Research / Annual reports / Main research results of the Institute for Condensed Matter Physics of NAS of Ukraine in 2017

# Main research results of the Institute for Condensed Matter Physics of NAS of Ukraine in 2017

One of the manifestations of a qualitative change in the properties of supercritical flows from the behavior of a gases type to the behavior of a liquid type is the manifestation of a "positive dispersion" of sound excitations in scattering experiments. To describe this effect, an approach based on analytic results obtained in the framework of a thermo-elastic model of generalized hydrodynamics is proposed. In parallel, the idea of the presence of the "Frenkel line" on the phase diagram of the streams, separating the gaseous flows from the fluid-type flows, developed. To analyze the conditions proposed for the "Frenkel line" definition, we have carried out the MD modeling of supercritical neon along an isothermal line, for which there are experiments on the scattering of X-rays. It is shown that the assumption of the existence of a low frequency cutoff for the frequency for transverse collective excitations (on which the "Frenkel line" approach are based) contradicts both the results of MD and the analytical results of the thermo-elastic model. The expressions for the positive dispersion of sound and specific heat in the "Frenkel line" approach are analyzed and its main contradictions with the generalized hydrodynamic theory are revealed (Bryk T.M., Mryglod I.M.).

Functional photo-guided materials are increasingly used in practice, in particular in medicine in the form of nano- and micro patterns, sensors, artificial muscles, etc. The basis of such materials is the absorption of light energy by sensitive groups, such as azobenzene chromophores, resulting in this energy being converted into mechanical work. In most cases such a conversion is carried out at a relatively weak intensity of irradiation and at temperatures significantly lower than the temperature of the transition of the polymer to the state of the glass. Recent experiments indicate that under the influence of light in the glass state there are significant stresses that can break the layer of graphene placed on the surface of the azobenzine-containing polymer. So far, there was no reasoned explanation for the mechanism of the emergence of such gigantic tensions. For this purpose, the kinetics of photojisomerisation and the time evolution of ordering in azo-benzene-containing polymer materials have been studied. Based on the kinetic photoisomerization equations, it is shown that light absorption is equivalent to the action of an effective potential that reorient azobenzine groups perpendicular to the polarization vector of light.The same conclusions are derived from the results of computer simulation of such systems. The correlation for the amplitude of this potential is obtained, depending on the optical and visco-elastic properties of the material. The photoinduced potential, in turn, leads to the emergence of a giant stress of about a few gigasPascal for typical azobenzine-containing polymers, which explains the experimental results. The proposed approach allows a deeper understanding of physical processes in photo-controlled functional materials, which is important for future applications (Il'nytskyi Y.M.).

In 1925, Ernst Ising investigated the possibility of a phase transition in a one-dimensional system. To his disappointment, there was none at positive temperature. This was the start of a vast literature in the statistical mechanics of critical phenomena, including a number of famous theorems on why it is impossible to have a phase transition in one-dimensional classical systems with short-range interactions. We have found a way around the no-go theorems and we show how one can induce a phase transition on 1D systems - of the type Ising was searching for. We considered nearest neighbours Potts model with invisible states in one dimension. If two neighbouring spins lie in the same visible state, then there is a contribution to the interaction energy. Thus, the number of invisible states does not change the interaction energy, but affects the entropy. Usually phase transitions arise through a competition between entropy and energy. It is now well known that the reason behind the no-go theorems is that there is an excess of entropy in 1D systems - entropy always wins over energy so that the delicate balance that gives a phase transition is never achieved. We analysed the exact solution using the partition function zeros. It was shown that the positive number of invisible states do not change the order of the phase transition and critical temperature. Two methods of shifting the phase transition into the region of positive temperatures were suggested – complex external magnetic field and negative numbers of invisible states. Both these methods, although assume unphysical values of parameters, can be mapped onto the quantum models, where the phase transition at positive temperatures is possible, even in one dimension. (P. Sarkanych, Yu. Holovatch in collaboration with R. Kenna). The results are published in: P. Sarkanych, Yu. Holovatch, R. Kenna. Exact solution of a classical short-range spin model with a phase transition in one dimension: the Potts model with invisible states, Phys. Lett. A, 381, (2017) 3589

Fig. The locus of the Fisher zeros for the Potts model with q=2 visible states and r=-6 invisible states for the system of N=100 particles. The intersection signals the existence of the phase transition at positive temperature t=0.2.

We consider the quantum Heisenberg model on a two-dimensional bilayer which consists of two square lattices. The properties of the model in the presence of an external magnetic field with two different orientations are studied. Such a spin model might be used to describe the properties of recently synthesized magnetic compound Ba2CoSi2O6Cl2. We first show that our theory describes the known experimental facts and then we propose new experimental studies to detect a field-driven phase transition, which leads to ordering in the compound at low temperatures. Since the compound is sensitive to the direction of magnetic field, along which it is applied, then, correspondingly, we will have different phases of ordering with characteristic critical parameters. This phase transition can be detected by measuring the specific heat c, which exhibits a logarithmic singularity in its temperature dependence at Ttrans(H) or in its field dependence at Htrans(T). These studies were performed by O. Krupnitska, V.Baliha, T.Krokhmalskii, and O.Derzhko in collaboration with Johannes Richter (Magdeburg University, Germany).

