O. V. Velychko

National Academy of Sciences of Ukraine, Kharkiv, Kharkivs'ka Oblast', Ukraine

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Publications (18)5.73 Total impact

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    I. V. Stasyuk, O. V. Velychko
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    ABSTRACT: The Bose-Einstein condensation in the hard-core boson limit (HCB) of the Bose-Hubbard model with two local states and the particle hopping in the excited band only is investigated. For the purpose of considering the non-ergodicity, a single-particle spectral density is calculated in the random phase approximation by means of the temperature boson Green functions. The non-ergodic contribution to the momentum distribution function of particles (connected with the static density fluctuations) increases significantly and becomes comparable with the ergodic contribution in the superfluid phase near the tricritical point.
    10/2012;
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    I. V. Stasyuk, O. V. Velychko
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    ABSTRACT: Phase transition into the phase with Bose-Einstein (BE) condensate in the two-band Bose-Hubbard model with the particle hopping in the excited band only is investigated. Instability connected with such a transition (which appears at excitation energies $\delta<\lvert t_0' \rvert$, where $\lvert t_0' \rvert$ is the particle hopping parameter) is considered. The re-entrant behaviour of spinodales is revealed in the hard-core boson limit in the region of positive values of chemical potential. It is found that the order of the phase transition undergoes a change in this case and becomes the first one; the re-entrant transition into the normal phase does not take place in reality. First order phase transitions also exist at negative values of $\delta$ (under the condition $\delta>\delta_{\mathrm{crit}}\approx-0.12\lvert t_0' \rvert$). At $\mu<0$ the phase transition mostly remains to be of the second order. The behaviour of the BE-condensate order parameter is analyzed, the $(\Theta,\mu)$ and $(\lvert t_0' \rvert,\mu)$ phase diagrams are built and localizations of tricritical points are established. The conditions are found at which the separation on the normal phase and the phase with the BE condensate takes place.
    03/2011;
  • I. V. Stasyuk, O. V. Velychko
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    ABSTRACT: Based on the mean field approximation, we investigate the transition into the Bose-Einstein condensate phase in the Bose-Hubbard model with two local states and boson hopping in only the excited band. In the hard-core boson limit, we study the instability associated with this transition, which appears at excitation energies δ < |t 0 |, where |t 0 | is the particle hopping parameter. We discuss the conditions under which the phase transition changes from second to first order and present the corresponding phase diagrams (Θ,µ) and (|t 0 |, µ), where Θ is the temperature and µ is the chemical potential. Separation into the normal and Bose-Einstein condensate phases is possible at a fixed average concentration of bosons. We calculate the boson Green’s function and one-particle spectral density using the random phase approximation and analyze changes in the spectrum of excitations of the “particle” or “hole” type in the region of transition from the normal to the Bose-Einstein condensate phase. KeywordsBose-Hubbard model–hard-core boson–Bose-Einstein condensate–energy spectrum
    Theoretical and Mathematical Physics 01/2011; 168(3):1347-1357. · 0.67 Impact Factor
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    O. V. Velychko, I. V. Stasyuk
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    ABSTRACT: Lithium intercalated anatase used in Li-ion batteries has some special features: coexistence of Li-rich and Li-poor phases as well as two possible positions for Li ions in the oxygen tetrahedron. A theoretical description of the compound considering those peculiarities is presented. As shown by the performed symmetry analysis, the intercalation induced lattice deformation can be accompanied by the ordering of antiferroelectric type (internal piezoeffect). In the following step, a qualitative illustration of the phase separation in the lithiated anatase is given within the Landau expansion at the proper choice of coefficients. A microscopic model for description of the compound is also proposed which combines features of the Mitsui and Blume-Emery-Griffits models and utilizes the symmetry analysis results. Various ground state and temperature-dependent phase diagrams of the model are studied to find a set of model parameters corresponding to the lithiated anatase. A phase separation into the empty and half-filled phases in a wide temperature range has been found closely resembling the phase coexistence in the intercalated crystal. In the framework of the model, the two-position Li subsystem could have the ordering of ferro- or antiferroelectric types which, however, has not been yet observed by the experiment. Comment: 19 pages, Reported at the Conference "Statistical Physics: Modern Trends and Applications" dedicated to the 100-th anniversary of N.N. Bogolyubov (June 23--25, 2009 Lviv, Ukraine)
    07/2009;
  • I. V. Stasyuk, I. I. Grygorchak, O. V. Velychko
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    ABSTRACT: Intercalation of gallium and indium selenide crystals by nickel initiates their electret polarization. The effect manifests itself at small concentrations of the intercalant. Temperature dependences of dielectric permittivity for both crystals demonstrate peaks in the same region of concentration. The effect is presumably connected with the ordering of local quasi-dipoles caused by the redistribution of intercalant atoms between octahedral and two tetrahedral positions. It is shown that a microscopic four-state model is able to reproduce a peak-like behaviour of the polarization at change of the intercalant chemical potential (concentration) and peculiarities of temperature dependences of the transverse dielectric susceptibility.
