BEGIN:VCALENDAR VERSION:2.0 PRODID:-//CERN//INDICO//EN BEGIN:VEVENT SUMMARY:Features of light coupling in low-loss 2D periodic structures supp orting phonon polariton DTSTART;VALUE=DATE-TIME:20181205T110500Z DTEND;VALUE=DATE-TIME:20181205T112500Z DTSTAMP;VALUE=DATE-TIME:20260316T200102Z UID:indico-contribution-1-11@indico.bitp.kiev.ua DESCRIPTION:Speakers: Yuliia Mankovska (Taras Shevchenko National Universi ty of Kyiv)\nUnderstanding of light-matter interaction in a wide frequency range is an important fundamental problem with significant potential appl ied impacts. For instance\, Plasmonics is a rapidly developing field at th e boundary of physical optics and condensed matter physics with many prosp ective applications. However\, the use of plasmons is limited from a pract ical point of view because their spectral signature is in the visible and near infrared region\, where metals have strong absorption leading to high losses. From this point of view\, the use of low-loss materials\, like po lar dielectrics\, is promising. Particularly\, in such materials surface p honon polaritons can be easily excited from the infrared to the terahertz frequencies resulting in a strong coupling of light and optical phonons in the crystal.\nIn this report\, we studied features of light localization in low loss 2D periodic structures formed by a polar crystal. Specifically \, we considered a periodic structure of silicon carbide nanodisks on a cr ystalline silicon substrate. We numerically solved Maxwell equations with appropriate boundary conditions. The strong dependence of phonon polariton spectral resonances with the period of the array was theoretically reveal ed. Furthermore\, experimental reflectivity measurements on the fabricated structure confirmed these tendencies.\n\nhttps://indico.bitp.kiev.ua/even t/2/contributions/11/ LOCATION: URL:https://indico.bitp.kiev.ua/event/2/contributions/11/ END:VEVENT BEGIN:VEVENT SUMMARY:Boundary conditions for the superconducting junctions at temperatu res close to critical DTSTART;VALUE=DATE-TIME:20181205T100500Z DTEND;VALUE=DATE-TIME:20181205T102500Z DTSTAMP;VALUE=DATE-TIME:20260316T200102Z UID:indico-contribution-1-19@indico.bitp.kiev.ua DESCRIPTION:Speakers: Oleksandr Pastukh (Lesya Ukrainka Eastern European N ational University)\nTo calculate the current-phase relation in supercondu cting junctions\, it is necessary to investigate the spatial behavior of t he order parameter in the superconducting regions of the junction. In the case of temperatures close to the critical one\, the Ginzburg-Landau theor y [1] is used for this purpose. However\, to apply this theory there is ne cessary to find the corresponding boundary conditions for the Ginzburg-Lan dau equation. Boundary condition can be found using the Wiener–Hopf meth od [2-3]\, however\, use of this method for complicated superconducting ju nctions is problematic.\nIn our investigation\, the problem of finding bou ndary conditions for the $\\\\$Ginzburg-Landau equation\, was considered i n the case of different superconducting junctions. In particular\, superco nducting junctions\, combining tunnel effects and the proximity effect\, w ith nonmagnetic impurities in superconducting regions were investigated. F or finding the boundary condition for the Ginzburg-Landau equation the met hod of quasiorthogonality to asymptotics was used [4]. In addition\, there were no restrictions on the values of the electron transmission coefficie nt and the thickness of the normal layer.\nIt has been shown that the boun dary condition for the Ginzburg-Landau equation contains unknown constants for the calculation of which the quasiorthogonality to the asymptotics me thod was used. This method proved to be quite effective for complicated su perconducting systems which contain the combination of dielectric layer an d normal layer. In addition\, boundary conditions obtained using this meth od\, are valid for the arbitrary concentration of nonmagnetic impurities.