The basic Feynman integrals required for a theoretical description of complicated anisotropic and constrained systems are calculated on a rigorous mathematical level. As a byproduct, some new relations between special mathematical functions are derived, and some new results for the integrals and double sums of interest in the number theory are suggested. The method of Feynman parametrization has been suggested and successfully employed for the study of such mathematical objects. This is an approach, standard in the quantum field theory and the theory of critical phenomena, but practically unknown in the mathematical community. The results can be successfully employed in investigations of non-trivial dependence of the critical exponents of strongly anisotropic m-axial systems in the Lifshitz point on the physical space dimension d in the area given in the Fig. The derivation of these results opens a perspective for further theoretical studies of such systems under spatial constraints and an appearance of fluctuation-induced forces between the surfaces. (Shpot M.A.).

Effects connected with the Bose-Einstein condensation in the mixtures of Bose- and Fermi- atoms in optical lattices are studied. The conditions are established, at which the phase separation with the appearance of the BE condensate droplets due to the change of the phase transition order takes place. The possibility of the so-called “reentrant” behaviour (when the BE condensate exists at the intermediate temperatures and disappears at the tending to the absolute zero) is foreseen. The phase behaviour of such mixtures, calculated analytically, is in agreement with the computer modulation data. Optical lattices with ultracold atoms are considered as perspective physical objects from the quantum computer creation point of view (V.O.Krasnov, I.V.Stasyuk).

In the ferroelectric glycinium phosphite (GPI) the phase transition to the ferroelectric phase is connected with ordering of protons on the hydrogen bonds. An important feature of GPI consists in possibility of reorientation of the local dipole moments (formed by protons and the adjacent glycine groups) by the external electric field E_{z} perpendicular to the ferroelectric axis OY. It manifests in the proportional to E_{z}^{2} decrease of phase transition temperature T_{c} and in the increasing with E_{z} anomalies of transverse permittivity ε_{zz}. We have developed proton ordering model for GPI by allowance for piezoelectric coupling of the proton subsystem with the lattice strain. Calculations of dielectric, piezoelectric and thermal characteristics are performed in the frames of the two-particle cluster approximation. At the proper choice of the model parameters the satisfactory quantitative description of the available experimental data has been obtained. It is shown that the quite good description of the temperature dependence of ε_{zz} can be achieved, when the presence of the small longitudinal component E_{y} (of the order of ~0.05E_{z}) is supposed. Such a component could appear due to incomplete reorientational relaxation of the glycine groups; one can not exclude, also, the possibility of some deflection of the applied transverse field from the OZ-axis during experiment.

It is shown, that hydrostatic pressure decreases temperature Tc. At the small pressures the phase transition remains of second order, and starting from some critical pressure ~4•108Pa the phase transition become of first order. Effect of hydrostatic pressure on thermodynamic characteristics significantly decreases at deuterating of GPI crystal. (Levitskii R.R., Vdovych A.S., Stasyuk I.V.).

The quantitative analysis of publications of "Journal of Physical Studies" is performed. The database containing bibliographic information about 962 papers published during 1996¬–2016 years is considered. The methods of complex network theory were used to investigate the authors collaboration. The co-authorship network itself contains 1344 nodes (authors) connected by 2386 links (common publications). Several co-authorship networks were constructed on the different levels: separate authors, countries, cities, scientific topics. The geography of collaboration as well as its dynamics were studied. The corresponding quantitative characteristics of networks were calculated. The interconnections between scientific topics within the journal were studied using PACS numbers for each paper. Discrete and continuous models were applied to describe how journal citations are distributed. The results obtained can be useful for assessing the role of a journal as a whole as well as for developing editorial policy for the future. The presented data can be applied for better understanding of general picture of science in Ukraine. (Yu. Holovatch, M. Krasnytska, O. Mryglod in collaboration with A. Rovenchak).

The results are published in: Yu. Holovatch, M. Krasnytska, O. Mryglod, A. Rovenchak. Twenty years of the ``Journal of Physical Studies''. An attempt of journalometrical analysis. Journ. Phys. Stud. 21 No. 4 (2017) 4001 [in Ukrainian]

Fig. Statistics of authors' publications on the level of Ukrainian cities. The radius of a circle for each city depends on the number n of publications as ln(n+1). The links between cities (lines) are non-weighted, representing the fact of collaboration only.

The new theoretical model of digital processing and color synthesis is applied both to the development of a new information technology of digital color printing, and to a new principle of reproduction of colors on the monitor, as well as information encoding. Innovative novelty of the proposed technologies is based on the new principle of color reproduction on the basis of analytical methods of analysis and synthesis of color images. In the case of color printing, each pixel of the image is represented only by two color and black paints. The technology provides optimal technological conditions for color printing and a significant saving (more than 50%) of colored paints. In the case of the proposed new innovative technology, for reproduction of an image on the screen of a color monitor based on the physiological model of Hubel, a minimum energy consumption is provided. Developed innovative technology for encoding color images based on real Hartley transformation allows encoding large-sized digital color images with high resolution, which provides a high level of protection of confidential information (Shovgenyuk M.).