Наукові публікації кафедри загальної та прикладної фізики

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Change in the entropy during a first-order phase transition induced by a magnetic field in an isotropic non-Heisenberg ferromagnet
(AIP Publishing, Low Temperature Physics 44, 322 (2018), 2018-04) Лаванов, Геннадій Юрійович
We describe a first-order phase transition in an isotropic non-Heisenberg ferromagnet induced by a magnetic field with a fourth-order spin exchange interaction at temperatures above the Curie point. The field behavior of the magnetization is analyzed and the temperature dependences of the critical fields of the stability of the paramagnetic and ferromagnetic phases as well as the critical field of the phase transition between them are determined. It is shown that this first-order phase transition has a finite amplitude of the magnetization jump and can occur in small magnetic fields. An H–T phase diagram is presented. It is found that the amplitude of the phase jump during the magnetic field-induced transition from the paramagnetic to the ferromagnetic phase can exceed the magnitude of the entropy change of an isotropic Heisenberg ferromagnet by two orders of magnitude when it is magnetized at the Curie point. It is shown that the expansion of the free energy model up to the 6th power in magnetization can only qualitatively describe the entropy behavior in the phase transition between the paramagnetic and ferromagnetic phases.
Conductivity of composites of thermotropic ionic liquid crystals
(Electronics and Control Systems 2015. N 2(44): 29-32, 2015) Gridyakina, Alexandra
This work presents the studies of the transport properties of samples of oriented binary composite of cobalt decanoate with lead decanoate. Found that samples of the mixture at the temperature range of liquid crystal existence are weak electrolytes, which have anisotropy of bulk conductivity caused by the ordering of molecules. The values of mobility and charge concentration were estimated.
Conductivity of thermotropic ionic liquid crystals
(Proceedings of the National Aviation University. 2015. N 3(64): 64–68, 2015) Gridyakina, Alexandra
The article represents the results of electrical conductivity studies of cobalt decanoate and lead decanoate oriented samples. It was determined that samples are weak electrolytes in the liquid crystal state region. The samples are characterized by great anisotropy of bulk conductivity caused by molecular order.
Cubic Susceptibility and Hyper-polarizability of the Lyotropic Liquid Crystal−Viologen System
(Ukr. J. Phys. 2008, Vol. 53, N 12, p.1167-1173, 2008) Bordyuh, Hanna; Polishchuk, Arkadiy
Nonlinear optical properties of the “lyotropic liquid crystal–viologen” system are investigated. The system shows electrochromism under the action of an external electric field. For the colored samples, the recording of dynamic holographic gratings is obtained, by using the pulse laser method. The main characteristics of a recording on such samples are studied. On their base in view of the electrooptical properties of the system, the mechanism of the recording of gratings is proposed. The recording is realized due to a change in the polarizability of the π-electron system of viologen derivatives under the action of an intense laser radiation. The main nonlinear optical parameters (n2, χ(3), γ) of the system under study are determined.
Dynamic grating recording in lyotropic ionic smectics of metal alkanoates doped with electrochromic impurities
(Elsevier, Optical Materials 31 (2009) 1109–1114, 2009) Bordyuh, Hanna; Polishchuk, Arkadiy
In this work, we investigated nonlinear-optical properties in lyotropic ionic liquid crystals (LILC) of potassium caprilate doped with electrochromic impurities (viologens). It is known that after applying a direct electric field to the LILC-viologen cells colouration of samples is observed [1]. Dynamic grating recording under the action of nanosecond pulse laser radiation was obtained for the coloured samples. We analyzed the origin of the optical nonlinearity in the investigated materials taking into account their electrochemical and electrooptical properties. To explain grating recording in LILC doped with viologens, we proposed the mechanism of photoconversion in viologen molecules.
Electric properties of ionic thermotropic liquid crystals
(Український фізичний журнал., 2016. Т. 61, № 6, 2016) Gridyakina, Alexandra
This work presents the results of studies of the electric properties of samples of oriented ionic thermotropic liquid crystals (ITLC) of cobalt decanoate, lead decanoate, and their binary mixture. It is found that all studied samples in the temperature range, where a liquid crystal exists, are weak electrolytes. All investigated compounds are characterized by an anisotropy of bulk conductivity caused by the ordering of molecules. The values of electric conductivity, activation energy, charge mobility, and concentration are estimated.
Electrooptical and Nonlinear Optical Properties of Lyotropic Liquid Crystals Doped with Electrochromic Viologen
(Український фізичний журнал, 2012) Bordyuh, Hanna; Polishchuk, Arkadiy
This work presents the results of experimental studies of the electrooptical and nonlinear optical properties of lyotropic ionic liquid crystals (LILC) with soluted electrochromic admixtures of viologens. It is established that the Smectic A ordering of the LILC of potassium caprylate is not disrupted by the introduction of viologens. Moreover, LILC-viologen composites obtain electrochromic properties inherent to viologens, which produce colored radical cations and dimers, while reducing. The presence of radical cations and dimmers is fixed by the optical absorption spectra. It is found that, under the action of an electric field, the LILC-viologen samples form a bilayer structure consisting of a liquid crystal layer and an absorptive layer of viologen redox products (radical cations and dimers). A dynamic grating recording is realized and studied in bilayer LILC-viologen cells. It is determined that the recording takes place in the colored layer of viologen redox products. A possible mechanism of grating recording in LILC-viologen cells is proposed.
