Seminář se koná každé úterý v 10:40 v posluchárně ÚTF MFF UK
v 10. patře katedrové budovy v Tróji, V Holešovičkách 2, Praha 8
V nejbližší budoucnosti nejsou plánované žádné semináře.
Magnetically induced reorientation (MIR) - one of the magnetic shape memory effects providing giant, up to 12% deformation in magnetic field will be introduced and put into the context of multiferroic materials. In the presentation I will follow the path from macroscopic phenomenological description down to the nanoscale providing necessary conditions for the effect in Ni-Mn-Ga Heusler alloys. I will examine material demand; in particular partial doping of Ni-Mn-Ga by transition metals in attempt to increase transformation temperatures, twin hierarchy from macro to nanoscale, magnetic ordering and extreme shear instability of the lattice. The presence of antiphase boundaries in ordered Heusler alloys, which can enhance the functionalities of the material, will be discussed. All these characteristics observed experimentally will be put to the perspective of theoretical calculation on suitable scale to provide inspiration for the state-of-art calculation which is still regrettably not to date for such interesting materials.
[Dr. Zavázal je matematik a diplomat. Po absolvování teoretické kybernetiky na MFF UK byl mimo jiné matematikem-analytikem Parlamentního institutu Federálního shromáždění ČSFR, poradcem ministra zahraničí ČR, nebo českým velvyslancem v Řecku, v Itálii a v Chorvatsku. Dnes je ředitelem Diplomatického servisu.]
Když jsem přemýšlel nad pojetím přednášky o mé kariéře, znovu jsem si uvědomil, že heslo Voskovce & Wericha na můj život opravdu přesně sedí. Na přednášce se o tom pokusím přesvědčit i posluchače. Jestliže máte jako generačně podmíněný životní axiom, že s KSČ a StB nechcete mít nic společného, zbývá vám k seberealizaci jen velmi zúžený koridor, do kterého se musíte vejít. Kdybych byl aspoň o 20 let mladší, možná bych nestudoval matematiku, ale třeba římskou historii. Potom bych ale nejspíše přišel i o druhou část mého profesionálního života, a to o manažerské funkce, kterými jsem prošel a dosud procházím. Hlavní výhybka v mé kariéře nastala začátkem roku 1993 s koncem ČSFR, kdy mně ministr Zieleniec nabídl, abych přišel dokončit dělení federálního majetku MZV. To jsem měl na starosti první rok působení na MZV a pak už k diplomacii chyběl jen krůček. Nakonec jsem v ní uvízl 30 let.
The concept of effective mass is frequently used for simplification of complex mechanical models. A puzzling use of effective mass concept appeared in theory of metamaterials. It is claimed in some metamaterials the corresponding effective mass can be frequency dependent, negative and it may not even be a scalar quantity. These findings have even led some authors to suggest that Newton's second law needs to be modified within the context of classical continuum mechanics. While there is nothing wrong with the mathematical procedures used to reach these conclusions, the accompanying physical interpretation thereof is absurd. We show that the puzzling concept of negative mass can be easily eliminated provided that we model the corresponding metamaterials using an effective constitutive relation. The effective constitutive relation is a rate-type constitutive relation, and the simple models we study give a clue how to design a non-trivial effective rate-type constitutive relation that is conservative, that is, possessing an associated conserved energy depending on the force/stress and its time derivatives. In short, the rate-type constitutive relations obtained in our study might be seen as a revival of hypoelastic materials.
Solar physics, like the entire astrophysics, experiences periods of great blossoming thanks to new space-born and ground-based instruments which revolutionized our view of the Sun in the past decade. The seminar aims to briefly review the recent main instruments for solar research, scientific motivation for their construction, and summary of their most significant results. Particular attention will be devoted to solar science with the so-far largest interferometric observatory ALMA, as it is the speaker's own topic.
I will review mostly (semi)analytical approaches to a generic model of an interacting single-impurity Anderson model with superconducting leads describing experimentally relevant setups consisting of nanoobjects ("quantum dots") coupled to superconducting contacts. Such systems might form a basis of future quantum information devices and, thus, their reliable theoretical description is highly needed. After general physical introduction to the topic I will focus on the technical aspects of the problem from the many-body quantum mechanical point of view.
