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У оквиру семинара Центра за неравнотежне процесе Института за физику у Београду, у четвртак, 23. јуна 2022. године у 14 часова у читаоници „др Драган Поповић“ путем Zoom платформе, др Стефан Мијин (Tokamak Science, Управа за атомску енергију Уједињеног Краљевства, Кулам научни центар), одржаће предавање:

Fluid and kinetic modelling of transport in the tokamak Scrape-Off Layer

САЖЕТАК:

As the JET tokamak nears the end of its lifetime and machines like JT-60SA and ITER slowly step onto the stage, we enter into the last planned generation of experimental machines before demonstration fusion plants, such as the EU’s DEMO and UK’s STEP. On that road there are still many unanswered physics questions, with a significant number of
them being part of the so-called exhaust problem. This problem stems from the transport of plasma out of the core and towards various material surfaces, where it interacts with neutral particles and impurities. The hot plasma carries immense amounts of heat to the surfaces, and these heat loads must be mitigated. To that end, the understanding of plasma and impurity transport in the boundary region of tokamaks is important for future reactor design and operations.

In this talk I will endeavour to give a broad overview of the exhaust problem, including theoretical and computational approaches to tackling various aspects of the problem. In that context I will present work done at UKAEA and Imperial College London on 1D fluid and kinetic modelling of electron transport in the tokamak Scrape-Off Layer (i.e. boundary region). I will show where kinetic effects can come into play and compare kinetic to fluid simulations both in steady state and during transient heating pulses. Finally, I will put the presented work into the broader context of ongoing and planned exhaust code development at UKAEA and discuss some important aspects of boundary plasma physics on
our minds.

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Meeting ID: 952 8120 0484
Passcode: 927407

Центар за чврсто стање и нове материјале Института за физику у Београду организује догађај који привлачи светску пажњу. Наиме, у четвртак и петак, 9. и 10. јуна 2022. године, Центар организује радионицу „Workshop in Strongly Correlated Electron Systems“. Радионица је посвећена делу академика Зорана В. Поповића, оснивача Центра и једног од пионира физике чврстог стања у Србији.

Радионица ће бити одржана у хибридном облику, односно у комбинацији предавања уживо и Zoom платформе. Програм радионице и у четвртак и у петак почиње у 10 часова у сали „Др Драган Поповић“ Института за физику.

Предавања ће покрити широк спектар тема из области јако корелисаних електронских система са циљем да се сагледа стање и нови трендови у експерименталним и теоријским истраживањима. У посебном фокусу биће гвожђе-халкогенидни суперпроводници и истраживања у оквиру пројекта StrainedFeSC који финансира Фонд за науку Републике Србије.

Радионица се организује уз финансијску подршку Фонда за науку Републике Србије кроз грант број 6062656, у оквиру програма ПРОМИС.

Програм радионице

Линк за праћење програма првог дана радионице

Линк за праћење програма другог дана радионице

У оквиру семинара Групе за гравитацију, честице и поља Института за физику у Београду, у петак, 10. јуна 2022. године у 11:15 путем BigBlueButton платформе, др Маја Бурић (Физички факултет у Београду), одржаће предавање:

Fuzzy BTZ

САЖЕТАК:

We introduce recently proposed fuzzy version of the BTZ black hole and discuss some of its physical properties.

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У оквиру семинара Центра за изучавање комплексних система Института за физику у Београду, у четвртак, 02. јуна 2022. године у 14 часова путем Zoom платформе, др Рене Мејер (Универзитет у Вирцбургу, Немачка), одржаће предавање:

Strongly correlated Dirac materials, electron hydrodynamics, and AdS/CFT

САЖЕТАК:

A current challenge in condensed matter physics is the realization of strongly correlated, viscous electron fluids. These fluids are not amenable to the perturbative methods of Fermi liquid theory, but can be described by holography, that is, by mapping them onto a weakly curved gravitational theory via gauge/gravity duality. The canonical system considered for realizations has been graphene, which possesses Dirac dispersions at low energies as well as significant Coulomb interactions between the electrons. In this talk, after introducing basic notions of hydrodynamics and holography, we will discuss Kagome systems which, with electron fillings adjusted to the Dirac nodes of their band structure, provide a much more compelling platform for realizations of viscous electron fluids, including non-linear effects such as turbulence. In particular, in stoichiometric Scandium (Sc) Herbertsmithite, the fine-structure constant, which measures the effective Coulomb interaction and hence reflects the strength of the correlations, is enhanced by a factor of about 3.2 as compared to graphene. We will present a holographic model which includes the leading finite coupling corrections and allows to estimate the ratio of shear viscosity over entropy density in Sc-Herbertsmithite, and find it about three times smaller than in graphene. These findings put, for the first time, the turbulent flow regime described by holography within the reach of experiments.

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Meeting ID: 881 4646 3326
Passcode: 928695

У оквиру семинара Центра за изучавање комплексних система Института за физику у Београду, у четвртак, 26. маја 2022. године у 14 часова путем Zoom платформе, др Марко Младеновић (Лабораторија за интегрисане системе, ЕТХ, Цирих), одржаће предавање:

Fully-atomistic simulations of valence change memories

САЖЕТАК:

The ability to vary their conductance makes valence change memories (VCMs) promising candidates for applications in emerging neuromorphic applications [1]. The operational mechanism of VCMs is based on the arrangement of oxygen vacancies in the presence of applied voltage: if a conductive path consisting of oxygen vacancies is established between two electrodes, the VCM is in its low resistance (ON) state, if the conductive path is broken, the system is in its high resistance (OFF) state. In this talk, a theoretical framework to model VCMs will be presented. The model is fully-atomistic and consists of three steps: (1) calculating the activation energies for events that occur in VCMs using density functional theory calculations; (2) simulating the rearrangement of oxygen vacancies using an in-house kinetic Monte Carlo code; and (3) calculating the transport properties using the quantum transport method based on non-equilibrium Green’s function formalism. Following a detailed description of the method, an overview of the results on hafnium oxide-based VCMs will be given.

[1] K.Portner et al, ACS Nano 15, 14776 (2021).

