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1
Content available remote

Numerical Test of Molecular-Field Renormalization Group

100%
Kamieniarz G.
Acta Physica Polonica A
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1997
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vol. 92
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issue 2
423-426
EN
A new transfer matrix approach has been worked out to test the predictions of the molecular-field renormalization group for square Ising clusters with a linear size up to L ≤ 11. The convergence of the finite-size critical couplings towards the exact value for the molecular-field renormalization group is shown and the limit of the ratio y_{h}/y_{t} consistent with the corresponding universal value is revealed.
2
Content available remote

Ground-State Configurations of 2D Clusters of Classical Charged Particles

64%
Sobczak P., Kamieniarz G.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
477-480
EN
The ground-state configurations of the classical point-charge particles were calculated using a new genetic-algorithm-based approach. The structures obtained confirmed the recent Monte Carlo findings, including the metastable states.
3
Content available remote

Simulations of the 3D Ising Model at Criticality

64%
Blöte H., Kamieniarz G.
Acta Physica Polonica A
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1994
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vol. 85
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issue 2
395-398
EN
We present some new finite-size results for the Binder cumulant that we obtained by means of a transfer-matrix perturbation expansion and by the cluster Monte Carlo methods - the Swendsen-Wang and Wolff algorithms and the largest-cluster method. A finite-size scaling analysis taking into account a correction to scaling locates the critical coupling at Kc = 0.221649(4).
4
Content available remote

Universal Sequence of the Ground States and Energy Level Ordering in Frustrated Antiferromagnetic Rings with a Single Bond Defect

52%
Antkowiak M., Florek W., Kamieniarz G.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
890-892
EN
The universal sequence of the ground states for antiferromagnetic frustrated rings with the odd number of the local spins s and a single bond defect α described by the isotropic Heisenberg Hamiltonian is discussed. The Lieb-Mattis energy level ordering in a pentanuclear ring is revealed and the arising magnetisation steps are demonstrated.
5
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Time Evolution of the Magnetic Chains with Uniaxial Anisotropy

52%
Woźniak D., Drzewiński A., Kamieniarz G.
Acta Physica Polonica A
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2015
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vol. 127
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issue 2
333-335
EN
In the framework of the matrix product states representation the effect of the single-ion anisotropy on the time evolution of the initial state after a sudden quench of the local magnetic field has been investigated. The overlap of the initial and time-evolved states, so called the Loschmidt echo, magnetization profiles and correlation functions are presented.
6
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DFT Study of Octanuclear Molecular Chromium-Based Ring Using New Pseudopotential Parameters

52%
Brzostowski B., Ślusarski T., Kamieniarz G.
Acta Physica Polonica A
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2012
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vol. 121
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issue 5-6
1115-1117
EN
We present a comprehensive study of magnetic properties of Cr_8F_8(Piv)_{16} molecular ring (in short Cr8) using the package SIESTA with several choices of chromium pseudopotential parameters. We use generalized gradient approximation to investigate properties of Cr8 ring approximated by replacing the pivallic group by H atoms (hydrogen saturation). For different choice of chromium pseudopotential we examine the electronic and magnetic properties of Cr8 molecule. We compare highest occupied molecular orbital and the lowest unoccupied molecular orbital gaps for both spin channels, magnetic moments, and exchange interaction parameter J. The influence of pseudopotential parameters on obtained results is discussed. Finally our results are compared with other theoretical approaches and experimental data.
7
Content available remote

Relaxation Dynamics in the Spin-1 Heisenberg Antiferromagnetic Chain after a Quantum Quench of the Uniaxial Anisotropy

52%
Woźniak D., Drzewiński A., Kamieniarz G.
Acta Physica Polonica A
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2016
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vol. 130
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issue 6
1395-1400
EN
In the framework of the matrix product state representation the effect of a sudden turning on of the uniaxial anisotropy on the time evolution of the Haldane state has been investigated. Depending on the value of the uniaxial anisotropy parameter, the calculations were derived within (or outside) the region where the Haldane gap survives. An exact expression for the time evolution of the Loschmidt echo has been derived and, moreover, its collapse and revival behaviour was captured. In addition, the non-local order parameter of a time evolving state was tracked, revealing two types of relaxations.
8
Content available remote

Highly Degenerated Ground States in Some Rings Modeled by the Ising Spins with Competing Interactions

52%
Florek W., Antkowiak M., Kamieniarz G., Jaśniewicz-Pacer K.
Acta Physica Polonica A
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2018
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vol. 133
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issue 3
411-413
EN
We discuss three Ising ring systems with competing interactions which are analogs of quantum systems and we show that they exhibit similar properties. In particular, the archetypal system of three antiferromagnetically coupled spins s has two magnetically degenerated ground states with |M|=s, when 0
9
Content available remote

Mapping of the DFT Spin Configuration Energies of Cr8Cd Molecular Ring onto the Energy Structure of Falicov-Kimball Model