Fast Nolinear Optical Mechanisms in Bi-Layered Cells Composed by Lyotropic Ionic Liquid Crystals with Dye and Viologen Films
(Taylor & Francis Group, LLC, Mol. Cryst. Liq. Cryst., Vol. 508, pp. 296[658]–308[670], 2009, 2009) Bordyuh, Hanna
Mechanisms of the fast optical nonlinearity are studied in two types of cells based on lyotropic ionic liquid crystals (LILC) of metal alkanoates. They are (I) bi-layer cell, which consists of a thin dye film covered by LILC, and (II) LILC with electrochromic impurity (viologen). Applying dc electric voltage to the cell II leads to adsorption of viologen redox products with formation of a thin nanosized layer on the cathode. The dynamic holographic technique with pulsed laser radiation of the nanosecond duration was used to investigate nonlinear optical properties in both types of cells. The mechanism of photoconversion of viologen redox products under strong laser radiation is proposed to explain grating recording in the cell II.
Holographic grating recording in “lyotropic liquid crystal – viologen” system
(Національний Авіаційний Університет, 2013) Bordyuh, Hanna; Polishchuk, Arkadiy
This work presents the results of nonlinear optical experiment run on the samples of lyotropic liquid crystal (LLC) with viologen admixtures. During the experiment we obtained dynamic grating recording on bilayered LLC-viologen samples and determined main characteristics of recoded gratings. It was found out that the recording takes place in a thin near-cathode coloured viologen layer. The analysis of kinetics of thermal gratings erasing showed that contribution of a thermal nonlinearity into general diffraction efficiency is negligible small. The last fact is connected with a separation of LLC-viologen samples under the action of an electric field and heat sink into the liquid crystal layer.
Isostructural magnetic phase transitions and the magnetocaloric effect in Ising ferromagnets
(Low Temperature Physics 40, 823 (2014), 2014-04) Лаванов, Геннадій Юрійович
It is shown that the first-order isostructural magnetic phase transition between antiferromagnetic phases with different magnitudes of the antiferromagnetism vector induced by an external magnetic field in Ising antiferromagnets can be related to entropy. It is found that, depending on the temperature, the entropy jump and the corresponding heat release can change their signs at the phase transition point. In the low-temperature region of the metamagnetic first-order phase transition the entropy jump is positive, while the entropy jump is negative near the triple point for isostructural magnetic phase transitions.
Liquid crystal materials with regulated optical properties
(Національний Авіаційний Університет, 2013) Bordyuh, Hanna
In this work we present the results of structural and electrooptical investigations of the new-created composites based on lyotropic liquid crystals (LLC) of potassium caprylate and viologens. Provided structural analysis showed the presence of a Smectic ordering in the investigated samples and allowed to determine its main parameters. It was shown that the electrochromic properties of viologens are preserved after their solution in the liquid-crystalline matrix. They reveal themselves as a coloration of the samples under applying of an electric field and are proved by the optical absorption spectra. Also we analyzed the processes taking place in LLC-viologen composites in the case of an electric field applying.
Magnetic quantum phase transitions and entropy in van vleck MAGNET
(Journal of Magnetism and Magnetic Materials Volume 416, 15 October 2016, Pages 466-474, 2016-10-15) Лаванов, Геннадій Юрійович
Field-induced magnetic quantum phase transitions in the Van Vleck paramagnet with easyplane single-ion anisotropy and competing Ising exchange between ions with the spin S=1 have been studied theoretically. The description was made by minimizing the Lagrange function at zero temperature (Т = 0) and the free energy at T  0. Stable and unstable solutions of equations corresponding to the case T  0 asymptotically transform into those following from the Lagrange function at Т = 0. First-order phase transitions from the Van Vleck paramagnet state into the ferromagnet one were found to take place at a sufficiently high single-ion anisotropy. The entropy of such a magnet was shown to grow with its magnetization, as it occurs for antiferromagnets. At the point of quantum phase transition, the entropy has a jump, which magnitude depends on the ratio between the Ising exchange and anisotropy constants, as well as on the temperature. The described magnetic phase transition was supposed to be accompanied by the magnetocaloric effect. In the case when the Ising exchange dominates over the single-ion anisotropy, the magnetization reversal of ferromagnetic state by an external field was shown to be a phase transition of the first kind, which does not belong to orientational ones and which should be regarded as a quantum order-order phase transition.
Mechanism of Dimerization of Viologens in Liquid Crystalline Medium
(Ukrainian Journal of Physics, 2017) Бордюг, Ганна Борисівна
This work presents the analysis of experimwental data on electrooptical properties of viologens incorporated into the liquid crystalline medium. These data along with theoretical considerations allow specifying the mechanism, which leads to the double color change in the liquid crystal-viologen samples stipulated by the application of increasing voltage values. Specifically, it is proved that the first color change takes place due to the one-electron reduction of viologen molecules, and the second color change is caused by the dimerization between viologen molecules, which were fully reduced under the action of a voltage applied, and initial viologen molecules.
