arXiv subjects

K. Szałowski

Publications and source records attributed to K. Szałowski.

At least 19 recordsLinked to original sources

arXiv2021

Two-spin and multi-spin quantum entanglement in V12 polyoxovanadate molecular nanomagnet

The paper reports a computational study of the quantum entanglement in V12 cluster molecular magnet. The low-temperature properties of the system are modelled with anisotropic quantum Heisenberg model on a tetramer of spins $S=1/2$ in the external magnetic field. The two-spin entanglement is quantified using the concurrence, whereas the fidelity serves as a measure of four-spin entanglement. The analytic and numerical results are derived and discussed, emphasizing the importance of real-space and spin-space interaction anisotropy and the role of quantum level crossings in the entanglement description in V12.

cond-mat.mes-hall↗

arXiv2021

Magnetocaloric and electrocaloric properties of the Hubbard pair cluster

The paper contains the discussion of the magnetocaloric and electrocaloric effect in a model dimer (pair cluster). The system of interest is modelled with a Hubbard Hamiltonian including the external electric and magnetic field. The thermodynamics of such pair is described exactly, on the grounds of the grand canonical ensemble, focusing on the half-filling of energy states. The quantities of interest, such as magnetic entropy, magnetic specific heat as well as isothermal entropy change resulting from the variation of either electric or magnetic field and appropriate Grüneisen ratios are calculated and discussed in a wide range of external fields. The importance of singlet to triplet transition for the observed behaviour is emphasized. The ranges of direct and inverse caloric effects are found and the manifestations of the magnetoelectric phenomena are described. In particular, the tunability of the magnetocaloric effect with electric field as well as tunability of the electrocaloric effect with magnetic field are demonstrated.

cond-mat.str-el↗

arXiv2020

The Pair Approximation method for the ferromagnetic Heisenberg model with spin $S=1$ and arbitrary range of interactions. Application for the magnetic semiconductor CrIAs

The Pair Approximation method has been formulated for the isotropic ferromagnetic Heisenberg model with spin $S=1$. The exchange interactions of arbitrary range have been taken into account. The single-ion anisotropy has been considered as well as the external magnetic field. Within the method, the Gibbs free-energy has been derived, from which all thermodynamic properties can be self-consistently obtained. In order to illustrate the developed formalism, the numerical calculations have been performed for CrIAs planar magnetic semiconductor, a hypothetical material whose existence has been recently predicted by the Density Functional Theory-based calculations. For this model material, all the relevant thermodynamic magnetic properties have been studied. The numerical results have been presented in the figures and discussed.

cond-mat.stat-mech↗

arXiv2020

Thermodynamic properties of the one-dimensional Ising model with magnetoelastic interaction

The Ising one-dimensional (1D) chain with spin $S=1/2$ and magnetoelastic interactions is studied with the lattice contribution included in the form of elastic interaction and thermal vibrations simultaneously taken into account. The magnetic energy term and the elastic (static) energy term based on the Morse potential are calculated exactly. The vibrational energy is calculated in the Debye approximation, in which the anharmonicity is introduced by the Gr{ü}neisen parameter. The total Gibbs potential, including both the magnetic field, as well as the external force term, is constructed and from its minimum the equation of state is derived. From the Gibbs energy all the thermodynamic properties are calculated in a self-consistent manner. The comprehensive numerical calculations are performed in a full temperature range, i.e., from zero temperature up to the vicinity of melting. In particular, a role of magneto-elastic coupling is emphasized and examined. The numerical results are illustrated in figures and discussed.