52%
Wojciechowski M., Brzostowski B., Lemański R., Kamieniarz G.
Acta Physica Polonica A
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2015
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vol. 127
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issue 2
410-412
EN
A comprehensive study of electronic and magnetic properties of a recently synthesized nonametalic chromium-based heteronuclear molecule Cr₈CdF₉(O₂C-C(CH₃)₃)₁₈ is presented, using DFT and Falicov-Kimball (FK) model approach. The magnetic moments are calculated and the spin charge density map is discussed. The exchange coupling parameters between transition metals ions are extracted, taking into account all the nonequivalent spin configurations. It is demonstrated that the energies of the spin configurations can be reproduced by the FK model with a set of parameters consistent with that for the parent Cr8 molecule. For molecular ring considered, the ground state corresponds to the antiferromagnetic configuration and the ferromagnetic configuration yields the highest energy.
10
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DFT Estimation of Exchange Coupling Constant of Cr₈ Molecular Ring using the Hybrid Functional B3LYP

52%
Wojciechowski M., Brzostowski B., Kamieniarz G.
Acta Physica Polonica A
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2015
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vol. 127
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issue 2
407-409
EN
A study of electronic and magnetic properties of an octametallic chromium-based homonuclear molecule Cr₈F₈(CO₂-C(CH₃)₃)₁₆ is presented, using density functional theory (DFT) approach and linearized augmented plane wave (LAPW) method with Perdew, Burke and Ernzerhof (PBE) and Becke 3-term correlation (B3LYP) functionals. The exchange coupling parameters between transition metals ions are extracted, taking into account two different (ferro- and antiferromagnetic) spin configurations. The value J=3.1 meV found for the hybrid B3LYP functional improves significantly the one obtained for the PBE functional and gives an evidence for the superiority of the former in simulation of molecular nanomgnets. Moreover, the hybrid functional yields excellent spin density localisation, an enhancement of the HOMO-LUMO gaps and the value 2.81 μ_B of magnetic moment at the chromium centre in good agreement with experiment.
11
Content available remote

The Study of Magnetic Molecules Containing Cr₉ Chromium-Based Rings within Density Functional Theory

52%
Brzostowski B., Wojciechowski M., Kamieniarz G.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
961-963
EN
We examine the electronic and magnetic properties of three frustrated Cr₉ molecules with a single bond defect using density functional theory method. Five non-equivalent broken-symmetry spin configurations with S=±3/2 for Cr are considered, the corresponding differences between the total energies are calculated and the exchange interaction parameters J extracted using different scenarios. We find that the couplings for all molecules are antiferrimagnetic. We also estimate the fundamental gaps and in addition, the high occupied molecular orbitals and low occupied molecular orbitals are plotted and discussed.
12
Content available remote

Zero Temperature Magnetic Frustration in Nona-Membered s=3/2 Spin Rings with Bond Defect

52%
Antkowiak M., Kozłowski P., Kamieniarz G.
Acta Physica Polonica A
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2012
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vol. 121
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issue 5-6
1102-1104
EN
Zero-temperature limits of the local and global thermodynamic quantities in the nine-membered antiferromagnetic s=3/2 spin ring are investigated by means of numerical exact diagonalization. An anisotropic Heisenberg model with tunable bond defect reflecting continuously varying topology (from closed to open ring) is exploited. The frustrated and non-frustrated phases are identified in the ground-state phase diagram determined by a bond-defect strength and magnetic field. Near the phase boundaries significance of the thermal fluctuations affecting the estimates of the local magnetic quantities found earlier at T=1 K is revealed. For the global quantities the effects of thermal fluctuations are found much weaker. A sequence of the local magnetic moments is analysed and their experimental verification at the edges of the non-frustrated and within the entire frustrated phase is suggested at sufficiently low temperature.
13
Content available remote

Advantages of the DFT Augmented Symmetry Approach to Simulations of the Chromium-Based Ring Cr₈ within B3LYP Functional

52%
Brzostowski B., Wojciechowski M., Kamieniarz G.
Acta Physica Polonica A
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2018
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vol. 133
|
issue 3
591-593
EN
The DFT estimates of magnetic couplings in molecular nanomagnets are computationally demanding and their values have not achieved the satisfactory accuracy in spite of a lot of effort. We concentrate here on comprehensive tests for predictions of the recently proposed augmented symmetry approach aiming at reducing the computational complexity of the DFT calculations which is particularly important for the Wien2k code. Using the B3LYP functional, we demonstrate the numerical stability of magnetic couplings, magnetic moments and the HOMO-LUMO gaps, changing the fraction of exact exchange α. We reach the significant gain in the computing time without a loss in the accuracy of the final results with respect to those obtained by the standard PBE approach. We conclude that the value α=0.25 leads to best estimation of magnetic couplings for hybrid functionals within Wien2k.
14
Content available remote