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Meeting ID: 810 5550 6980
Passcode: 683804

У оквиру семинара Института за физику у Београду, у среду, 25. маја 2022. године у 12 часова путем Zoom платформе, предавање под насловом:

Beyond Quantum Mechanics

одржаће проф. др Дејан Стојковић (Универзитет у Бафалу, САД)

САЖЕТАК:

In this talk we will present a new formalism, which is based on comparing sequences populated by zeros and ones. Two or more sequences can be compared by defining measures that count the frequency with which various features occur, such as the similarities and differences between them. One such measure can be interpreted as angular momentum j, while another can be interpreted as its projection m along the z axis. Using this formalism, we will produce a simple derivation of the rules for angular momentum addition (identical to those in quantum mechanics) that ultimately yield the Clebsch-Gordan coefficients. This might suggest that elementary particles (i.e., states labeled with quantum numbers j and m) are emergent phenomena for an observer who can observe only the relationships between sequences, but not their exact composition. The probabilistic nature of quantum mechanics is thus a simple consequence of obscuring information about the sequences. Further, calculating probabilities within this formalism reduces to counting. Such a counting procedure leads to the squares of the usual Clebsch-Gordan coefficients, which is tightly connected with the question of why we must square the wave function to obtain probabilities in quantum mechanics. Finally, in our formalism, the role of the reference sequence is crucial, which may shed some new light on the role of the observer and/or vacuum in quantum mechanics. If we interpret the reference sequence as an observer, then clearly an observer becomes an integral part of the system which is observed.

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Meeting ID: 879 2836 3524
Passcode: 826930

У оквиру семинара Групе за гравитацију, честице и поља Института за физику у Београду, у петак, 20. маја 2022. године у 11:15 путем BigBlueButton платформе, др Марко Војиновић (Институт за физику у Београду), одржаће предавање:

A review of a few results at the interplay between quantum gravity and quantum information theory

САЖЕТАК:

I will give a review of two (if time permits maybe three) results obtained in collaboration with Nikola Paunkovic, on research at the intersection between quantum gravity (QG) and quantum information theory (QI). The first result deals with the possible space of states in an abstract QG formalism, and the restrictions imposed by gauge symmetry, specifically diffeomorphisms. It turns out that these restrictions generically rule out gravity-matter product states as gauge-noninvariant. The second result deals with the motion of a point-particle in QG, in particular in a background gravitational field constructed as a quantum superposition of two distinct spacetime geometries. The resulting trajectory of the particle fails to be described by a geodesic equation with respect to either geometry, giving rise to a correction term due to quantum interference between two geometries. We will discuss the consequences of this for the weak equivalence principle. Finally, we may briefly mention a few other recent results trending in the QI community, such as the quantum switch protocol, del Santo-Dakic protocol, and their relationship to QG.

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У оквиру семинара Нискофонске лабораторије за нуклеарну физику Института за физику у Београду, у понедељак, 23. маја 2022. године у 11 часова у Библиотеци Института за физику и путем Zoom платформе, др Игор Аникин (Богољубов лабораторија за теоријску физику, Обједињени институт за нуклеарна истраживања у Дубни), одржаће предавање:

Transverse-momentum dependent Distributions: Functional Complexity manifesting in Gluon Poles and Inverse Radon Transforms

САЖЕТАК:

As well-known, QCD can be applied to the description of hard reactions with the help of the factorization theorem.
The factorization theorem, as an extended method of asymptotical estimations, states that the short (hard) and long (soft) distance dynamics can be separated out provided the momentum transfer is large. In this connection, any types of parton distributions are stemming from the factorization procedure applied for a given hard process. In the talk, we discuss the functional complexity which is associated with the transverse momentum dependent parton distributions. Working with the semi-exclusive and inclusive reactions, we demonstrate the manifestation of complexity in the gluon pole contributions and in the inverse Radon transforms applied to the transverse momentum dependent generalized
parton distributions. The experimental observables which are directly probing the gluon poles have been suggested.

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Meeting ID: 914 2401 6765
Passcode: 110488

У оквиру семинара Центра за изучавање комплексних система Института за физику у Београду, у четвртак, 05. маја 2022. године у 14 часова путем Zoom платформе, др Предраг Поповић (Институт за физику Земље у Паризу), одржаће предавање:

Critical percolation threshold controls Arctic sea ice melt pond evolution

САЖЕТАК:

Arctic sea ice covers a vast area of nearly 15 million square kilometers at its peak annual extent. However, in recent years, it has been rapidly disappearing at a rate underestimated by most climate models. A likely reason for this underestimate are processes on scales smaller than tens of kilometers, which the large-scale models cannot resolve. One such small-scale process that contributes to the disappearance of the Arctic ice are meter-sized melt ponds that form on the ice’s surface during summer and absorb significant amounts of solar radiation. During the summer, these ponds evolve in a complicated manner that we do not yet fully understand. In this talk, we will show how the critical point of a connectivity transition – the percolation threshold – controls the melt pond evolution. The ponds evolve by draining through large holes that open over the summer on the ice’s surface so that above the percolation threshold, they can drain easily, while below the threshold, their drainage is impaired. In this way, the percolation threshold is an approximate upper bound on the pond coverage fraction. In fact, in a manner typical for critical phenomena, pond drainage is universal. This universality enables us to write an equation for their evolution below the threshold that compares well with field and satellite observations over most of the summer. Our work, thus, might help resolve some of the uncertainty present in the large-scale climate models and help better predict the fate of the Arctic ice.

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Meeting ID: 845 0021 3601
Passcode: 019682

У оквиру семинара Групе за гравитацију, честице и поља Института за физику у Београду, у петак, 29. априла 2022. године у 11 часова путем BigBlueButton платформе, Михаило Ђорђевић (Институт за физику у Београду), одржаће предавање:

Coleman-Mandula theorem

САЖЕТАК:

Coleman-Mandula theorem is a no-go theorem in theoretical physics. It states that space-time symmetries and internal symmetries cannot be combined in any but a trivial way. The main consequence of the theorem is impossibility of finding functional dependence between parameters of irreducible representations of the Poincare group (mass and spin), and parameters of irreducible representations of internal symmetry group (charge, color etc.). Finding a „loophole“ of the theorem would give a new insight into the relations between properties of elementary particles, and by doing that, it would also reduce the number of fundamental constants of the theory. In this seminar, we will give a precise formulation of the theorem, show its detailed proof based on very general physical assumptions and talk about possible ways of the generalization of the theorem and of „loopholing“ it.