\ n \n[1] A. V. Svidzinskii\, Spatially Innhomogeneous Problems in the Theor y of Superconductivity\, Nauka\, Moscow (1982).\n[2] R.O. Zaitsev Boundary conditions for the superconductivity equations at temperatures close to c ritical // Sov. Phys. JETP 21\, 1178 (1965).\n[3] A. Barone\, Yu. N. Ovchi nnikov Boundary conditions and critical current of SNS junctions // Zh. Ek sp. Teor. Fiz. 77\, 1463 (1979).\n[4] A. V. Svidzinsky\, and V. E. Sakhnyu k\, Condens. Matter Phys. 3\, 683 (2000).\n\nhttps://indico.bitp.kiev.ua/e vent/2/contributions/19/ LOCATION: URL:https://indico.bitp.kiev.ua/event/2/contributions/19/ END:VEVENT BEGIN:VEVENT SUMMARY:An effective theory for Heisenberg antiferromagnet on one-dimensio nal frustrated lattices at high magnetic fields DTSTART;VALUE=DATE-TIME:20181205T090500Z DTEND;VALUE=DATE-TIME:20181205T092500Z DTSTAMP;VALUE=DATE-TIME:20260316T200102Z UID:indico-contribution-1-7@indico.bitp.kiev.ua DESCRIPTION:Speakers: Olesia Krupnitska (Institute for Condensed Matter Ph ysics\, NAS of Ukraine)\nWe consider the spin-1/2 antiferromagnetic Heisen berg model on one-dimen-sional frustrated lattices (double tetrahedra chai n [1]\, deformed octahedral chain [2]) placed in an external magnetic fiel d with almost dispersionless (almost flat) lowest magnon band. The main go al of our study is to develop a systematic theory for the low-temperature high-field properties of these models\, using the localized magnons approa ch [3\,4]. We construct an effective description of one-dimensional chains with triangular and quadrangular traps by means of the localized magnons concept within the strong coupling approximation. The obtained effective m odels are much simpler than the initial ones: firstly\, the effective mode ls have smaller number of sites and secondly\, and most importantly\, they are unfrustrated. As a result\, one can apply well elaborated methods of the quantum spin systems theory to discuss the properties of the initial frustrated quantum antiferromagnets at high fields and low temperatures. W e perform extensive exact diagonalization calculations to check the validi ty of the obtained effective Hamiltonians by comparison with the initial m odels.\n\n[1] M. Maksymenko\, O. Derzhko and J. Richter\, Acta Physica Pol onica A **119**\, 860 (2011)\; Eur. Phys. J. B **84**\, 397 (2011).\n[2] J . Strečka et al.\, Phys. Rev. B **95**\, 224415 (2017)\; Physica B **536* *\, 364 (2018).\n[3] J. Schulenburg et al.\, Phys. Rev. Lett. **88**\, 167 207 (2002).\n[4] O. Derzhko\, J. Richter\, and M. Maksymenko\, Int. J. Mod . Phys. B **29**\, 1530007\n(2015).\n\nhttps://indico.bitp.kiev.ua/event/2 /contributions/7/ LOCATION: URL:https://indico.bitp.kiev.ua/event/2/contributions/7/ END:VEVENT BEGIN:VEVENT SUMMARY:On consistency of classical homogenization $\\\\$ models for the p ermittivity of statistically homogeneous mixtures DTSTART;VALUE=DATE-TIME:20181205T130000Z DTEND;VALUE=DATE-TIME:20181205T132000Z DTSTAMP;VALUE=DATE-TIME:20260316T200102Z UID:indico-contribution-1-15@indico.bitp.kiev.ua DESCRIPTION:Speakers: Andrii Semenov (Odessa I.I.Mechnikov National Univer sity)\nThe standard effective-medium methods to electrodynamic homogenizat ion of heterogeneous media can be divided into two classes: symmetrical an d asymmetrical. They are assumed to be independent and applicable to diffe rent types of systems\, making a basis for different further modifications that take into account specific features of a given system. Despite the f act that these modifications are able to restore experimental data rather well\, various authors note that the basic assumptions behind the methods are not consistent.\nThe goal of this report is to scrutinize these two ho mogenization methods\, their consistency\, and ranges of validity. To obta in the most general results\, a simple system of impenetrable dielectric b alls embedded in a uniform host medium has been considered. For its analys is\, a generalized differential scheme was developed for the effective qua sistatic permittivity of macroscopically homogeneous and isotropic dielect ric mixtures [1]. The scheme is based upon the compact group approach (CGA ) [2] reformulated in a way that allows one to analyze the role of differe nt contributions to the effective permittivity of the system and modify it for different system structures.