Fundamental Gaps in Cr_{8}, Cr_{7}Ni and Cr_{7}Cd Molecules

52%
Brzostowski B., Wojciechowski M., Kamieniarz G.
|
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vol. 126
|
issue 1
234-235
EN
In this paper we investigate fundamental gaps of three octametallic Cr-based molecular rings Cr_{8}F_{8}(Piv)_{16}, Cr_{7}NiF_{8}(Piv)_{16} and Cr_{7}CdF_{8}(Piv)_{16} using the SIESTA package. We find that for the ground-state antiferromagnetic configurations, the gap of the homometallic ring is significantly higher than those of the heterometallic rings. In addition, the HOMO and LUMO orbitals are plotted and discussed.
15
Content available remote

Application of the Package SIESTA to Linear Models of a Molecular Chromium-Based Ring

52%
Ślusarski T., Brzostowski B., Tomecka D., Kamieniarz G.
|
|
issue 5
967-968
EN
We investigate for the first time the electronic and magnetic properties of the linear models of Cr_8F_8(Piv)_{16} molecular ring using the SIESTA package. In the first step the proper values of the SIESTA parameters and the optimal basis set needed for the convergence of the total energy are established. Next the estimates of the magnetic coupling J confirming the previous density functional theory calculations are presented.
16
Content available remote

Structural and Magnetic Properties of Co Thin Films on Au(111) Substrates

52%
Walczak A., Ślusarski T., Lehmann-Szweykowska A., Kamieniarz G.
|
EN
The structural and magnetic properties of thin Co films grown on Au(111) substrates are investigated using ab initio local-spin-density calculations in the generalized gradient approximation. It is shown that a large lattice intermismatch between Co and Au(111) leads to a strong contraction of the interlayer distances within the Co layer, which adopts either the hexagonal close-packed (α-phase) or the face-centered cubic (β-phase) structure of bulk Co. Magnetic moments in the Co layer are increased beyond their value in bulk Co, the enhancement being strongest at the free surface and at the Co/Au interface, and more modest in the central part of the adlayer. Moderating influence of capping Ag layers on the structure and the magnetic properties of the films has also been considered. It is shown that Ag capping reduces the magnetic moments in comparison to uncovered Co/Au systems.
17
Content available remote

Critical Behaviour of Some 2D Lattice Models

52%
Kamieniarz G., Blöte H., Dekeyser R.
Acta Physica Polonica A
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1994
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vol. 85
|
issue 2
389-393
EN
The density series of the non-interacting hard-square lattice gas model are reanalyzed by the ratio, Dlog Padé and differential approximant methods. The problem of poor consistency between series and other results is resolved. Transfer matrix calculations are performed, implementing both finite-size scaling and conformal invariance. Very accurate estimates of the critical exponents y_{t}, and y_{h} are obtained in agreement with Ising universality. Furthermore, an improvement of the value of the critical density ρ_{c} is found. In addition, the universal critical-point ratios of the square of the second and the fourth moment of the magnetization for ferromagnetic Ising models on the square and on the triangular lattice with periodic boundary conditions are reported.
18
Content available remote

Effective Field Methods for Magnetic Systems

52%
Kamieniarz G., Dekeyser R., Musiał G.
Acta Physica Polonica A
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1994
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vol. 85
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issue 2
413-417
EN
The self-consistency conditions inherent in the mean-field approximation are extended to take into account some pair correlations. Ii is shown that this extension is systematically improvable and applicable to the spin systems with short-range interactions in the entire temperature region. With two correlations included and the renormalization-group ideas implemented, the critical properties of the Ising model are satisfactorily revealed. Moreover, the improved estimates of the thermodynamical quantities for the quantum linear XY model from the finite-size calculations are found down to very low temperatures.
19
Content available remote

Numerical Simulations of Linear S=1 Ferro- and Antiferromagnets

46%
Kamieniarz G., Matysiak R., Caramico D'Auria A., Esposito F., Esposito U.
Acta Physica Polonica A
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1997
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vol. 92
|
issue 2
387-389
EN
The finite-temperature static properties of the spin S=1 antiferromagnetic Heisenberg chains are extensively simulated using the quantum transfer matrix method. The zero-field susceptibility and specific heat as well as the field-dependent magnetization data are evaluated to select the microscopic parameters of a number of real quasi-one-dimensional compounds and to verify some theoretical approaches.
20
Content available remote

Improved Phase Diagrams of Ashkin-Teller Model

46%
Pawlicki P., Kamieniarz G., Kozlowski P., Dekeyser R., Rogiers J.
Acta Physica Polonica A
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1997
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vol. 92
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issue 2
453-458
EN
The phase diagram of the Askhin-Teller model in two dimensions is determined. Numerical calculations are performed for the simple square L × L lattice using transfer matrix technique. Exploiting finite-size scaling all unknown critical lines were obtained with good accuracy. An extended version of the Ashkin-Teller model is also considered within the molecular field renormalization group method and the critical surface for three-parameter odd-parity Hamiltonian is calculated.
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