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У оквиру дуплог семинара Центра за изучавање комплексних система Института за физику у Београду, у четвртак, 21. априла 2022. године у 14 часова путем Zoom платформе, др Марија Вранић (GoLP/Институт за плазму и нуклеарну фузију, Високи технички институт, Универзитет у Лисабону), одржаће предавање:

Sources of electrons, positrons, and gamma-rays from lasers within plasma channels

САЖЕТАК:

The recent rise in laser intensities accompanied by the ongoing construction of new laser facilities such as ELI, the Vulcan 10 PW, and APOLLON will place intensities above 10^23 W/cm^2 within reach. While propagating through a pre-formed plasma channel, a laser of such intensity allows for direct laser acceleration (DLA) of leptons in the radiation reaction dominated regime. The DLA scheme has already been demonstrated to provide high-charge electron beams (at a nC level) with moderate laser intensities  (10^20 W/cm^2). In this talk we will show what can be accomplished with near-future laser facilities. We find that increasing the laser power is bound to augment the charge content even further. The field structure formed due to electron beam loading allows for accelerating positrons. What is more, the interaction in the radiation dominated regime will provide a high flux of emitted photons, in hard x-ray and gamma-ray range. These photons can then be used as a seed for electron-positron pair creation, as well as a radiation source for applications.

Након тога др Томас Грисмајер (GoLP/Институт за плазму и нуклеарну фузију, Високи технички институт, Универзитет у Лисабону), одржаће предавање:

Electron-positron-photon cascades in ultrastrong electromagnetic fields

САЖЕТАК:

Electron-positron pair plasmas are tightly related to extreme astrophysical objects such as pulsar magnetospheres or gamma-ray bursts. Due to the inherent difficulties of studying these remote objects, it is desirable to study dense pair plasmas in the laboratory, both for fundamental purposes and astrophysical applications. The magnitude of the laser fields expected in the next generation of laboratories overlaps with the estimated fields of milliseconds pulsars. Producing pair plasmas in ultrastrong fields may demonstrate that we can mimic the conditions appropriate to these astrophysical environments in laboratories. The pair creation in such energy density environments is caused by the decay of gamma rays in intense fields. As pulsars efficiently convert the large-scale Poynting flux to gamma rays, the laboratory analog of a pulsar isexpected to efficiently convert optical laser light into gamma rays. This process usually leads to quantum electrodynamics (QED) cascades, as the pairs created re-emit hard photons that decay anew in pairs, eventually resulting in electron-positron-photon plasmas. In this talk we will present the QED cascades in the laboratory where the electromagnetic trap is comprised of colliding lasers, and in Pulsar magnetospheres.

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Meeting ID: 827 0093 8432
Passcode: 008340

У оквиру семинара Групе за гравитацију, честице и поља Института за физику у Београду, у петак, 15. априла 2022. године у 11 часова путем BigBlueButton платформе, Душан Ђорђевић (Физички факултет у Београду), одржаће предавање:

Noncommutative D=5 Chern-Simons Gravity: Kaluza-Klein Reduction and Chiral Gravitational Anomaly

САЖЕТАК:

Chern-Simons (CS) theory has greatly influenced physics and physicists during the past half-century. In this seminar, we will review some basic facts about Chern-Simons Lagrangians defined on some (2n-1)-dimensional manifold. We will then show how CS theory can be connected with gravity. Our focus will be on an SO(4,2) CS gravity in five dimensions and its connections with four-dimensional gravitation. Furthermore, we will analyse the noncommutative deformation of SO(4,2) CS gravity using the geometric Siberg-Witten map. This theory has a nontrivial first-order correction in \theta. By performing Kalutza-Klein dimensional reduction and suitable symmetry breaking, we obtain an effective, four-dimensional gravity theory that has Einstein-Hilbert action with (negative) cosmological constant as its classical (commutative) limit. We then proceed to analyse some concrete solutions of the field equations. We show that AdS space-time remains a solution after including the first-order corrections. On the other hand, AdS-Schwarzschild black hole solution develops a torsion that we explicitly compute. Moreover, we will see that this modified solution implies the existence of a chiral gravitational anomaly if one couples massless Dirac fermions to this NC background.

This seminar is based on arXiv:2203.05020

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У оквиру семинара Центра за изучавање комплексних система Института за физику у Београду, у четвртак, 14. априла 2022. године у 14 часова путем Zoom платформе, др Лук Радемакер (Департман за теоријску физику, Универзитет у Женеви), одржаће предавање:

Symmetry breaking and Chern insulators in twisted graphene structures

САЖЕТАК:

Twisted bilayer graphene (tBG) and variants like twisted monolayer-bilayer graphene (tMBG) were proposed to be a platform for strongly correlated physics akin to the cuprate family. However, we will show that many of the observed interacting phenomena can be explained in terms of breaking of spin/valley symmetry. To do so, we will first describe how flat bands in tBG can be directly observed using ARPES [1]. An effective description of the flat bands is hampered by band renormalization due to charge transfer [2] and band topology. The combination of interactions and topology naturally leads to spin/valley symmetry breaking with a quantum anomalous Hall effect in tMBG [3]. In tBG in large magnetic fields, similar spin/valley symmetry breaking creates a series of Chern insulator states [4]. We will conclude with a brief discussion of the possibility of genuine strong correlations in Moiré structures.

[1] S. Lisi, X. Lu, T. Benschop, et al., Nature Phys. 17, 189 (2021).
[2] L. Rademaker, D. A. Abanin, P. Mellado, Phys. Rev. B 100, 205114 (2019).
[3] L. Rademaker, I. V. Protopopov, D. A. Abanin, Phys. Rev. Research 2, 033150 (2020).
[4] Y. Saito, J. Ge, L. Rademaker, et al., Nat. Phys. 17, 478 (2021).