cond-mat.stat-mech↗

arXiv2019

Magnetocaloric properties of V6 molecular magnet

The paper presents a theoretical study of magnetocaloric properties of polyoxovanadate molecular magnet V6 containing 6 vanadium ions carrying quantum spins $S = 1/2$. The characteristic property of such structure is the presence of two weakly interacting spin triangles with all-antiferromagnetic couplings. The properties of the system are described using the exact numerical diagonalization approach applied to the quantum Heisenberg model and utilizing a field ensemble formalism. The dependence of the magnetic entropy and magnetic specific heat on the temperature and external magnetic field is calculated and extensively discussed. The magnetocaloric properties are quantified by isothermal entropy change and entropy derivative over the magnetic field. An interesting behaviour of isothermal entropy change is found, with high degree of tunability of the magnetocaloric effect with the initial and final magnetic field values.

cond-mat.mtrl-sci↗

arXiv2019

Hubbard pair cluster with elastic interactions. Studies of thermal expansion, magnetostriction and electrostriction

The pair cluster (dimer) is studied within the framework of the extended Hubbard model and the grand canonical ensemble. The elastic interatomic interactions and thermal vibrational energy of the atoms are taken into account. The total grand potential is constructed, from which the equation of state is derived. In equilibrium state, the deformation of cluster size, as well as its derivatives, are studied as a function of the temperature and the external magnetic and electric fields. In particular, the thermal expansion, magnetostriction and electrostriction effects are examined for arbitrary temperature, in a wide range of Hamiltonian parameters.

cond-mat.stat-mech↗

arXiv2019

Spin-glass-like ordering in a frustrated $J_1-J_2$ Ising antiferromagnet on a honeycomb lattice

We study the nature of a low-temperature phase in the frustrated honeycomb-lattice Ising model with first- and second-neighbor antiferromagnetic (AF) interactions, $J_1$ and $J_2$, respectively, for $R = J_2/J_1 > 1/4$. It is known that for $R < 1/4$ there is a phase transition at low temperatures to the AF phase. Nevertheless, little is known about the critical behavior of the model for $R > 1/4$, except for recent effective field results which detected no phase transition down to zero temperature. Our Monte Carlo results suggest that for $R > 1/4$ there is at least one peculiar phase transition accompanied by a spin-glass-like freezing to a highly degenerate state consisting of frozen domains with stripe-type AF ordering separated by zero-energy domain walls. In spite of the local ordering within the respective domains there is no ordering among them and thus, unlike in the corresponding square-lattice model with $R > 1/2$, there is no conventional magnetic long-range ordering spanning the entire system.

cond-mat.stat-mech↗

arXiv2018

Magnetization distribution in a spin ladder-shaped quantum nanomagnet

The quantum nanomagnets show interesting site-dependent magnetic properties as a function of the temperature and the external magnetic field. In the paper we present the results of calculations for a finite quantum spin ladder with two legs, consisting of 12 spins $S=1/2$. We describe our system with isotropic quantum Heisenberg model and perform exact numerical diagonalization of the Hamiltonian to use canonical ensemble approach. Our analysis focuses on the site-dependent magnetization in the system, presenting magnetization distributions for various interaction parameters. We discuss extensively the temperature and magnetic field dependences of individual site magnetizations. The interesting behaviour, with pronounced non-uniformity of magnetization across the ladder, is found.

cond-mat.mes-hall↗

arXiv2018

Hubbard pair cluster in the external fields. Studies of the polarization and susceptibility

The electric and magnetic polarizations as well as the electric and magnetic susceptibilities of the Hubbard pair-cluster embedded in the external fields were studied by the exact method. Based on the grand canonical ensemble for open system, the numerical calculations were performed for the electron concentration corresponding to the half-filling case. It has been found that the electric and magnetic properties are strictly interrelated, what constitutes a manifestation of a magnetoelectric effect, and the detailed explanation of such behaviour was given. In particular, near the ground state where the transitions are induced by the external fields, discontinuous changes of the studied quantities have been found. They have been associated with the occurrence of the singlet-triplet transitions. An anomalous behaviour of the electric and magnetic polarizations as a function of the temperature, occurring below the critical magnetic field, was illustrated. In the presence of the competing electric and magnetic fields, the influence of Coulombic repulsion on the studied properties was discussed.

