Lithuanian Journal of Physics

Title

Lietuvos mokslų akademija — 2024-02-23

The XYZ model in the mean-field approximation in terms of Pauli spin matrices

E. Albayrak — 2024-02-23

The mean-field approximation (MFA) of spin-1/2 XYZ model was studied by using the Pauli spin matrices and their exponentials which led to very nice hyperbolic tangent functions in nonlinear form. The magnetization components Mx, My and Mz were obtained for the ferromagnetic (FM) case and then it was modified for the antiferromagnetic (AFM) case by introducing the sublattices. The bilinear exchange interaction parameters Jx, Jy and Jz, and the external magnetic fields Hx, Hy and Hz were considered along the three-dimensions for various coordination numbers q = 3, 4 and 6. The thermal variations of the magnetizations and thus the phase diagrams were obtained to illustrate the behaviours of phase transition lines in the AFM case.

Predicting nonradiative decay barrier of BODIPY dye in polar environment by applying ONIOM multiscale method

D. Narkevičius — 2024-02-23

Fluorescent molecular sensors are widely used in biological research. They allow straightforward viscosity, temperature or polarity measurements at the microscopic level, including live cells. Maps of desired physical properties can be obtained by applying fluorescence lifetime imaging microscopy (FLIM) to cells.

One of the most important properties of a cell is viscosity, as it affects other parameters, such as the rate of biochemical reactions and particle diffusion. Boron-dipyrromethene (BODIPY) compounds are widely used for viscosity measurements, but current variants have the undesirable sensitivity to polarity, and more suitable alternatives are being sought using theoretical computations. The polarizable continuum model (PCM) used in previous studies did not adequately take into account the influence of the polar environment when calculating the BODIPY activation energy associated with polarity sensitivity.

After applying the multilayer ONIOM method in polar and non-polar environments, the calculated maximum wavelengths of the fluorescence spectra of the 8PhBODIPY compound were closer to the experimental results compared to PCM. The activation energy was also calculated, its value in polar and non-polar environments qualitatively corresponded to the experimental results, and the quantitative agreement was reached using the empirical correction.

Second-order Rayleigh–Schrödinger perturbation theory for the GRASP2018 package: Core–valence correlations

G. Gaigalas — 2024-02-23

The General Relativistic Atomic Structure package [GRASP2018, C. Froese Fischer, G. Gaigalas, P. Jönsson, and J. Bieroń, Comput. Phys. Commun. (2019), DOI: 10.1016/j.cpc.2018.10.032] is based on multiconfiguration Dirac– Hartree–Fock and relativistic configuration interaction (RCI) methods for energy structure calculations. The atomic state function used in the program is built from the set of configuration state functions (CSFs). The valence–valence, core–valence and core–core correlations are explicitly included through expansions over CSFs in RCI. We present a combination of RCI and the stationary second-order Rayleigh–Schrödinger many-body perturbation theory in an irreducible tensorial form to account for electron core–valence correlations when an atom or ion has any number of valence electrons. This newly developed method, which offers two ways of use, allows a significant reduction of the CSF space for complex atoms and ions. We also demonstrate how the method and program works for the energy structure calculation of Cl III ion.

Optical characteristics of structures with silicon nanowires and metal nanoparticles

O. Havryliuk — 2024-02-23

To calculate the optical parameters, the finite difference method in the time domain (FDTD) was used, which can be applied to solve Maxwell’s equations. A large number of combinations of a planar structure with metal nanoparticles and a structure with nanowires and metal nanoparticles (NPs) were calculated. The height of nanowires h varied from 50 to 3000 nm, the period of the structure P was 100–600 nm, and the diameter of metal nanoparticles d was 50–400 nm. The reduction of light reflection was determined by the anti-reflection effect of the Si-NWs array itself and the direct scattering effect of metal nanoparticles. It was shown that all structures gave significantly lower reflection coefficients compared to that of a solid silicon plate.

Numerical simulation of proton backscattering spectra in GEANT4 toolkit

D. Lingis — 2024-02-23

Rutherford backscattering spectroscopy (RBS) is a widely used technique for the atomic-scale analysis of sample composition, lattice displacement and impurity profiling. RBS is based on the elastic scattering of incident charged particles by target nuclei and the subsequent detection of scattered particles. The interpretation of RBS spectra, however, poses challenges due to overlapping peaks, corresponding to scattering from different atomic species, and uncertainties from energy loss, scattering geometry and detector response. To address this, an open source simulation model based on the versatile GEANT4 simulation toolkit has been developed. The flexibility of the open source enables users to tailor the model to its specific requirements, such as the use of specific particle stopping powers, cross-sections, and physics processes. This work presents the results of the comparison between the experimental and simulated backscattering spectra in crystalline silicon, silicon carbide and silicon dioxide samples by 1–2.5 MeV energy protons, obtained in random orientation conditions. The results demonstrate the capability of the model to accurately simulate backscattering spectra in both amorphous materials and single crystals. The overall agreement between the simulated and experimental results is highly promising for future development and use in the interpretation and simulation of RBS spectra.

New approach to evaluating the thermodynamic consistency of melts in the ‘metal-slag’ system based on interatomic interaction parameters

D. Togobitska — 2024-02-23

The article presents a new approach to evaluating the thermodynamic state of the ‘metal–slag’ system during metal smelting in oxidizing-reducing conditions. The interaction between the metal and slag is analyzed using the model of the structure of metallurgical melts, which considers cooperative ion exchange processes and interatomic interaction parameters. As a result of analyzing experimental data on the compositions of reacting melts during pig iron and steel smelting, criteria were developed for assessing the degree of achieving the equilibrium in the system regarding sulfur. The charge state parameters of the metallic system ZY and the slag system Δe, the slag stoichiometry index ρ, and the charge state parameter of the melt components Zi were used. The regularity of a consistent formation of metallic and slag melts has been established, which is evidenced by a significant correlation between the chemical equivalent of the metal composition ZY and the slag Δe. Analytical dependences were obtained in the form of ZY = f (Δe, ρ). The identified patterns and criteria can be integrated into automated process control systems for regulating the slag regime and producing high-quality pig iron and steel.