\nIt is shown that within the CGA\, the o nly physically consistent homogenization type is symmetrical effective-med ium homogenization. Applicability of this approach to the core-shell model and the description of numerical and experimental data for conductivity o f composite electrolytes was demonstrated in [3]. The asymmetrical (differ ential) homogenization type can be obtained by replacing the electromagnet ic interaction between previously added constituents and those being added by the interaction of the latter with recursively formed effective medium . Under this assumption\, each portion of inclusions has different polariz ation\, and the previously added portions do not interact with the new one s. This can be valid only for narrow concentration ranges and low contrast constituents\, even for the generalized versions of the original mixing r ules\, as can be proved using the Hashin-Shtrikman bounds. Therefore\, the asymmetric approach is approximate in the long-wave limit\, and one shoul d be cautious when using the differential models since they can lead to un predictable errors and wrong results.\n\n[1] A. K. Semenov\, *J. Phys. Com mun.*\, **2** (2018) 035045.\n[2] M. Ya. Sushko\, *Zh. Eksp. Teor. Fiz.*\, **132** (2007) 478 [*JETP* **105** (2007) 426]\; *Phys. Rev. E* **96** (2 017) 062121.\n[3] M. Ya. Sushko\, A. K. Semenov\, *arXiv*:1810.11892 [cond -mat.mtrl-sci].\n\nhttps://indico.bitp.kiev.ua/event/2/contributions/15/ LOCATION: URL:https://indico.bitp.kiev.ua/event/2/contributions/15/ END:VEVENT BEGIN:VEVENT SUMMARY:Equilibrium states of antiferromagnetic ring-shaped and helix-shap ed spin chains with hard-tangential anisotropy DTSTART;VALUE=DATE-TIME:20181205T084500Z DTEND;VALUE=DATE-TIME:20181205T090500Z DTSTAMP;VALUE=DATE-TIME:20260316T200102Z UID:indico-contribution-1-12@indico.bitp.kiev.ua DESCRIPTION:Speakers: Denys Kononenko (Taras Shevchenko National Universit y of Kyiv)\nFor the last decade\, active research on magnetic nanosystems of curved geometry was motivated by their outstanding properties and great application potential [1]. For instance\, recent theoretical studies of l ow-dimensional magnets with complex geometry propose a description of fasc inating geometry-induced effects including pattern formation and magnetoch iral effects in quasi-one-dimensional wires [2]\, for review see [1]. Desp ite these advances in the study of curvilinear low-dimensional ferromagnet s\, significant knowledge gaps exist in the study of curvilinear antiferro manetic systems.\nThe purpose of the current study is the theoretical inve stigation of equilibrium states in antiferromagnetic ring-shaped and helix -shaped spin chains with hard-tangential anisotropy. For this purpose we u se both analytical methods and computer spin-lattice simulations in SLaSi software package [3]. In our study\, we consider two sublattice antiferrom agnet in the frame of the sigma-model approach where its statics and dyna mics are described in terms of Neel vector only. \nWe analytically show t hat the global energy minimum of the antiferromagnetic ring-shaped spin ch ain is reached when Neel vector is perpendicular to the ring plane. An equ ilibrium phase diagram is constructed for the antiferromagnetic helix-shap ed spin chain: (i) a quasi-binormal state is realized in the case of relat ively large curvatures and (ii) spatial-periodic state is typical in the o pposite case. Both states are described analytically and well confirmed by SLaSi.\nStability regions of both ground states are determined using spin -lattice simulator SLaSi.\n\n[1] R. Streubel\, P. Fischer\, F. Kronast\, V . P. Kravchuk\, D. D. Sheka\, Y. Gaididei\, O. G. Schmidt and D. Makarov\ , J. Phys. D\, 49\, 363001\, (2016). \n[2] D. D. Sheka\, V. P. Kravchuk\, Y. Gaididei\, J. Phys. A\, 48\, 125202\, (2015).