    Ferroelectrics 01/2008; 362(1):115-122. · 0.42 Impact Factor
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    I. V. Stasyuk, O. V. Velychko
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    ABSTRACT: A simple four-sublattice order-disorder model is developed for description of phase transitions and dielectric properties of the Rochelle salt crystal. The model is developed as a generalization of the semimicroscopic Mitsui model. The symmetry properties of lattice and spatial orientations of effective dipoles connected with the asymmetric structure units in the elementary cell are taken into account. The model allows to investigate the temperature and field behaviour of transverse (besides longitudinal) components of dielectric susceptibility. The influence of the transverse electric field $\vec{E}\parallel\vec{b}$ on the phase transition points and spontaneous polarization is studied.
    01/2005;
  • I. Stasyuk, Z. Czapla, S. Dacko, O. Velychko
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    ABSTRACT: The dielectric properties of glycinium phosphite (GPI) crystals as a function of temperature and electric field magnitude are investigated. The electric field E is applied perpendicularly to the ferroelectric b-axis in the direction of hydrogen-bonded phosphite chains in the crystal (the c-axis). The shift of the paraelectric-ferroelectric phase transition to lower temperatures proportionally to Ec2 (where Ec is an effective field in the sample) is observed. Strong anomalies in the field dependence of the permittivity \varepsilon '_c in the temperature region T \leq T_{\mathrm {c}} are revealed. It is shown that the observed jump-like changes of \varepsilon '_c are caused by the phase transition from the ferro- to paraelectric phase induced by the electric field. Such a transition is connected with the rearrangement of protons on hydrogen bonds and the reversal of the corresponding dipole moments, at which the compensation of their components along the b-axis takes place. The theoretical description of the observed dielectric anomalies, given on the basis of the phenomenological Landau free energy approach, is in good agreement with the experimental data.
    Journal of Physics Condensed Matter 01/2004; 16(12):1963-1979. · 2.22 Impact Factor
  • I. STASYUK, O. VELYCHKO
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    ABSTRACT: The microscopic model based on the consideration of the proton ordering is proposed for theoretical study of glycine phosphite (GPI) crystals with a complex structure of the hydrogen bond network. The phase transition into the ferroelectric state is described and changes of the dielectric susceptibility of the crystal are investigated in the presence of the transverse external electric field acting along the c-axis.*Paper originally presented at 10th European Meeting on Ferroelectricity, Cambridge, U.K., August 3–8, 2003.
    Ferroelectrics. 01/2004; 300(1):121-124.
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    ABSTRACT: Within the developed microscopic models we analyze the role of different mechanisms of external pressure influence on phase transitions in deuterated crystals of the KD 2 PO 4 family; crystals of DMAGaS type, and H-bonded ferroelectrics of the Me 3 H(XO 4 ) 2 family.