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Meeting ID: 897 1269 4555
Passcode: 580212

У оквиру семинара Групе за гравитацију, честице и поља Института за физику у Београду, у петак, 01. априла 2022. године у 11 часова путем BigBlueButton платформе, др Бранислав Цветковић (Институт за физику у Београду), одржаће други део предавања:

Entropy in Poincaré gauge theory: Hamiltonian approach

САЖЕТАК:

Canonical generator G of local symmetries in Poincaré gauge theory is constructed as an integral over a spatial section Σ of spacetime. Its regularity (differentiability) on the phase space is ensured by adding a suitable surface term, an integral over the boundary of Σ at infinity, which represents the asymptotic canonical charge. For black hole solutions, Σ has two boundaries, one at infinity and the other at horizon. It is shown that the canonical charge at horizon defines entropy, whereas the regularity of G implies the first law of black hole thermodynamics.

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У оквиру семинара Групе за гравитацију, честице и поља Института за физику у Београду, у петак, 25. марта 2022. године у 11 часова путем BigBlueButton платформе, др Бранислав Цветковић (Институт за физику у Београду), одржаће предавање:

Entropy in Poincaré gauge theory: Hamiltonian approach

САЖЕТАК:

Canonical generator G of local symmetries in Poincaré gauge theory is constructed as an integral over a spatial section Σ of spacetime. Its regularity (differentiability) on the phase space is ensured by adding a suitable surface term, an integral over the boundary of Σ at infinity, which represents the asymptotic canonical charge. For black hole solutions, Σ has two boundaries, one at infinity and the other at horizon. It is shown that the canonical charge at horizon defines entropy, whereas the regularity of G implies the first law of black hole thermodynamics.

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У оквиру семинара Центра за изучавање комплексних система Института за физику у Београду, у четвртак, 24. марта 2022. године у 14 часова путем Zoom платформе, др Марија Митровић Данкулов (Лабораторија за примену рачунара у науци, Центар за изучавање комплексних система, Институт за физику у Београду), одржаће предавање:

Universal patterns of multiscale stochastic processes in the SARS-CoV-2 pandemic

САЖЕТАК:

SARS-CoV-2 pandemic has influenced every aspect of our lives in the previous two years. The course of an epidemic in one country depends on many factors, including different epidemic measures. The epidemic measures taken varied from country to country and during the epidemic. Apart from the measures themselves, the evolution of the epidemic depends significantly on different underlying stochastic processes. Understanding the global nature of these processes and how they, combined with different measures, influence the course of epidemics is crucial for predicting its evolution. In this talk, we will present the analysis [1] of the world data of SARS-CoV-2 infection events for two distinct periods during the epidemic: the first eight months of the outbreak phase and the first eight months of the immunization phase. Using methods from time series analysis and complex network theory, we will analyze the global features of mentioned stochastic processes. Our results reveal the robust communities of different countries and regions clustered according to similar shapes of infection fluctuations – the content of communities differs for two different phases of the epidemic. Further, examination of the communities reveals the appearance of large clusters that span different geographic locations, suggesting that regional similarities are not the only driver of the evolution of the epidemic. Detailed time series analysis reveals cyclic trends and that persistent fluctuations around the local trend occur in intervals smaller than 14 days. Our results provide the basis for further research into the interplay between biological and social factors that are the primary cause of epidemic infection cycles.

[1] M. Mitrović Dankulov, B. Tadić, R. Melnik, medRxiv:2021.12.20.21268095 (2021).

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Meeting ID: 878 5066 2165
Passcode: 569569

У оквиру семинара Групе за гравитацију, честице и поља Института за физику у Београду, у петак, 11. марта 2022. године у 11 часова путем BigBlueButton платформе, др Игор Прлина (Браун универзитет), одржаће предавање:

Landau Singularities in Planar Massless Theories

САЖЕТАК:

In this work we present our contribution to the method of using Landau singularities for probing scattering amplitudes in planar massless quantum field theories. We start by proposing a simple geometric algorithm for determining the complete set of branch points of amplitudes in planar N = 4 super-Yang-Mills theory directly from the amplituhedron, without resorting to any particular representation in terms of local Feynman integrals. This represents a step towards translating integrands directly into integrals. In particular, the algorithm provides information about the symbol alphabets of general amplitudes. First we illustrate the algorithm applied to the one-and two-loop MHV amplitudes. Then we demonstrate how to use the reformulation of amplituhedra in terms of ‘sign flips’ in order to streamline the application of this algorithm to amplitudes of any helicity. In this way we recover the known branch points of all one loop amplitudes, and we find an ’emergent positivity’ on boundaries of amplituhedra. Lastly, we look beyond planar N = 4 super-Yang-Mills theory, and analyze Landau singularities of general massless planar theories. In massless quantum field theories the Landau equations are invariant under graph operations familiar from the theory of electrical circuits. Using a theorem on the Y-∆ reducibility of planar circuits we prove that the set of first-type Landau singularities of an n-particle scattering amplitude in any massless planar theory, in any spacetime dimension D, at any finite loop order in perturbation theory, is a subset of those of a certain n-particle ⌊(n−2)^2/4⌋-loop „ziggurat“ graph. We determine this singularity locus explicitly for D = 4 and n = 6 and find that it corresponds precisely to the vanishing of the symbol letters familiar from the hexagon bootstrap in SYM theory. Further implications for SYM theory are discussed.

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У оквиру семинара Центра за изучавање комплексних система Института за физику у Београду, у четвртак, 03. марта 2022. године у 14 часова путем Zoom платформе, др Михаило Чубровић (Лабораторија за примену рачунара у науци, Центар за изучавање комплексних система, Институт за физику у Београду), одржаће предавање:

Quantum chaos, gravity and factorization (part 2)

САЖЕТАК:

In this talk we will argue that the factorization of partition functions in ensembles of interacting quantum systems is a fundamental measure of chaos for a broad range of strongly and weakly interacting, few- and many-body examples. We will first introduce the factorization puzzle in the context of recent advances from the black hole information literature: the problem is likely solved if the black hole geometry is corrected by spacetime wormholes, but wormholes imply that the partition function of two independent, mutually non-interacting and non-entangled systems does not equal the product of their separate partition functions. Making use of a matrix (IKKT) model of D-branes in type IIB string theory, we will show that the factorization is restored when half-wormholes and in general fractional wormholes are taken into account, which corresponds to the regime of strong microscopic chaos on D-branes. Afterwards we will consider the opposite case: ensembles of few-body quantum mechanical systems (Schroedinger equation in the Henon-Heiles potential, quantum kicked top, random matrices) factorize into products of individual partition functions when the system is quantum-chaotic and obeys the Wigner-Dyson energy level statistics. At the end, we will discuss potential applications to strongly correlated electron systems, which bring some new difficulties because of exchange correlations brought about by Fermi statistics.