\n[3] http://slasi.knu.ua /\n\nhttps://indico.bitp.kiev.ua/event/2/contributions/12/ LOCATION: URL:https://indico.bitp.kiev.ua/event/2/contributions/12/ END:VEVENT BEGIN:VEVENT SUMMARY:Ground-state phases of frustrated bilayer quantum Heisenberg antif erromagnets DTSTART;VALUE=DATE-TIME:20181205T092500Z DTEND;VALUE=DATE-TIME:20181205T094500Z DTSTAMP;VALUE=DATE-TIME:20260316T200102Z UID:indico-contribution-1-16@indico.bitp.kiev.ua DESCRIPTION:Speakers: Vasyl Baliha (Institute for Condensed Matter Physics of the NASU)\nWe consider the quantum antiferromagnetic Heisenberg model on the square-lattice and honeycomb-lattice bilayers in the absence of an external magnetic field. We use a variational approach to construct the gr ound-state phase diagrams of such model on different bilayers. For simplic ity\, we choose two one-parameter variational wave functions\, which can d escribe states of model in two regions with different relationships betwee n lattice parameters. We calculate variational energies and compare them. This allows us to find borders between different states\, where one of the variational energies is minimal. Finally\, we construct ground-state phas e diagrams in different coordinates. At the end of this study we compare our findings with the earlier results obtained by more sophisticated appro aches [1\, 2]. We observe good agreement with these results. This studies were performed together with O. Derzhko\, J. Richter and O. Götze.\n\n[1] H. Zhang et al.\, Phys. Rev. B 93\, 235150 (2016)\;\n[2] J. Stapmanns et al.\, Phys. Rev. Lett. 121\, 127201 (2018).\n\nhttps://indico.bitp.kiev.ua /event/2/contributions/16/ LOCATION: URL:https://indico.bitp.kiev.ua/event/2/contributions/16/ END:VEVENT BEGIN:VEVENT SUMMARY:The third sound as an electric field generator DTSTART;VALUE=DATE-TIME:20181205T104500Z DTEND;VALUE=DATE-TIME:20181205T110500Z DTSTAMP;VALUE=DATE-TIME:20260316T200102Z UID:indico-contribution-1-10@indico.bitp.kiev.ua DESCRIPTION:Speakers: Aleksandr Konstantinov (B.Verkin Institute for Low T emperature Physics and Engineering of the National Academy of Sciences of Ukraine)\nIt is predicted that oscillations of temperature during propaga tion of third sound in a thin superfluid film cause appearance of an alter nating electric field in the surrounding space\, i.e. the third sound acts as a generator of the electric field [1].\nAs known\, the helium atoms ha ve no dipole\, quadrupole and other multipole moments. The situation chang es in the vicinity of the wall. Interaction of helium with the container w alls leads to the emergence of a nonzero average dipole moment of the heli um atoms directed along the normal to the wall. This leads to polarization of the film. Oscillations of its surface are accompanied by polarization oscillations\, which cause the appearance of the electric fields over the film. Specificity of a superfluid film is associated with the ability to c ause oscillations of the film surface by periodically heating the film bou ndary. As a result\, due to the anomalously large thermomechanical effect\ , a small temperature difference $\\Delta T$ leads to large fluctuations i n film height and electrical potential $\\Delta \\varphi$. It turns out th at the differential thermal e.m.f. $\\Delta \\varphi/\\Delta T$ exceeds i ts value for typical metals. The predicted effect can be considered as an electrical analogue of the fountain effect.