    Ferroelectrics 01/2003; 288(1):133-145. · 0.42 Impact Factor
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    ABSTRACT: This paper presents the results of investigations of the dielectric, birefringent and electrooptic properties of NH2(CH3)2Me(SO4)2 · 6H2O (Me = Al, Ga) crystals. The data obtained are compared with the predictions of a phenomenological theory. In particular, the temperature behaviour of the dielectric susceptibility components and their anomalies in the vicinity of phase transition points were investigated in details. The specific behaviour of the studied parameters in the low temperature region was related to the coexistence of ferroelectric and antiferroelectric ordering of the dimethylammonium groups carrying a dipole moment.
    physica status solidi (b) 01/2001; 228(3):785-798. · 1.49 Impact Factor
  • I. V. Stasyuk, O. V. Velychko
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    ABSTRACT: A configurational model for the quantum statistical description of the process of metal ion complex formation in water solution is proposed. Special attention is paid to the formation of hydroxocomplexes. In the framework of the model determination of such chemical characteristics of reaction as Bjerrum function, partial mole fraction and formation constants is made. It is established that regions of existence of different complex forms at variation of solution pH reaction depend on differences of configuration energies and temperature. Existence of a saturation effect which leads to a sharp change of ligand concentration in solution when the metal ion concentration crosses a certain value is established. Dependence of this effect on temperature and other parameters of the system is investigated.
    Physics and Chemistry of Liquids 11/2000; 38(6):743-757. · 0.52 Impact Factor
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    ABSTRACT: A simple description of thermodynamics of DMAAS and DMAGaS ferroelectric crystals by means of Landau expansion is proposed. Conditions of occurrence of phase transitions are established and their temperatures are obtained. The influence of external hydrostatic pressure on phase transitions is described. The temperature behaviour of dielectric susceptibility components and their anomalies in the vicinity of phase transition points are investigated. Obtained results are compared with experimental data.
    02/2000;
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    I. V. Stasyuk, O. V. Velychko
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    ABSTRACT: A four-state model is proposed for description of the sequence of phase transitions in ferroelectric crystals of DMAGaS and DMAAlS type. The ordering processes in the subsystems of DMA groups are considered as a main reason of such transformations. The interaction between groups in their various orientational states is taken into account in the dipole-dipole approximation. Obtained thermodynamical characteristics of the model (spontaneous polarization, occupancy of orientational states, dielectric susceptibility, phase diagram) are in good agreement with experimental data. The experimental fact of suppression of ferroelectric phase at increase of hydrostatic pressure is explained under assumption that pressure changes the difference between energies of various orientational states of DMA groups mostly.
    01/2000;
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    I. V. Stasyuk, O. V. Velychko
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    ABSTRACT: The four--state model is proposed for the description of phase transition in ferroelectric crystals of the DMAGaS and DMAAlS type. Thermodynamic functions of the model are obtained in the mean field approximation. The phase transition between paraelectric and ferroelectric phases is investigated. It is established that the order of the phase transitions depends on the relations between model parameters.
    Journal of Physical Studies 01/2000; 4:92-99.
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    I. Stasyuk, Z. Czapla, S. Dacko, O. Velychko
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    I. V. Stasyuk, O. V. Velychko
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    ABSTRACT: Measurements of dielectric properties of Ni-intercalated GaSe and InSe have been performed. The present study is aimed at the investigation of the low-admixture region where the intercalation induced electret effect occurs. The effect exhibits pronounced peak-like concentration dependences and a non-monotonous temper- ature behaviour with maximum magnitudes at low temperatures. Intercalation leads to over tenfold increase of dielectric permittivity over the whole measured frequency range with up to several orders at low frequencies for GaSe. Temperature dependences of the permittivity demonstrate well-defined peaks with localizations and heights strongly depending on the concentration. A microscopic model of order-disorder type has been pro- posed that considers redistribution of intercalant atoms between non-polar octahedral and polar tetrahedral positions in the crystal van der Waals gaps. Such a redistribution can occur in the form of phase transi- tion to the polar phase (corresponding to the electret effect) which is stabilized by the internal field. For the case of octahedral positions being more preferable, the model predicts a peak-like dependence of the crystal polarization on chemical potential due to passing through the interjacent polar phase in accordance with the measured behaviour of the electret effect. The calculated temperature dependences of dielectric susceptibility qualitatively reproduce experimental results for permittivity as well.
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