Main research results of the Institute for Condensed Matter Physics of NAS of Ukraine in 2012

a) Main fundamental results

We propose a new approach for handling the salvation interactions in multiscale molecular dynamics of complex biochemical systems in the canoncal-isokinetic ensemble. This has allowed us to overcome the limitations on time step and efficiency, inherent in the standard methods. On an example of the alanine dipeptide in aqueous solution it is shown that the new approach can use huge outer time steps h (hundreds of picoseconds), speeding up the calculations by a factor of 100 to 500 compared to the best schemes known previously (see figure). The new approach can be modified and extended to more complicated models of biological liquids including, in particular, proteins and other macromolecules (Omelyan I.P.).

Static scattering function on f-arm star polymers in solutions in presence of spatial inhomogeneities, correlated according the power law ∼r^-a was analyzed. Such study is important for understanding the behavior of macromolecules in colloidal solutions, in vicinity of porous membranes and in cellular environment. In particular, we studied the ratio g(f) of scattering intensities on star and linear macromolecules of the same molecular weight, which is an experimentally measurable quantity. Applying the direct polymer renormalization approach, we found, that increasing of correlations in porous structure (decreasing of the correlation parameter a leads to smaller distinction between the size measure of a star and linear polymers of the same molecular weight (Yu. Holovach, V. Blavatska).

Досліджено так званий "плоскозонний феромагнетизм", що виникає на класі фрустрованих граток. Принцип Паулі забезпечує мінімізацію кулонівської енергії відштовхування завдяки зменшенню перекриття хвильових функцій з утворенням феромагнітного стану з електронів, що локалізовані в сусідніх областях гратки. Математично проблема зведена до нового класу задач про протікання – Паулі-скорельованої перколяції; система стає феромагнітною, як тільки концентрація електронів дозволяє утворення безмежно великого кластера. Показано, що перехід до феромагнетизму є переходом першого роду з широкою областю співіснування пара- і феромагнітної фаз (Держко О.В., Максименко М.В.).

The X-ray photoemission spectroscopy (XPS), X-ray absorption near-edge spectroscopy (XANES) and X-ray emission spectroscopy (XES) were examined for the Falicov-Kimball model extended by the interaction with the deep core hole. An exact results in terms of the functional determinants were obtained by using solution from dynamical mean-field theory. These three spectra are closely related to one another and display orthogonality catastrophe behavior in the metallic phase at T=0. Our results show that at finite temperatures the sharp edge peak exists both for the metallic and Mott insulator phases but transforms in the power law singularity and δ-peak at zero temperature, respectively.
The features of the resonant inelastic x-ray scattering (RIXS) in the Mott insulator were analyzed. The spectra contain the low-energy peak from the inband electron-hole excitations, high-energy peak from the overgap interband electron-hole excitations, and quasielastic peak at very low frequencies from the density fluctuations, which gives the main contribution in the dynamical structure factor. The account of the nonradiative Auger and fluorescence relaxation processes (E_h → E_h − iΓ) strongly reduces the high-energy response and does not influence the quasielastic features (A.M. Shvaika in collaboration with J. Freericks and Nandan Pakhira (Georgetown University, U.S.A.)).

On the base of collective variables method it was established connection between different statistical-field formulations of theory of non-homogeneous fluids widely used in modern investigations of liquids. Separately it was examined the case of transfer to functional field theory. It was shown that collective variables method has some advantages and can be purposed even in cases when another methods become inapplicable.(Mryglod I., Patsahan O.).

З використанням потенціалів внесеного атома (embedded atom potentials) методом молекулярної динаміки вперше розраховано спектри колективних збуджень та динамічні структурні фактори для рідкого заліза. Отримані результати, зокрема для швидкості поширення звуку та співвідношення теплоємностей, добре відтворюють експериментальні дані для розплаву заліза вище від точки плавлення і правильно відтворюють особливості динамічних структурних факторів, що спостерігаються в експериментах із непружнього рентгенівського розсіяння. Таке узгодження, виявлене для рідкого заліза при нормальному тиску, дозволяє стверджувати, що використані потенціали можуть бути застосовні для дослідження рідкого заліза при високих тисках, що важливо для проблем геофізики (Брик Т.М.).

The nematic phase of the liquid crystal with an anisotropic Yukawa potential of interaction has been studied. In the bulk case the pair correlation function has been calculated. The orientational distribution function with the account of the Gaussian fluctuations has been found. The latter has been shown to reduce to a renormalized mean field Maier-Saupe form. Subsequently, corrections to the order parameter due to fluctuations have been found. Analytical expressions for thermodynamic functions (the free energy, the pressure, the chemical potential, the internal energy) and the elasticity constant in the random phase approximation have been obtained. For the first time in the approximations considered the contact theorem for an anisotropic fluid at a hard wall has been proved. It has been shown that the density contact value is determined by the bulk pressure and does not depend on the angle between the director and the surface. It has also been demonstrated that near the surface the axial symmetry vanishes and additional planar phases can appear. For the case of the director being oriented perpendicular to the surface, the profiles of the density and of the order parameter S have been calculated. With the decrease of the bulk order parameter, in the vicinity of the wall the fluid has been found to become more dilute and less orientationally ordered. As the nematic phase becomes stable at S = 0.43, the possibility of the nematic-isotropic phase transition near the surface has been predicted (M. Holovko, the corr. member of the NAS of Ukraine, and I. Kravtsiv).