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Meeting ID: 832 1825 2482
Passcode: 282961

У оквиру семинара Групе за гравитацију, честице и поља Института за физику у Београду, у петак, 25. фебруара 2022. године у 11 часова путем BigBlueButton платформе, Илија Иванишевић (Институт за физику у Београду), одржаће предавање:

Courant algebroids in bosonic string theory

САЖЕТАК:

We analyze bosonic string symmetry transformations and their relations with  T-duality in the framework of generalized geometry. The generator governing symmetry transformations can be described simply as the inner product of generalized vectors and its algebra produces the Courant bracket as the T-dual invariant extension of the Lie bracket, while T-duality can be interpreted as an isomorphism between Courant algebroids. We show how different O(D, D) transformations give rise to different twists of the Courant bracket that feature string theory relevant fluxes.

arXiv:1903.04832
arXiv:2010.10662
arXiv:2103.09585
arXiv:2202.03227

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У оквиру семинара Центра за изучавање комплексних система Института за физику у Београду, у четвртак, 24. фебруара 2022. године у 14 часова путем Zoom платформе, др Михаило Чубровић (Лабораторија за примену рачунара у науци, Центар за изучавање комплексних система, Институт за физику у Београду), одржаће предавање:

Quantum chaos, gravity and factorization

САЖЕТАК:

In this talk we will argue that the factorization of partition functions in ensembles of interacting quantum systems is a fundamental measure of chaos for a broad range of strongly and weakly interacting, few- and many-body examples. We will first introduce the factorization puzzle in the context of recent advances from the black hole information literature: the problem is likely solved if the black hole geometry is corrected by spacetime wormholes, but wormholes imply that the partition function of two independent, mutually non-interacting and non-entangled systems does not equal the product of their separate partition functions. Making use of a matrix (IKKT) model of D-branes in type IIB string theory, we will show that the factorization is restored when half-wormholes and in general fractional wormholes are taken into account, which corresponds to the regime of strong microscopic chaos on D-branes. Afterwards we will consider the opposite case: ensembles of few-body quantum mechanical systems (Schroedinger equation in the Henon-Heiles potential, quantum kicked top, random matrices) factorize into products of individual partition functions when the system is quantum-chaotic and obeys the Wigner-Dyson energy level statistics. At the end, we will discuss potential applications to strongly correlated electron systems, which bring some new difficulties because of exchange correlations brought about by Fermi statistics.

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Meeting ID: 869 2228 8686
Passcode: 245902

У оквиру семинара Групе за гравитацију, честице и поља Института за физику у Београду, у петак, 18. фебруара 2022. године у 11 часова путем BigBlueButton платформе, Илија Иванишевић (Институт за физику у Београду), одржаће предавање:

Courant algebroids in bosonic string theory

САЖЕТАК:

We analyze bosonic string symmetry transformations and their
relations with  T-duality in the framework of generalized geometry. The
generator governing symmetry transformations can be described simply as
the inner product of generalized vectors and its algebra produces the
Courant bracket as the T-dual invariant extension of the Lie bracket,
while T-duality can be interpreted as an isomorphism between Courant
algebroids. We show how different O(D, D) transformations give rise to
different twists of the Courant bracket that feature string theory
relevant fluxes.

arXiv:1903.04832
arXiv:2010.10662
arXiv:2103.09585
arXiv:2202.03227

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У оквиру семинара Групе за гравитацију, честице и поља Института за физику у Београду, у петак, 18. фебруара 2022. године у 11 часова путем BigBlueButton платформе, Драган Прекрат (Физички факултет у Београду), одржаће предавање:

Phase transitions in matrix models on the truncated Heisenberg space

САЖЕТАК:

The important generic problem with QFTs on noncommutative spaces is the UV/IR mixing of divergences which interferes with their renormalization. The Grosse-Wulkenhaar model solved this problem by introducing a harmonic oscillator term that can be interpreted as a coupling between the curvature and the field. In this talk, we will examine the success of this model from the phase transition point of view. We will show that renormalizable and nonrenormalizable versions of the 2D noncommutative phi4 model have different phase diagrams.
We will also consider the implications of the striped-phase shifting on the renormalization properties of other models.

arXiv:2104.00657 [hep-th]
arXiv:2002.05704 [hep-th]

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У оквиру семинара Групе за гравитацију, честице и поља Института за физику у Београду, у петак, 21. јануара 2022. године у 11 часова путем BigBlueButton платформе, Бојана Бркић (Физички факултет у Београду), одржаће предавање:

Laplacian on fuzzy de Sitter space

САЖЕТАК:

In this seminar, we shall consider the geometric property of fuzzy de Sitter space by studying the Laplacian. The latter is defined within the noncommutative frame formalism and it is not hermitian, thus, inducing the nonunitary evolution. We shall solve the eigenproblem of the symmetrically ordered quantum-mechanical Laplacian and discuss its spectrum.

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У оквиру семинара Центра за изучавање комплексних система Института за физику у Београду, у четвртак, 23. децембра 2021. године у 14 часова путем Zoom платформе, др Аксел Пелстер (Департман за физику, Технички универзитет у Кајзерслаутерну, Немачка), одржаће предавање:

On the theoretical description of photon Bose-Einstein condensates

САЖЕТАК:

Since the advent of experiments with photon Bose-Einstein condensates in dye-filled microcavities in 2010 [1], many investigations have focused upon the emerging effective photon-photon interaction. Despite its smallness, it can be identified to stem from two physically distinct mechanisms [2]. On the one hand, a Kerr nonlinearity of the dye medium yields a photon-photon contact interaction, whose microscopic theoretical description is based on a Lindblad master equation [3]. On the other hand, a heating of the dye medium leads to an additional thermo-optic interaction, which is both delayed and non-local [4]. The latter turns out to represent the leading contribution to the effective interaction for the current 2D experiments.

