Research / Annual reports / Main research results of the Institute for Condensed Matter Physics of NAS of Ukraine in 2016

# Main research results of the Institute for Condensed Matter Physics of NAS of Ukraine in 2016

Water, as one of the most important for life molecular fluids, has always attracted researchers for explanation of their numerous anomalous properties. However, ab initio studies in frames of the electron density functional theory for structural and dynamic properties of water did not result for long time in a reasonable agreement with experiments. This was connected with a high melting temperature of the hexagonal ice, which for regular exchange-correlation functionals was higher than 400К. One of the problems of the electron density functional theory is incorrect asymptote for van-der-Waals potentials for dimers, hence, in order to make a correction for the correct behavior ~r-6 it was suggested to add to the expression for total energy a so-called dispersion correction, which corrects forces acting between particles in ab initio computer simulations. In paper [1] we used the dispersion corrections of 3-rd generation D3 for ab initio simulations of two-phase equilibrium between ice and water. A largest by the date for ab initio simulations two-phase system of 389 water molecules was used, which was studied at 7 temperatures between 250К and 400К in constant temperature-volume NVT and constant temperature-pressure NPT ensembles. Results obtained from the ice-water equilibrium showed, that dispersion corrections D3 lead to reduction of the ice melting temperature by about 100 dergees, which was estimated as 325К for the BLYP+D3 functional. This allows to study properties of water by ab initio simulations with D3 dispersion corrections at conditions, which are essentially closer to the experimental ones.

Fig. Snapshot of two-phase ice-water equilibrium of 389 water molecules in ab initio simulations with BLYP+D3 dispersion corrections.

A.P. Seitsonen and T. Bryk. “Melting temperature of water: DFT-based molecular dynamics simulations with D3 dispersion correction”, Phys.Rev. B, 94, 184111 (2016)

An exact time-resolved photoemission spectroscopy is determined for a nesting driven charge density wave (described by the spinless Falicov-Kimball model within dynamical mean-field theory). The pump-probe experiment involves two light pulses: the first is an ultrashort intense pump pulse that excites the system into nonequilibrium, and the second is a lower amplitude, higher frequency probe pulse that photoexcites electrons. Three different cases were examined: the strongly correlated metal, the quantum-critical charge density wave, and the critical Mott insulator. Our results show that the quantum critical charge density wave has an ultraefficient relaxation channel resulting in little net excitation. In contrast, the metal and the Mott insulator show excitations that are closer to what one expects from these systems. In addition, the pump field produces spectral band narrowing, peak sharpening, and a spectral gap reduction, all of which rapidly return to their field free values after the pump is over. (O.P. Matveev, A.M. Shvaika in collaboration with J. Freericks (Georgetown University, U.S.A.) and T. Devereaux (Stanford University, U.S.A.))

We consider the quantum Heisenberg model on a two-dimensional bilayer which consists of two honeycomb lattices. Such a spin model might be used to describe the properties of some magnetic isolators. We have discovered theoretically that in the presence of an external magnetic field the system shows such phase transitions as the famous Ising-Onsager phase transition at finite (but low) temperature and a spin-flop transition. This is unexpected, since the interaction in the model is isotropic. This becomes possible because the magnetic field drives into the game a certain class of low-energy states which are corresponding for the mentioned phase transitions. These studies were performed by T.Krokhmalskii, V.Baliha, and O.Derzhko in collaboration with Johannes Richter (Magdeburg University, Germany).

A field-theoretical approach is applied to study the structure of two model fluids with Yukawa-like pair potentials in the vicinity of a hard wall.

The first model investigates a Maier-Saupe nematogenic fluid. A non-linear integral equation for the singlet distribution function is obtained. Analytical estimations of the density and order parameter profiles are compared to the numerical solution of the mean field equation. A fundamental role of fluctuations for the contact theorems for the density and order parameter profiles is revealed. In order to account for the contribution from fluctuations into the structural properties of the model, the solution of the Riemann boundary problem for the pair correlation function is generalized for the anisotropic case.

The second model considers a fluid with soft repulsion between particles and a damped oscillatory tail of the potential due to the presence of a solvent. The system is described beyond the mean field approximation with the Gaussian fluctuations taken into account. Analytical expressions for the pair correlation function, the density profiles, and the adsorption coefficient are derived. These results are analyzed for different values of the temperature, density, and parameters of the pair potential. The adsorption coefficient as a function of density is found to exhibit qualitatively different kinds of behavior depending on the softness of the short-range repulsive potential.

In order to estimate the validity of the applied approximations, the theoretical results are tested against the original computer simulations data. Compared to the mean field prediction, the Gaussian approximation is shown to provide a much better description of the fluid density profile.(M.F. Holovko, T.Patsahan, I.Kravciv)

Fig. Simulation snapshot of the nematogenic fluid between two hard walls

Di Caprio D., Kravtsiv I., Patsahan T., Holovko M., Oscillating Yukawa fluid at a hard wall: field theory description, Mol. Phys., 114, 2016, 2500-2515.

The critical behaviour of magnets with non-collinear ordering was considered. In physical realizations, such ordering may be caused by the lattice symmetry or the competing types of inter-particle interactions. Classical example of systems which order non-collinearly are helical magnets Ho, Dy, beta-MnO2 and three-dimensional stacked antiferromagnets VCl_2, VBr_2, CsMnBr_3. Numerous experimental and MC studies performed so far do not lead to the definitive conclusion about the order of transition into the non-collinear state. Using the non-perturbative renormalization group approach we have analysed a model for such systems in d dimensions with the 2N component order parameter. The border line Nc(d) separating the region of values N, where the first order phase transition is realized, N Nc(d) was obtained. Our results show the absence of critical behavior for physical cases (N=2, 3 and d=3), supporting outcome of ϵ=4−d-expansion. (M. Dudka in collaboration with B. Delamotte, D. Mouhanna, S. Yabunaka).

The results are published in:

B. Delamotte, M. Dudka, D. Mouhanna, and S. Yabunaka, Functional renormalization group approach to non-collinear magnets. Phys. Rev. B 93, (2016) 064405

Fig. Borderline Nc(d) separating region of N where the first order phase transition occurs, N

We were interested in analysis of academic communities to a particular urgent topic which abruptly arises as a scientific problem. To this end, we have chosen the disaster that occurred in 1986 in Chornobyl, Ukraine, considered as one of the most devastating nuclear power plant accidents in history. The academic response was evaluated using scientific-publication data concerning the disaster using the Scopus database to present the picture on an international scale and the bibliographic database ‘‘Ukrainika naukova’’ to consider it on a national level. We measured distributions of papers in different scientific fields, their growth rates and properties of co-authorship networks. Elements of descriptive statistics and tools of complex network theory were used to highlight the interdisciplinary as well as international effects. Our analysis allows comparison of contributions of the international community to Chornobyl-related research as well as integration of Ukraine in international research on this subject. Furthermore, the content analysis of titles and abstracts of the publications allowed detection of the most important terms used for description of Chornobyl-related problems. (O. Mryglod, Yu. Holovatch in collaboration with R. Kenna, B. Berche).

The results are published in:

O. Mryglod, Yu. Holovatch, R. Kenna, B. Berche. Quantifying the evolution of a scientific topic: reaction of the academic community to the Chornobyl disaster. Scientometrics 106 (2016) 1151–1166

Fig. Collaboration network of countries for Chornobyl-related research based on Scopus data (publication period: 1986–2015). The nodes, which belong to the same component are shown in a same color online.