Mechanism of dimerization of viologens in liquid crystalline medium
(Ukr. J. Phys. 2017. Vol. 62, No. 4 294-298, 2017) Bordyuh, Hanna
This work presents the analysis of experimental data on electrooptical properties of viologens incorporated into the liquid crystalline medium. These data along with theoretical considerations allow specifying the mechanism, which leads to the double color change in the liquid crystal – viologen samples stipulated by the application of increasing voltage values. Specifically, it is proved that the first color change takes place due to the one-electron reduction of viologen molecules, and the second color change is caused by the dimerization between viologen molecules, which were fully reduced under the action of a voltage applied, and initial viologen molecules.
Mechanism of dimerization of viologens in liquid crystalline medium
(Національний Авіаційний Університет, 2015) Bordyuh, Hanna
This work presents the analysis of experimental data on optical properties of viologens introduced to the liquid crystalline medium stipulated by the application of increasing voltage values. These data along with quantum mechanical theory allowed to specify the mechanism of dimerization of violoden molecules which leads to the colour change of the liquid crystal-viologen sample. Specifically, the dimerization of viologen molecules in liquid crystalline medium occurs due to interaction between viologen molecules, which were fully reduced under the action a voltage applied, and initial viologen molecules.
Mechanism of optical nonlinearity in “Lyotropic Liquid Crystal-Viologen” system
(2014) Bordyuh, Hanna; Polishchuk, Arkadiy
In the present work we analyze the characteristics of holographic grating recording and consider a mechanism of optical nonlinearity in the lyotropic liquid crystal (LLC) —viologen samples. Taking into account structural and electrooptical properties of the admixture molecules it is possible to suggest that the recording is realized due to the change of polarizability of -electron system of coloured viologen derivatives under the action of laser radiation. The main nonlinear optical parameters such as nonlinear refraction coefficient, cubic nonlinear susceptibility, and hyperpolarizability were calculated.
Nonlinear Optical Response of Smectic Glasses Based on Cobalt Alkanoates
(Український фізичний журнал, 2012) Gridyakina, Alexandra; Polishchuk, Arkadiy
The nonlinear-optical response of anisotropic smectic glasses based on cobalt-alkanoates is studied using the method of dynamic holography. Laser-induced dynamic gratings under the action of nanosecond laser pulses are observed and analyzed for such materials. It is found that a cubic optical nonlinearity of all studied anisotropic glasses is of electronic origin in the nanosecond diapason and caused by a nonlinear polarization of cobalt-alkanoates complexes.
Nonlinear optical response of smectic structure glasses based on cobalt alkanoates
(IEEE, 2010, 2010) Gridyakina, Alexandra
Nonlinear-optical response of metal-alkanoates anisotropic glasses based on cobalt-alkanoate was studied using method of the dynamic holography. Fundamental optical (refractive index and absorption coefficient) and nonlinear-optical (nonlinear susceptibilities and polarizabilities) parameters of the mesomorphic glasses based on cobalt-alkanoates can be tuned via varying chain length anion (interlayer distance of homologous lines) and\or by changing of the cation composition (exploiting binary or ternary systems).
Novel Nanocomposite Materials Based on Mesomorphic Glasses of Metal Alkanoates: Structure and Nonlinear Optical Properties
(High Energy Chemistry, 2009, Vol. 43, No. 7, pp. 532–535, 2009) Gridyakina, Alexandra; Polishchuk, Arkadiy
Novel nanocomposite materials based on an ordered mesomorphic glass of metal alkanoates have been proposed for fast transfer and processing of optical information. Metal alkanoate salts when heated form a smectic mesophase which can be easily supercooled forming an ordered mesomorphic glass at room temperature. The glass has the structure of the smectic-A phase: electrostatic cation–anion layers alternate with bilayers of alkane chains. The mesomorphic glass is a universal matrix for organic and inorganic nanoinclusions and nanosized crystals. In this paper, the structure of the cells of the mesomorphic glasses with various inclusions and their nonlinear optical properties, which become evident upon the impact of pulsed laser radiation with an intensity of up to 5.5 MW/cm2, have been investigated.
Optical properties of mesophases and mesomorphic glasses of Co2+ alkanoates homologous series
(Proceedings of the National Aviation University. 2013. N 4 (57): 48–52, 2013) Gridyakina, Alexandra; Polishchuk, Arkadiy
The results of spectral studies of thermotropic ionic liquid crystals (TILC) of homologous series of cobalt alkanoates and their mesomorphic glasses are presented. It is shown that the dominant coordination of the cobalt ions in the investigated media is an octahedron. A significant decrease the absorption coefficient with increasing of the aliphatic chain length in the homologous series of cobalt alkanoates is caused by the decrease of the volume concentration of the cobalt cation complexes and the dipole moments of d-d-transitions.

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