A new experimental platform, which is currently built up in Kaiserslautern, will be devoted to analyse the dimensional crossover in trapped photon gases from 2D to 1D. As the photon-photon interaction is generically quite weak, they behave nearly as an ideal Bose gas. Moreover, since the current experiments are conducted in a microcavity, the longitudinal motion is frozen out and the photon gas represents effectively a two-dimensional trapped gas of massive bosons, where the anisotropy of the confinement allows for a dimensional crossover. A detailed investigation for such a system allows to determine its effective dimensionality from thermodynamic quantities [5].

Furthermore, we investigate how the effective photon-photon interaction changes when the system dimension is reduced from 2D to 1D [6]. To this end, we consider an anisotropic harmonic trapping potential and determine via a variational approach how the properties of the photon Bose-Einstein condensate in general, and both aforementioned interaction mechanisms in particular, change with increasing anisotropy. We find that the thermo-optic interaction strength increases at first linearly with the trap aspect ratio and later on saturates at a certain value of the trap aspect ratio. Furthermore, in the strong 1D limit the roles of both interactions get reversed as the thermo-optic interaction remains saturated and the contact Kerr interaction becomes the leading interaction mechanism. Finally, we discuss how the predicted effects can potentially be measured experimentally.

[1] J. Klaers, J. Schmitt, F. Vewinger, and M. Weitz, Nature 468, 545 (2010).
[2] J. Klaers, J. Schmitt, T. Damm, F. Vewinger, and M. Weitz, Appl. Phys. B 105, 17 (2011).
[3] M. Radonjic, W. Kopylov, A. Balaz, and A. Pelster, New J. Phys. 20, 055014 (2018).
[4] E. Stein, F. Vewinger, and A. Pelster, New J. Phys. 21, 103044 (2019).
[5] E. Stein and A. Pelster, arXiv:2011.06339 (2020).
[6] E. Stein and A. Pelster, arXiv:2109.11211 (2021).

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Meeting ID: 826 2047 2000
Passcode: 680361

У оквиру семинара Групе за гравитацију, честице и поља Института за физику у Београду, у петак, 17. децембра 2021. године у 11 часова путем BigBlueButton платформе, Стефан Ђорђевић (Физички факултет у Београду), одржаће предавање:

Black hole entropy and the information loss paradox

САЖЕТАК:

In this part of the seminar on the black hole information paradox, we consider some concrete models of 2d dilaton gravity (CGHS, BPP) that allow us to explicitly calculate the Page curve using the island formula. As a new result, we will show how to perform this calculation for an eternal black hole in 2d dilaton gravity model obtained from 4d Einstein-Hilbert action via dimensional reduction.

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У оквиру семинара Групе за гравитацију, честице и поља Института за физику у Београду, у петак, 10. децембра 2021. године у 11 часова путем BigBlueButton платформе, Стефан Ђорђевић (Физички факултет у Београду), одржаће предавање:

Black hole entropy and the information loss paradox

САЖЕТАК:

In this part of the seminar on the black hole information paradox, we consider some concrete models of 2d dilaton gravity (CGHS, BPP) that allow us to explicitly calculate the Page curve using the island formula. As a new result, we will show how to perform this calculation for an eternal black hole in 2d dilaton gravity model obtained from 4d Einstein-Hilbert action via dimensional reduction.

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У оквиру семинара Центра за изучавање комплексних система Института за физику у Београду, у четвртак, 09. децембра 2021. године у 14 часова путем Zoom платформе, др Ненад Вукмировић (Лабораторија за примену рачунара у науци, Центар за изучавање комплексних система, Институт за физику у Београду), одржаће предавање:

Charge pair transport pathway in organic solar cells

САЖЕТАК:

Solar cells based on organic semiconductors have been widely investigated in the last three decades with a continuous improvement in their performance and lifetime and improved understanding of the processes that take place in the device. The first part of the talk will consist of a brief review of this research field and a review of our previous efforts aimed at understanding the key process in the solar cell – the separation of photo-generated charge pairs (excitons). Our recent results will be presented in the second part of the talk [1, 2]. We model the system using a microscopic effective Hamiltonian that takes into account the most relevant processes that determine the dynamics of excitons. The results for energy and time resolved dynamics of charge pairs from the moment of their photogeneration to the moment of their extraction or recombination will be presented. The results indicated that charge separation proceeds via the cold pathway in which donor excitons convert to relaxed charge-transfer excitons which further transform to the states of separated charges [1]. We further analyze these results from the perspective of non-equilibrium thermodynamics. We determine the difference between equilibrium and non-equilibrium dependence of free energy on charge pair separation distance. Our results indicated that this difference is largest when charge separation efficiency is large which led us to conclude that charge separation in efficient organic solar cells proceeds via a cold but nonequilibrated pathway [2].

[1] V. Janković and N. Vukmirović, J. Phys. Chem. C 124, 4378 (2020).
[2] W. Kaiser, V. Janković, N. Vukmirović, and A. Gagliardi, J. Phys. Chem. Lett. 12, 6389 (2021).

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Meeting ID: 899 4920 6726
Passcode: 819948

У оквиру семинара Групе за гравитацију, честице и поља Института за физику у Београду, у петак, 3. децембра 2021. године у 11 часова путем BigBlueButton платформе, Драгољуб Гочанин (Физички факултет у Београду), одржаће предавање:

Black hole entropy and the information loss paradox

САЖЕТАК:

This talk will be devoted to the so-called wormhole replica trick and the notion of an island – a kind of a quantum extremal surface recently advocated by Maldacena. We will explain how the application of the holographic principle leads to the unitary Page curve. A generalization of the original formula for a generalized entropy is achieved via the notions of a quantum extremal surface and an island. Using the gravity path integral we will present additional arguments in favour of this approach.