cond-mat.str-el↗

arXiv2018

Phase Diagram of the $J_1$ - $J_2$ Frustrated Anisotropic Antiferromagnet with Spin $S=1$ on the Quadratic Lattice

In the paper the phase diagram of $J_1-J_2$ frustrated antiferromagnet with spin $S=1$ and single-ion anisotropy is studied on the planar quadratic lattice in the cluster approximation. The Bogolyubov inequality is adopted for the Gibbs energy calculation for the case of $2 \times 2$ and $4 \times 4$ clusters. On this basis, the ranges of existence of the anfiferromagnetic, superantiferromagnetic and paramagnetic phases are investigated for the antiferromagnetic nearest-neighbour ($J_1<0$) and next-nearest-neighbour ($J_2<0$) interactions. In particular, the occurrence of tricritical and triple points is discussed and a comparison between the results for $2 \times 2$ and $4 \times 4$ clusters is made. The results are also compared with the classical MFA method, adopted here for the model in question, as well as with selected literature results for particular choices of interaction parameters.

cond-mat.stat-mech↗

arXiv2018

Hubbard pair cluster in the external fields. Studies of the magnetic properties

The magnetic properties of the two-site Hubbard cluster (dimer or pair), embedded in the external electric and magnetic fields and treated as the open system, are studied by means of the exact diagonalization of the Hamiltonian. The formalism of the grand canonical ensemble is adopted. The phase diagrams, on-site magnetization, spin-spin correlations, mean occupation numbers and hopping energy are investigated and illustrated in figures. An influence of temperature, mean electron concentration, Coulomb $U$ parameter and external fields on the quantities of interest is presented and discussed. In particular, the anomalous behaviour of the magnetization and correlation function vs. temperature near the critical magnetic field is found. Also, the effect of magnetization switching by the external fields is demonstrated.

cond-mat.stat-mech↗

arXiv2018

Thermodynamic model of a solid with RKKY interaction and magnetoelastic coupling

Thermodynamic description of a model system with magnetoelastic coupling is presented. The elastic, vibrational, electronic and magnetic energy contributions are taken into account. The long-range RKKY interaction is considered together with the nearest-neighbour direct exchange. The generalized Gibbs potential and the set of equations of state are derived, from which all thermodynamic functions are self-consistently obtained. Thermodynamic properties are calculated numerically for FCC structure for arbitrary external pressure, magnetic field and temperature, and widely discussed. In particular, for some parameters of interaction potential and electron concentration corresponding to antiferromagnetic phase, the existence of negative thermal expansion coefficient is predicted.

cond-mat.stat-mech↗

arXiv2017

Ground-state magnetic properties of spin ladder-shaped quantum nanomagnet: Exact diagonalization study

The paper presents a computational study of the ground-state properties of a quantum nanomagnet possessing the shape of a finite two-legged ladder composed of 12 spins $S=1/2$. The system is described with isotropic quantum Heisenberg model with nearest-neighbour interleg and intraleg interactions supplemented with diagonal interleg coupling between next nearest neighbours. All the couplings can take arbitrary values. The description of the ground state is based on the exact numerical diagonalization of the Hamiltonian. The ground-state phase diagram is constructed and analysed as a function of the interactions and the external magnetic field. The ground-state energy and spin-spin correlations are extensively discussed. The cases of ferro- and antiferromagnetic couplings are compared and contrasted.

cond-mat.stat-mech↗

arXiv2017

Thermodynamics of a model solid with magnetoelastic coupling

In the paper a study of a model magnetoelastic solid system is presented. The system of interest is a mean-field magnet with nearest-neighbour ferromagnetic interactions and the underlying s.c. crystalline lattice with the long-range Morse interatomic potential and the anharmonic Debye model for the lattice vibrations. The influence of the external magnetic field on the thermodynamics is investigated, with special emphasis put on the consequences of the magnetoelastic coupling, introduced by the power-law distance dependence of the magnetic exchange integral. Within the fully self-consistent, Gibbs energy-based formalism such thermodynamic quantities as the entropy, the specific heat as well as the lattice and magnetic response functions are calculated and discussed. To complete the picture, the magnetocaloric effect is characterized by analysis of the isothermal entropy change and the adiabatic temperature change in the presence of the external pressure.