\nUsing the method of I.E. Dzya loshinskii\, the average electric field over the system "atom-solid" was c alculated. It is shown that this electrical field is equivalent to the fie ld of the dipole moment of the atom (induced by substrate) and its image. The polarization of the system\, associated with its inhomogeneity (analog ue of the flexoelectric effect)\, was also considered. It has been establi shed that the flexoelectric effect leads only to a small renormalization o f the dipole moment induced by the substrate. The equations describing the propagation of the third sound through the film have been solved. The eff ects of evaporation and condensation of helium atoms were taken into accou nt. The relationship between oscillating parts of the height and temperatu re of the film was found for the low-temperature and high-temperature regi ons. For helium film covering a hollow cylinder on the outside or inside\, an analytical expression for the electric field in space has been found. The coefficient of proportionality between the electric potential and the oscillating part of the film temperature (differential thermal e.m.f) was calculated.\n[1] S. I. Shevchenko and A. M. Konstantinov\, J. Low Temp. Ph ys. (2018).\n\nhttps://indico.bitp.kiev.ua/event/2/contributions/10/ LOCATION: URL:https://indico.bitp.kiev.ua/event/2/contributions/10/ END:VEVENT BEGIN:VEVENT SUMMARY:Features of thermal transport in strained and compressed crystalli ne silicon DTSTART;VALUE=DATE-TIME:20181205T132000Z DTEND;VALUE=DATE-TIME:20181205T134000Z DTSTAMP;VALUE=DATE-TIME:20260316T200102Z UID:indico-contribution-1-9@indico.bitp.kiev.ua DESCRIPTION:Speakers: Oleksii Nepochatyi (Faculty of Physics\, Taras Shevc henko National University of Kyiv)\nTuning of thermal properties of variou s materials is a key challenge in material research. First and foremost\, such necessity arises because of continuous miniaturization of constitute components of various devises. As a result\, issue connected with ameliora tion of heat management started to be more and more crucial. Therefore\, a ny possibilities of increasing or reduction of thermal conductivity in sem iconductor material are very important. \nOne of the possibilities for var iation of thermal transport is the change of elastic properties of the med ia. Particularly\, it is well-known that straining and compression of crys talline silicon lead to modification of heat conduction. However\, one nee ds more physical insight regarding phonon transport for efficient manipula tion of heat fluxes.\nIn our study\, we consider thermal conductivity of s trained and compressed silicon as a function of strain and temperature. We use ab-initio equilibrium molecular dynamics approach for evaluation of t hermal conductivity. Additionally\, we extracted phonon density of states and dispersion curves from molecular dynamics simulations. These data were utilized for direct calculations of thermal conductivity with the use of kinetic theory approach. Comparison of molecular dynamics simulation and direct approach allows us to decompose different factors affecting the the rmal conductivity of strained/$\\\\$compressed silicon.\n\nhttps://indico. bitp.kiev.ua/event/2/contributions/9/ LOCATION: URL:https://indico.bitp.kiev.ua/event/2/contributions/9/ END:VEVENT BEGIN:VEVENT SUMMARY:A current density dependence on phase differences in layered super conducting structures of SISIS type DTSTART;VALUE=DATE-TIME:20181205T102500Z DTEND;VALUE=DATE-TIME:20181205T104500Z DTSTAMP;VALUE=DATE-TIME:20260316T200102Z UID:indico-contribution-1-13@indico.bitp.kiev.ua DESCRIPTION:Speakers: Arsen Shutovskyi (Lesya Ukrainka Eastern European Na tional University)\nThe explored layered superconducting structure contain s three massive superconductors separated by two thin insulator films. Wit hout loss of generality we may consider that the superconductors are diffe rent. An insulator film is mathematically expressed via the Dirac delta fu nction [1]. An order parameter and a current density depend on an applicat e\, because we have a one dimensional problem.