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:sr]У оквиру семинара Центра за изучавање комплексних система Института за физику у Београду, у четвртак, 25. новембра 2021. године у 14 часова путем Zoom платформе, др Томасо Макри (Департман за теоријску и експерименталну физику, Федерални универзитет у Рио Гранде до Норте, Натал, Бразил), одржаће предавање:

Solitons and polarons in ultracold bosonic mixtures

САЖЕТАК:

The realization of multicomponent systems offers a natural playground to observe nonequilibrium effects in a more general framework. Restricting to two-component BECs, one notices already a rich variety of phases in the ground state. In purely repulsive mixtures, one observes a homogeneous superfluid or a phase separation when inter-species repulsion overcomes the intra-species interaction strength. In the attractive regime a series of recent experiments showed the formation of dilute self-bound droplet states in a two-component BEC both in a tight optical waveguide and in free space, closely following the theoretical predictions. In the quasi one-dimensional geometry, upon varying the mean-field interaction from the weakly to the strongly attractive regime, one observes a smooth crossover between bright soliton states and self-bound droplets. In the first part of this talk we will investigate the quench dynamics of a two component Bose mixture and study the onset of modulational instability, which leads the system far from equilibrium. Analogous to the single-component counterpart, this phenomenon results in the creation of trains of bright solitons. In the second part we will discuss the properties of impurities in uniform and self-bound states of heteronuclear mixtures inspired by recent experiments. Finally, we will focus on the formation of exotic bound states for an impurity interacting with a self-bound droplet.

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Meeting ID: 813 8100 5164
Passcode: 667308

У оквиру семинара Групе за гравитацију, честице и поља Института за физику у Београду, у петак, 19. новембра 2021. године у 12 часова путем BigBlueButton платформе, др Воја Радовановић (Физички факултет у Београду), одржаће предавање:

Black hole entropy and the information loss paradox

САЖЕТАК:

The most important unsolved problem in black hole physics is the information loss paradox, i.e. whether the process of formation and evaporation of a black hole is unitary in accordance with quantum mechanics. The unitarity implies that von Neumann entropy of the Hawking radiation should initially increase but subsequently fall back down, following the so-called Page curve.

In the first talk of this series we consider black hole thermodynamics and discuss various concepts of entropy: fine-grained, coarse-grained and entanglement entropy. In addition, we calculate entanglement entropy for Rindler observer and show how to generalize this result to curved space-time, where it is used to define generalized entropy of a black hole.

The upcoming talks will be devoted to the so-called wormhole replica trick and the notion of an island – a kind of a quantum extremal surface recently advocated by Maldacena. We study a particular case of 2D dilaton gravity obtained by dimensional reduction of the Einstein-Hilbert action for Schwarzschild metric. We calculate the von Neumann entropy of the Hawking radiation and reproduce the Page curve.

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:sr]У оквиру семинара Центра за изучавање комплексних система Института за физику у Београду, у четвртак, 18. новембра 2021. године у 14 часова путем Zoom платформе, др Јакша Вучичевић (Лабораторија за примену рачунара у науци, Центар за изучавање комплексних система, Институт за физику у Београду), одржаће предавање:

Universal magnetic quantum oscillations in correlated lattice systems

САЖЕТАК:

Magnetic field is well known to have drastic effects on the transport properties of electronic systems. Similar to the quantum Hall effect in continuum systems, the Shubnikov-de Haas (SdH) effect in crystalline materials is an oscillatory behavior of conductivity as a function of the applied magnetic field. This phenomenon is well understood in good metals, and has been very useful in mapping out the Fermi surface in various materials. In recent experiments on synthetic moire systems, a new type of magnetic quantum oscillations has been observed that differs from SdH oscillations in two important aspects: the so called Brown-Zak (BZ) oscillations appear at elevated temperature and have no dependence on the size of the two-dimensional Fermi sea. In this talk, we will present the dynamical mean field theory (DMFT) results for the square-lattice Hubbard model in a magnetic field that clearly display the phenomenology of the BZ oscillations [1, 2]. A detailed analysis reveals an essential role of incoherence for the BZ oscillations, which may allow one to use this phenomenon as a tool for characterization of dominant scattering mechanisms in lattice systems.

[1] J. Vučičević and R. Žitko, Phys. Rev. Lett. 127, 196601 (2021).
[2] J. Vučičević and R. Žitko, Phys. Rev. B 104, 205101 (2021).

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Meeting ID: 863 3529 3898
Passcode: 054147

У оквиру семинара Групе за гравитацију, честице и поља Института за физику у Београду, у петак,5. новембра 2021. године у 11 часова путем BigBlueButton платформе, Илија Бурић (Департман за физику, Универзитет у Пизи), одржаће предавање:

Conformal bootstrap – a non-perturbative approach to conformal field theories

САЖЕТАК:

Conformal bootstrap is a collection of methods for the study of conformal field theories, which are based on minimal assumptions such as unitarity and self-consistency. After reviewing the basic structure of conformal field theories, I will illustrate some of these methods on one numerical and one analytic example.

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У оквиру семинара Центра за изучавање комплексних система Института за физику у Београду, у четвртак, 28. октобра 2021. године у 14 часова путем Zoom платформе, Милан Крстајић (Лабораторија Кларендон, Универзитет у Оксфорду), одржаће предавање:

Experimental platform for a box-trapped dipolar quantum gas

САЖЕТАК:

Dipolar quantum gases make up an increasingly popular branch within an already thriving field of ultracold quantum gases. The interest in such systems lies in a multitude of quantum phenomena that stem from dipole-dipole interactions, particularly from their long-range and anisotropic nature. This talk describes the design and building of an experimental platform for investigating many-body physics in homogeneous dipolar quantum gases of erbium. We will present results on the progress towards reaching the goal of an erbium Bose-Einstein condensate in an optical box potential, including the performance of laser cooling methods and the optical systems for optical trapping and transport. At present, the experiment is capable of producing clouds with 100 million atoms at the temperature of 15 microkelvin in the magneto-optical trap, which translates into about 15 million atoms at 50 microkelvin following transfer into an optical dipole trap. We will discuss details on the remaining steps to complete the platform and present an overview of the research topics our platform will make accessible, including roton physics and supersolidity, out-of-equilibrium many-body phenomena and dynamics of phase transitions in systems with long-range interactions. We will also briefly touch upon the subject of expanding the apparatus to a dual-species experiment with potassium, which will enable investigating systems with impurities. Finally, we will extend the discussion to results from numerical simulations on the stability of a purely dipolar quantum gas against collapse in a general power-law trap (of which the optical box is an example), showing that intermediate power-law traps lead to maximally homogeneous density distributions for near-critical interaction strengths.