We will introduce the conditional mutual information, a.k.a. transfer entropy which generalizes the notion of Granger causality for nonlinear systems. The information-theoretic approach, defining causality as information transfer, has been successful in many applications and generalized to multivariate data and causal networks. This approach, rooted in the information theory of Shannon, usually ignores two important properties of complex systems: the systems evolve on multiple time scales and their variables have heavy-tailed probability distributions giving a higher probability to occurrence of extreme events than in systems described by the normal distribution. Multiple time scales can be tackled using a scale-wise decomposition (e.g., wavelet transform) and information transfer between time scales can be observed, e.g., in atmospheric dynamics or El Nino Southern Oscillations. Reformulation of the conditional mutual information in the framework of Rényi information theory provides a tool for uncovering the causes of extreme events.
Lightning discharges emit electromagnetic pulses, which propagate through the plasma medium as dispersed whistlers. Short duration trans-ionospheric whistlers have been detected for the first time at Jupiter by the Juno spacecraft. They not only constitute the largest existing database of Jovian lightning but they also provide us with inputs for investigation of the ionospheric plasma. Dispersion properties of new propagation modes of whistlers can be explained by peculiar properties of the mode structure and group velocity for extremely low plasma densities. The radio and plasma waves instrument on the newly launched JUICE interplanetary probe will be able to characterize wave propagation and mode structure of whistler-mode waves.
Prezentuje se stručná historie standardního modelu elektroslabých interakcí, za který byla před 45 lety udělena Nobelova cena. Přitom se připomíná řada výročí zlomových objevů, jako Fermiho teorie slabých interakcí, koncept Yang-Millsova pole, kvarkový model, narušení CP, Higgsův mechanismus nebo objev c-kvarku.
The ordinary chondrite meteorites comprise 70% of all falls. Hereinafter, we explain their origin as ejecta from three relatively recent collisions in the asteroid belt --- related to the Massalia, Koronis, and Karin asteroid families. Our conclusions are supported by various kinds of observations, ranging from reflectance spectroscopy of near-Earth asteroids, infrared photometry of dust bands, radiometric dating of meteorites to bolide orbits prior to atmospheric entry.
Current advancements in artificial intelligence are reflected in the joint Nobel Prize in Physics awarded to two pioneers of artificial neural network research, Geoffrey Hinton and John Hopfield. In this talk, we will briefly trace the history of neural networks, from early theoretical concepts to the resurgence of deep learning in recent decades. We will discuss the development of the backpropagation algorithm by Hinton and his colleagues, and its pivotal role in the success of deep neural networks, as well as the Hopfield network model, which connects neural networks with physical systems—particularly the dynamics of energy minimization in the brain. The talk will also address current challenges and the ongoing evolution of neural network technologies, which have revolutionized fields such as artificial intelligence, pattern recognition, natural language processing, and cognitive science.
In this talk I will show how Brownian motion on a short scale can give rise to relativistic motion on scales larger than the particle's Compton wavelength. I will start by discussing complex dynamical systems whose statistical behaviour can be explained in terms of a superposition of simpler underlying dynamics - the so-called superstatistics paradigm. I then show that the combination of two cornerstones of contemporary physics - namely Einstein's special relativity and quantum mechanical dynamics - is mathematically identical (when analytically continued to the Euclidean regime) to a complex dynamical system described by two interlocked processes operating at different energy scales. The combined dynamics obeys special and doubly special relativity, even though neither of the two underlying dynamics does. This implies that Einstein's special relativity might well be an emergent concept in the quantum realm. To model the double-stochastic process in question, I consider quantum mechanical dynamics in a background space consisting of a number of small crystal-like domains varying in size and composition, known as polycrystalline space (or Voronoi tessellation). There, the particles exhibit a Brownian motion. The observed relativistic dynamics then arises solely from a particular grain distribution in the polycrystalline space. In a cosmological context, such a distribution could form during the formation of the early universe. Salient issues such as Hausdorff dimensions of path-integral trajectories, connection with the Feynman chessboard model and implications for quantum field theory and cosmology (leptogenesis) will be also briefly discussed.
Jiří Horáček David Heyrovský