A temperature behavior of the frustrated translational mode (T-mode) of a light particle, coupled by different regimes of ohmicity to the surface, is studied within a formalism of the generalized diffusion coefficients. The memory effects of the adsorbate motion are considered to be the main reason of the T-mode origin. We provide an explanation of the experimentally observed blue- or red-shift of the T-mode on the basis of a comparative analysis of two typical times of the system evolution: a time τ_ν of decay of the “velocity-velocity” autocorrelation function, and a correlation time τ_cor of the thermal bath random forces. It is shown that in the sub-Ohmic regime with s<<1 a strong inequality τ_ν << τ_cor, is valid, leading to the blue-shift of the T-mode since a non-relaxed energy of the lattice is being delivered to the adparticle increasing its effective vibrational frequency ω_T(T). On the contrary, in the strong super-Ohmic regime with s ≈ 2 the inequality τ_ν >> τ_cor becomes valid. At the timescales of about τ_cor the adparticle does not have enough time to be thermalized and starts to transfer its energy to the lattice, yielding the observed red-shift of the T-mode.

Temperature dependence of the T-mode frequency at different values of the ohmicity indexes s, dimensionless coupling constant Gx, and vibrational frequency Ω/ωmax.

Since there is a one-to-one correspondence between the system ohmicity and the kind of the coupling, one can deduce about a nature of the “adsorbate-substrate”' interaction from the temperature induced shift of the T-mode. We have also obtained analytical expressions for the T-mode shift and width at weak coupling in zero temperature and high temperature limits. This method allows to extrapolate the results obtained at T≠0 to the zero temperature limit, yielding information about the “adsorbate-substrate”' interaction strength and vibrational frequency from the IHAS data (V. Ignatyuk).

It is established that the sequence of structural phase transitions observed in Cs₂HgCl₄ crystal is stimulated by the same soft optic phonon branche of the B_3g symmetry in the Brillouin zone center (k = 0). The softening of this mode in the Brillouin zone center may evoke the different lattice instabilities which, depending on the temperature and hydrostatic pressure, may occur at k = 0 or its vicinity k ≠ 0, i.e. leading to an appearance of the spatially homogeneous or inhomogeneous phases, respectively. Another type of lattice instability, also related to the soft optic and acoustic modes of the same symmetry, is realized in Cs₂HgCl₄ crystal via the direct second-order ferroelastic phase transition at hydrostatic pressures above 140 MPa. It is again caused by a lowering of the soft optic branch of B3g symmetry, but this time preferably at the Brillouin zone center as sketched in figure 3(e) (left panel). Due to the bilinear coupling between these branches, the lowering of the soft optic branch leads to the critical slowing down of the acoustic phonons near the k = 0 point. (Ya. Shchur, S. Kamba, O.V. Parasyuk, A.V. Kityk)

Within the viscoelastic approximation for the ion-polarization model of ionic melts a spectrum of collective excitations is investigated. It was shown that taking into account polarizability of ions leads to renoralization of the sound damping coefficient. Among three relaxation modes one is due to a dipolar diffusion of the polarized ions, shifted from its hydrodynamic value because the interaction with other relaxation modes. A pair of the remaining relaxation modes are due to electric conductivity. (Vasylenko A.I., Markiv B.B., Tokarchuk M.V.)

b) Main applied results

Computer programme ICaS Color Print 2.0 was developed and registered by State service of intellectual properties of the Ukraine (cert. № 46371 by 13.11.2012 ). New informational technology of color separation ICaS-ColorPrint was tested in working environment. Results of work were exhibited on international specialized exhibition “High technology 2012”, Kyjiv, September 2012 (M.V.Shovhenyuk).

Рідкокристалічний еластомер (РКЕ) поєднує еластичність полімерної матриці із оптичними властивостями рідкокристалічних груп і має перспективи застосування в якості "штучних м’язів". Для передбачення взаємозв’язку між топологією полімерної матриці і макрореакцією цього мета-матеріалу на зовнішні збурення виконано симуляції методом молекулярної динаміки. Отримано ефект пам’яті при переході РКЕ через точку рідкокристалічного переходу. Встановлено, що причиною цього є суттєве сповільнення дифузії мономерів через наявність у РКЕ додаткових зв’язків порівняно із розплавом таких же молекул. При прикладенні до РКЕ одновісного навантаження встановлено формування нематичної фази в ізотропній системі та ефекти як плавної переорієнтації нематичного директора, так і формування V-подібної "шеврон"-структури, коли навантаження прикладено у монодоменній смектичній фазі. Розвинутий підхід дозволяє розгляд фото-активних еластомерів, які характеризуються оборотніми фото-деформаціями, що мають технологічне застосування (Ільницький Я.М.).