset of long-range magnetic order at 3.2 K was observed by AC susceptibility. The study of the magnetocaloric effect from magnetization measurements yields a large entropy change -ΔS_{M} = 13.65 J K¯¹mol¯¹ (-ΔS_{M} = 12.25 J kg¯¹K¯¹) at the field change from 0 T to 3 T at temperature 4 K. The analysis of the critical behavior of -ΔS_{M} suggests the value of critical exponent n = 0.577 at the ordering temperature characteristic for three-dimensional magnets with Ising anisotropy.
13
Content available remote

Low-Temperature Thermodynamics of Spin-1/2 Orthogonal-Dimer Chain with Ising and Heisenberg Interactions

80%
Verkholyak T., Strečka J.
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vol. 126
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issue 1
22-23
EN
We consider an exactly solvable version of the quantum spin-1/2 orthogonal-dimer chain with the Heisenberg intra-dimer and Ising inter-dimer couplings. The investigated quantum spin system exhibits at zero temperature fractional plateaux at 1/4 and 1/2 of the saturation magnetization and it has a highly degenerate ground state at critical fields where the magnetization jumps. We study the field dependence of the specific heat at low temperature. The lattice-gas description is formulated in a vicinity of critical fields to explain the low-temperature behaviour of specific heat.
14
Content available remote

Frustration-Driven Quantum Phase Transition in the 1D Extended Anisotropic Heisenberg Model

80%
Plekhanov E., 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
429-432
EN
By using density matrix renormalization group technique we study the 1D extended anisotropic Heisenberg model. We find that starting from the ferromagnetic phase, the system undergoes two quantum phase transitions induced by frustration. By increasing the next-nearest-neighbor interaction, the ground state of the system changes smoothly from a completely polarized state to a next-nearest-neighbor correlated one. On the contrary, letting the in-plane interaction to be greater than the out-of-plane one, the ground state changes abruptly.
15
Content available remote

Ground-State Properties of the Spin-1/2 Heisenberg-Ising Bond Alternating Chain with Dzyaloshinskii-Moriya Interaction

80%
Strečka J., Gálisová L., Derzhko O.
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issue 5
742-744
EN
Ground-state energy is exactly calculated for the spin-1/2 Heisenberg-Ising bond alternating chain with the Dzyaloshinskii-Moriya interaction. Under certain condition, which relates a strength of the Ising, Heisenberg and Dzyaloshinskii-Moriya interactions, the ground-state energy exhibits an interesting nonanalytic behavior accompanied with a gapless excitation spectrum.
16
Content available remote

Anisotropy, Geometric Structure and Frustration Effects in Molecule-Based Nanomagnets

80%
Kamieniarz G., Kozłowski P., Antkowiak M., Sobczak P., Ślusarski T., Tomecka D., Barasiński A., Brzostowski B., Drzewiński A., Bieńko A., Mroziński J.
Acta Physica Polonica A
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2012
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vol. 121
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issue 5-6
992-998
EN
In this paper we review our results of comprehensive study of molecular nanomagnets recently synthesized in the form of the chromium-based molecules and bimetallic copper-containing chains as well as we present some new findings. We focus on effects of anisotropy, geometry and frustration appearing in various thermodynamic properties of the nanoscale magnets which are described by Heisenberg-like spin models and simulated by accurate numerical methods. We show that bond-dependent exchange anisotropy is needed to model magnetic torque in the Cd-doped chromium ring. We argue that only in the limit of infinite rings (n → ∞) frustration can be considered as the opposite to bipartiteness in the odd numbered (3 ≤ n ≤ 9) s = 3/2 quantum spin rings. We analyse the influence of exchange interactions and anisotropy on magnetic susceptibility of bimetallic (S = 3/2, s = 1/2) chains composed of Cu ions linked to different 3d ions by fitting experimental data. We reach the remarkable consistency of the density functional theory estimates of the magnetic couplings in Cr_8 molecule and provide strong support to the spin models exploited in the literature.
17
Content available remote

On the Exact Solution of the Mixed-Spin Ising Chain with Axial and Rhombic Zero-Field Splitting Parameters

80%
Dančo M., Strečka J.
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issue 5
728-729
EN
Ground-state phase diagram of the mixed spin-1/2 and spin-1 Ising chain with axial and rhombic zero-field splitting parameters is exactly calculated within the framework of the transfer-matrix method. It is shown that the rhombic zero-field splitting parameter prefers the magnetically ordered phase instead of the disordered phase.
18
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Approximate Approach to Magnetic and Thermodynamic Properties of Mixed Spin (1/2-S) Chains with AB and AB_2 Topology

80%
Pełka R., Pinkowicz D.
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issue 5
959-961
EN
Motivated by the rapid development in the synthesis of novel molecule-based magnets, we have investigated magnetic and thermodynamic properties of mixed spin (s-S) exchange coupled chains displaying a simple linear AB or a knotted AB_2 arrangement. Approximate approach for s = 1/2 and S ≥ 5/2, treating at an intermediate step spin S operator as a commuting variable and using the transfer matrix technique, is used. Susceptibility, magnetization and heat capacity of both spin systems are evaluated numerically from the corresponding free energy for S = 5/2. Uniform ferromagnetic and antiferromagnetic couplings are discussed. The procedure reproduces the right values of saturation magnetization and the entropy content of the systems, corroborating its correctness. χ T curves are shown to depend crucially on the μ_{B} H/J ratio. For zero-field heat capacity a double-peak structure is revealed for the AB chain, whereas for the AB_2 chain only one broad anomaly is observed.
19
Content available remote

On Theoretical Study of Magnetic Behavior of Diamond Doped with Transition Metals

80%
Hussain F., Ullah M., Ullah H., Rashid M., Imran M., Ahmad E.
Acta Physica Polonica A
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2015
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vol. 127
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issue 3
823-826
EN
We study the magnetic behavior of the diamond system. In this system diamond bulk is involved which is doped with different transition metals, namely Cu, Cd, Hg, and Zn. The VASP code is employed for all calculations which are based on density functional theory. The substitutional point defects is introduced in the diamond bulk and occupied by the transition metals. Results exhibit that all transition metals show ferromagnetism behavior and Cu is a good advocate of conductivity among all transition metals. The range of magnetic moments is 2.89, 1.99, 1.96, and 1.80 μ_{B} per Cu, Cd, Hg, and Zn atom in diamond bulk, respectively. Strong magnetic behavior points out that these materials could be used for spintronics.
20
Content available remote

Polaron States in a CuO Chain

80%
Bieniasz K., Oleś A.
Acta Physica Polonica A
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2015
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vol. 127
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issue 2
269-271
EN
We introduce a one-dimensional model for a CuO chain, with holes and S=1/2 spins localized in 3d_{x²-y²} orbitals, and p_σ oxygen orbitals without holes in the ground state. We consider a single hole doped at an oxygen site and study its propagation by spin-flip processes. We develop the Green function method and treat the hole-spin coupling in the self-consistent Born approximation, similar to that successfully used to study polarons in the regular t-J model. We present an analytical solution of the problem and investigate whether the numerical integration is a good approximation to this solution.
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