Гост Института за физику у Београду, проф. др Александр Морозов (Department of Physics, Rutgers University, USА) у оквиру традиционалног колоквијума Института у среду, 20. јуна 2018. године у 13.30 часова у сали „Звонко Марић“ одржаће предавање:

Rapid exploration of complex networks by random walks

САЖЕТАК:

Humanity has become increasingly dependent on large-scale networks. In addition to the original computer-based networks, such as the World Wide Web and the Internet, several online social networks have emerged, notably Twitter and Facebook. Our daily professional and personal lives are influenced by knowledge-sharing online services, such as Wikipedia and YouTube. More generally, complex networks describe a broad spectrum of systems in nature, science, technology, and society. Many of these networks are large and evolving, making investigation of their statistical properties a challenging task. In particular, estimating the network size becomes non-trivial if the network is too large to visit every node. In this talk we will describe a novel methodology, based on random walks, for the inference of statistical properties of complex networks with weighted or unweighted edges. The statistics of interest include, but are not limited to, the node degree distribution, the average degree of nearest-neighbor nodes, and the node clustering coefficient. We will show how our formalism can yield high-accuracy estimates of these statistics, and of the network size, after only a small fraction of network nodes has been explored. We will first demonstrate our computational framework on several standard examples, including random, scale-free, and small-world networks. Next, we will discuss how our method can be used to explore Wikipedia, study propagation of infectious diseases on contact networks, and obtain census-type population data from small samples.

У оквиру семинара Центра за чврсто стање и нове материјале, у петак, 15. јуна у 15 часова на Институту за физику у Београду у читаоници библиотеке „Др Драган Поповић“ предавање под насловом:

Active matter and Curvature

одржаће др Растко Скнепнек (University of Dundee).

САЖЕТАК

Coupling between active motion and curvature is an integral
part of many fundamental biological processes such as gastrulation and
intestinal crypt fission. However, to date very little is known about
how curvature affects active motion. Here we use a particle-based model
to study the interplay between activity and curvature in dense systems.
Using detailed numerical simulations and simple physical arguments, we
show that the presence of curvature results in a number of steady-state
configurations that have no analogues in flat geometries. These states
are particularly interesting if topological constraints require the
presence of defects in the ground states in the passive limit. We focus
on polar and nematic active systems confined to move on the surface of a
sphere and show that activity can lead to the formation of moving band
and multidefect states. We extend our model to self-propelled filaments
confined to a plane or the surface of a sphere. We show that the
activity leads to an effective softening of the polymer chain. As a
result of this softening, with the increase in activity, the system
transitions between a jammed polymer-melt state, an active turbulent
state characterised by a proliferation of hair-pin defects, to a region
dominated by phase segregation (MIPS), finally followed by the onset of
a homogenous state characterised by spiral motion of individual
polymers.

У петак 15. јуна на Институту за физику у читаоници библиотеке „Др Драган Поповић“ са почетком у 11 часова, Марија Врањеш Милосављевић ће одржати семинар под насловом:

„Потрага за суперсиметријом на експерименту АТЛАС – прошлост, садашњост и будућност“

САЖЕТАК:
Потрага за суперсиметричним (SUSY) сигналом омогућава директну проверу теоријских предвидјања проширења Стандардног модела елементарних честица, која до сада нису експериментално потврдјена.
Биће приказан преглед суперсиметричних модела, експерименталних техника и статистичких метода у потрагама за суперсиметричним честицама на АТЛАС експерименту од периода пре почетка рада Великог сударача хадрона (ЛХЦ), током прве фазе прикупљања података из протон-протон судара на енергији од 8 TeV, као и последњи резултати добијени у текућој, другој фази рада ЛХЦ-а на енергији од 13 TeV. Анализа и интерпретација података у оквиру SUSY модела, као и припрема посебног скупа резултата који се могу додатно изучавати од стране шире HEP зајенице, биће дискутовани на примеру потраге за SUSY честицама у догађајима са великим бројем хадронских џетова и великом недостајућом енергијом. Такође, биће изложен приступ интерпретирања свих публикованих резултата АТЛАС експеримента у потрагама за SUSY честицама у оквиру 19-параметарског pMSSM модела (phenomenological Minimal Supersymmetric extension of the Standard Model).

Чланови Матичног одбора за физику посетиће Институт за физику у Београду у петак, 15. јуна 2018. године. Тог дана, у 12:00 часова у сали „Звонко Марић“ биће организован разговор са свим заинтересованим истраживачима.