cond-mat.stat-mech↗

arXiv2016

Hubbard pair cluster in the external fields. Studies of the chemical potential

The chemical potential of the two-site Hubbard cluster (pair) embedded in the external electric and magnetic fields is studied by exact diagonalization of the Hamiltonian. The formalism of the grand canonical ensemble is adopted. The influence of temperature, Hubbard on-site Coulombic energy $U$ and electron concentration on the chemical potential is investigated and illustrated in figures. In particular, a discontinuous behaviour of the chemical potential (or electron concentration) in the ground state is discussed.

cond-mat.stat-mech↗

arXiv2016

Self-consistent model of a solid for the description of lattice and magnetic properties

In the paper a self-consistent theoretical description of the lattice and magnetic properties of a model system with magnetoelastic interaction is presented. The dependence of magnetic exchange integrals on the distance between interacting spins is assumed, which couples the magnetic and the lattice subsystem. The framework is based on summation of the Gibbs free energies for the lattice subsystem and magnetic subsystem. On the basis of minimization principle for the Gibbs energy, a set of equations of state for the system is derived. These equations of state combine the parameters describing the elastic properties (relative volume deformation) and the magnetic properties (magnetization changes). The formalism is extensively illustrated with the numerical calculations performed for a system of ferromagnetically coupled spins $S$=1/2 localized at the sites of simple cubic lattice. In particular, the significant influence of the magnetic subsystem on the elastic properties is demonstrated. It manifests itself in significant modification of such quantities as the relative volume deformation, thermal expansion coefficient or isothermal compressibility, in particular, in the vicinity of the magnetic phase transition. On the other hand, the influence of lattice subsystem on the magnetic one is also evident. It takes, for example, the form of dependence of the critical (Curie) temperature and magnetization itself on the external pressure, which is thoroughly investigated.

cond-mat.stat-mech↗

arXiv2016

Exact Diagonalization Study of an Extended Hubbard Model for a Cubic Cluster at Quarter Filling

In the paper the thermodynamics of a cubic cluster with 8 sites at quarter filling is characterized by means of exact diagonalization technique. Particular emphasis is put on the behaviour of such response functions as specific heat and magnetic susceptibility. The system is modelled with extended Hubbard model which includes electron hopping between both first and second nearest neighbours as well as coulombic interactions, both on-site and between nearest-neighbour sites. The importance of hopping between second nearest neighbours and coulombic interactions between nearest neighbours for the temperature dependences of thermodynamic response functions is analysed. In particular, the predictions of Schottky model are compared with the calculations based on the full energy spectrum.

cond-mat.stat-mech↗

arXiv2016

Frustration in an exactly solvable mixed-spin Ising model with bilinear and three-site four-spin interactions on a decorated square lattice

Competitive effects of so-called three-site four-spin interactions, single ion anisotropy and bilinear interactions is studied in the mixed spin-1/2 and spin-1 Ising model on a decorated square lattice. Exploring the decoration-iteration transformation, we have obtained exact closed-form expressions for the partition function and other thermodynamic quantities of the model. From these relations, we have numerically determined ground- state and finite-temperature phase diagrams of the system. We have also investigated temperature variations of the correlation functions, internal energy, entropy, specific heat and Helmholtz free energy of the system. From the physical point of view, the most interesting result represents our observation of a partially ordered ferromagnetic or phase in the system with zero bilinear interactions. It is remarkable, that due to strong frustrations disordered spins survive in the system even at zero temperature, so that the ground state of the system becomes macroscopically degenerate with non-zero entropy. Introduction of arbitrarily small bilinear interaction completely removes degeneracy and the entropy always goes to zero at the the ground state.

cond-mat.stat-mech↗