\nAccording to the supercond uctivity theory every physical quantity can be calculated using the Green function method [2]. The Green functions satisfy the closed system of line ar differential equations. The mentioned equations have the second derivat ive in the configuration representation. Using the Fourier transform we ha ve constructed the closed system of linear matrix differential equations i n the t-representation [3]. The obtained equations have only the first der ivative. Since a momentum is close to the Fermi momentum our calculations become rather simplified. Solving the linear matrix differential equations in the t-representation we have introduced the undefined integration cons tants. These constants can be defined through application of Green functio n continuity. This means that we need to have a boundary condition. An ord er parameter as a complex function is usually defined by magnitude and pha se. The model with a piecewise constant order parameter allows to suppose that the outside superconductors have equal order parameter magnitudes and unequal order parameter phases. The inside superconductor has a zero orde r parameter phase. The order parameter magnitude of the inside superconduc tor is not necessary equal to the order parameter magnitudes of the outsid e superconductors.\nSubstituting the obtained integration constants into t he Green functions we have obtained the current density dependence on an a pplicate. Calculating the current density on the junction we have obtained the current density expression as a phase difference function.\n\n[1] С відзинський А. В. Математичні методи тео ретичної фізики. У 2-х т. – Вид. 4-те\, допов н. і переробл. – К.: І-нт теорет. фізики ім. М. М. Боголюбова\, 2009.\n[2] Свидзинский А. В. Пространственно-неоднородные задачи те ории сверхпроводимости. – М.: Наука\, 1982. – 310 с.\n[3] Свідзинський А. В. Мікроскопічн а теорія надпровідності: монографія. – Л уцьк: ВНУ ім. Лесі Українки\, 2011. – 422 с.\n\nht tps://indico.bitp.kiev.ua/event/2/contributions/13/ LOCATION: URL:https://indico.bitp.kiev.ua/event/2/contributions/13/ END:VEVENT BEGIN:VEVENT SUMMARY:Influence of correlated hopping on the optical conductivity spectr a DTSTART;VALUE=DATE-TIME:20181205T112500Z DTEND;VALUE=DATE-TIME:20181205T114500Z DTSTAMP;VALUE=DATE-TIME:20260316T200102Z UID:indico-contribution-1-21@indico.bitp.kiev.ua DESCRIPTION:Speakers: Danylo Dobushovskyi (Institute for Condensed Matter Physics of the National Academy of Sciences of Ukraine)\nOptical conductiv ity spectra are studied for the Falicov-Kimball model with correlated hopp ing on the Bethe lattice. An expression for the current-current correlatio n function is derived using dynamical mean field theory. Besides\, the Nyq uist plots were built and used to distinguish different contributions in t he optical conductivity spectra. \n In the metallic phase without correlat ed hopping\, both the current-current correlation function $\\chi(\\Omega) $ and optical conductivity $\\sigma(\\Omega)$ display Drude peak at low fr equencies. The shape of Drude peak is described by the Debye relaxation eq uation \n\n$\\chi_{\\text{D}}(\\Omega) = \\chi_{\\infty} + \\frac{\\chi_0- \\chi_{\\infty}}{1-\\Omega\\tau_{\\text{D}}}$\n\nOn the other hand in the presence of small correlated hopping\, the shape of Drude peak deviates f rom the Debye relaxation peak\, and an additional wide peak is observed on the optical conductivity spectra and on Nyquist plot when Fermi level is in the vicinity of the two particle resonance [1]. At larger values of the correlated hopping parameter\, the density of states contains three bands [1] and the corresponding optical spectra and Nyquist plots display a mor e complicated shape with additional peaks. For the case of strong local co rrelations\, the overall picture strongly depends on the doping level. For a small doping\, when the chemical potential is placed in the wide lower Hubbard band the obtained results are much closer to the case of the doped Mott insulator without correlated hopping\, whereas for a large doping\, when the chemical potential is placed in the narrow upper Hubbard band\, t he spectral weight of the Drude peak is strongly reduced and it is separat ed by a gap from the charge-transfer peak.\n\n[1] D.A. Dobushovskyi\, A.M. Shvaika\, V. Zlatic\, Phys. Rev. B. 95\, 125133 (2017).\n\nhttps://indico .bitp.kiev.ua/event/2/contributions/21/ LOCATION: URL:https://indico.bitp.kiev.ua/event/2/contributions/21/ END:VEVENT END:VCALENDAR