Ово ће бити прилика за отворену дискусију са члановима Матичног одбора за физику о свим битним питањима везаним за рад Одбора, чија је надлежност старање о квалитету научноистраживачког рада и развоју научноистраживачке делатности у републици Србији у области физике. Неке од отворених тема укључују изборе у звања и рад на пројектима, као и за размену информација и предлога које би Одбор могао да узме у обзир у наредном периоду.

Сви заинтересовани истраживачи су добродошли.

У оквиру СЦЛ семинара Центра за изучавање комплексних система, у среду, 13. јуна у 14 часова у читаоници библиотеке „Др Драган Поповић“, др Ивана Савић (Тиндал национални институт, Корк, Ирска) ће одржати предавање:

Electronic and Thermoelectric Transport in PbTe: Insights from First Principles Calculations

САЖЕТАК:

Thermoelectric materials could play an important role in the development of energy harvesting technologies due to their ability to convert waste heat into electricity [1]. It is challenging to control and enhance the efficiency of thermoelectric energy conversion in a material because of the conflicting requirements for the desired physical properties i.e. high electrical conductivity and low thermal conductivity. Exploiting the fascinating properties of materials near soft mode phase transitions is an emerging concept in the quest to increase thermoelectric efficiency [2,3]. The underlying idea is that soft phonon modes lead to intrinsically low thermal conductivity, while possibly preserving high electronic conductivity. In this talk I will focus on the unusual electronic transport properties of n-type PbTe, which is a classic thermoelectric material that exists near a soft optical mode phase transition. Our first principles calculations show that most scattering mechanisms in PbTe are very weak, with the exception of that due to polar optical phonons [4,5]. Our results suggest that good thermoelectric materials may be found among weakly polar materials with symmetry forbidden scattering mechanisms.

[1] G. J. Snyder and E. S. Toberer, Nat. Mater. 7, 105 (2008).
[2] L.-D. Zhao et al, Nature 508, 373 (2014).
[3] R. M. Murphy, E. D. Murray, S. Fahy, and I. Savic, Phys. Rev. B 93, 104304 (2016); Phys. Rev. B 95, 144302 (2017).
[4] A. R. Murphy, F. Murphy-Armando, S. Fahy, and I. Savic, submitted.
[5] J. Cao, J. D. Querales-Flores, A. R. Murphy, S. Fahy, and I. Savic, in preparation.

У оквиру СЦЛ семинара Центра за изучавање комплексних система, у среду, 6. јуна у 14 часова у читаоници библиотеке „Др Драган Поповић“, Вељко Јанковић (СЦЛ, Институт за физику у Београду) ће одржати предавање:

Some like it hot, others prefer it cold: Mechanism of free-charge generation in organic photovoltaics

САЖЕТАК:

The fundamental physical mechanisms governing light-to-charge conversion in photoexcited donor/acceptor (D/A) organic solar cells have recently been heavily debated. Having in mind that a photoexcitation of OSCs initially generates strongly bound excitons, experimental evidence demonstrating that almost all of the absorbed photons are converted to free carriers in a process that exhibits weak dependence on the internal electric field and temperature [1] seems surprising. Results of ultrafast spectroscopic studies point towards a subpicosecond time scale of free-charge generation, which occurs by virtue of high-energy („hot“) delocalized interfacial charge transfer (CT) states [2]. On the other hand, there are suggestions that free carriers are predominantly generated on much longer time scales out of the lowest-energy („cold“) CT state, which is strongly bound and localized [3]. Our comprehensive investigation of exciton dynamics at a model D/A heterojunction reveals that, on a picosecond time scale following the photoexcitation, the majority of photogenerated carriers remain bound in form of donor or CT excitons [4]. In this talk we will present our latest results that concern charge separation on longer time scales out of the aforementioned bound CT and donor excitons, which demonstrate that the synergy between carrier delocalization and moderate disorder can explain quite high and weakly field- and temperature-dependent free-charge yields observed in experiments [5].

[1] S. H. Park et al. Nat. Photonics 3, 297 (2009).
[2] G. Grancini et al., Nat. Mater. 12, 29 (2013).
[3] K. Vandewal et al., Nat. Mater. 13, 63 (2014).
[4] V. Janković and N. Vukmirović, J. Phys. Chem. C 121, 19602 (2017).
[5] V. Janković and N. Vukmirović, J. Phys. Chem. C 122, 10343 (2018).