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