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Meeting ID: 898 9938 6885
Passcode: 860765

У оквиру семинара Групе за гравитацију, честице и поља Института за физику у Београду, у среду, 27. октобра 2021. године у 13 часова путем BigBlueButton платформе, Волфганг Мартин Виланд (Аустријска академија наука, Институт за квантну оптику и квантну информацију, Беч), одржаће предавање:

How the Immirzi Parameter deforms the SL(2,R) Boundary Symmetries on the Light Cone

САЖЕТАК:

This talk describes how the Barbero–Immirzi parameter deforms the SL(2,R) symmetries on a null surface boundary. Our starting point is the definition of the action and its boundary terms. The action that we use is the usual Holst action. Compared to metric gravity it contains an additional coupling constant — the Barber Immirzi parameter. Given the action and the boundary conditions, we introduce the covariant phase space and explain how the Holst term alters the boundary symmetries on a null surface. This alteration only affects the algebra of the edge modes on a cross-section, whereas the algebra of the radiative modes is unchanged by the addition of the Barbero–Immirzi parameter. To compute the Poisson brackets explicitly, we work on an auxiliary phase space, where the SL(2,R) symmetries of the boundary fields are manifest. The physical phase space is obtained by imposing both first-class and second-class constraints. All gauge generators are at most quadratic in terms of the fundamental SL(2,R) variables. Finally, we discuss various strategies to quantise the system.

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У оквиру семинара Центра за изучавање комплексних система Института за физику у Београду, у четвртак, 14. октобра 2021. године у 14 часова путем Zoom платформе, др Ива Бачић (Лабораторија за примену рачунара у науци, Центар за изучавање комплексних система, Институт за физику у Београду и Департман за науку о мрежама и подацима, Централно-европски универзитет, Будимпешта), одржаће предавање:

Onset of physicality in a random network growth model

САЖЕТАК:

Networks are abstract representations of complex systems in which any physicality of their constituents is disregarded. In a number of real networks, however, nodes and links are physical objects embedded in space that cannot intersect with each other. If the size of nodes and links is small compared to the available space, physicality likely has little effect on the network; however, if the volume of the network is comparable to volume of the available space, physicality will affect the structure, the evolution, and the function of the networks. Examples of such systems include neurons in the brain, the vascular system, mycelial networks, three-dimensional integrated circuits, and subways or other similar infrastructures. This observation prompts the question: when does physicality matter? In this talk we will propose a tractable random growth model of physical networks that provides insight into this question. The model describes linear physical networks, where links are non-overlapping straight cylinders. Network growth is achieved by sequentially adding nodes to randomly chosen points within the unit cube, and connecting to a randomly chosen accessible node from the existing network, with non-crossing conditions taken into account. We will identify two critical transitions: (i) the onset of the weakly physical regime, where the average link length becomes shorter, although the volume of the network is still negligible compared to the available volume, and (ii) the onset of the strongly physical regime, where the network occupies a finite fraction of the cube’s volume. We will demonstrate the validity of the analytical arguments by extensive numerical simulations.

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Meeting ID: 841 4527 0449
Passcode: 551996

У оквиру семинара Групе за гравитацију, честице и поља Института за физику у Београду, у петак, 8. октобра 2021. године у 11:15 путем BigBlueButton платформе, Данијел Обрић (Институт за физику у Београду), одржаће предавање:

T-dualization of bosonic string and type II superstring in presence of coordinate dependent background fields

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Abstract: In this talk we will discuss one of the ways how coordinate noncomutativity arises in the context of string theory. Starting point of the talk will be a short introduction to string theory, where we will discuss basic formalism for both bosonic string and supersymmetric string, we will also discuss dualities that are present in the theory. After this, discussion will focus on procedure for obtain in T-dual theories. This procedure will be applied first on bosonic string and later on superstring, where, after obtaining T-dual theories, we will discuss what effect did T-duality have on structure of Poisson brackets.

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У оквиру семинара Центра за изучавање комплексних система Института за физику у Београду, у четвртак, 30. септембра 2021. године у 14 часова путем Zoom платформе, др Јелена Смиљанић (Лабораторија за примену рачунара у науци, Центар за изучавање комплексних система, Институт за физику у Београду и Департман за физику, Универзитет у Умеу, Шведска), одржаће предавање:

Mapping flows in undersampled networks

САЖЕТАК:

Analyzing flow pathways in networks often highlights subgroups of nodes where flows stay for relatively long times, which indicates that the networks have community structure. Detecting these community patterns is key to revealing critical organizational principles. However, empirical network data often contain measurement errors such as missing links. Uncertainties in the network structure can cause detection of inaccurate community patterns with misleading information about the system. In this talk we will present a community detection method that enables more reliable descriptions of complex systems with incomplete data [1, 2].

[1] J. Smiljanić, D. Edler, and M. Rosvall, Phys. Rev. E 102, 012302 (2020).
[2] J. Smiljanić, C. Blöcker, D. Edler, and M. Rosvall, arXiv:2106.14798 (2021).

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Meeting ID: 868 3259 2313
Passcode: 283188

Осма међународна школа и конференција о фотоници Photonica 2021 коју организују Српска академија наука и уметности, Институт за физику у Београду и Оптичко друштво Србије, свечано ће бити отворена у понедељак, 23. августа 2021. године у 8 и 30. Присуство у Свечаној сали САНУ ће у складу са епидемиолошком ситуацијом бити омогућено ограниченом броју лица.

Конференција ће трајати до петка, 27. августа, а на њој ће учествовати велики број истраживача из Србије и света, док ће велики број њих имати онлајн излагање. Паралелно са конференцијом одржава се и радионица HEMMAGINERO о снимању хемоглобина и еритроцита.

Програм конференције Photonica 2021