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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:20230606T014053Z
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
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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:20230606T014053Z
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/
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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:20230606T014053Z
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:20230606T014053Z
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:20230606T014053Z
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/
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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:20230606T014053Z
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:20230606T014053Z
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:20230606T014053Z
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:20230606T014053Z
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:20230606T014053Z
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/
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