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
Conical intersections play a crucial role in photophysics and photochemistry. At conical intersection seams of electronic potential-energy surfaces, the molecular dynamics is dominated by a complete breakdown of the celebrated Born-Oppenheimer (BO) approximation. Starting from a historical overview of the BO approximation and the concept of conical intersections, this lecture traces the development of methods which allow the computational study of the ultrafast (femtosecond) nonadiabatic dynamics at conical intersections. A brief overview is also given of computational methods for the simulation of time-resolved nonlinear spectra. It is demonstrated that photochemical dynamics can nowadays be scrutinized in unprecedented detail by the interplay of laser spectroscopy and computational chemistry.
When a gas of identical atoms about a million times thinner than air is cooled down to about a millionth of a degree above absolute zero, quantum mechanical effects become important and the gas can enter various exotic phases of matter such as the Bose-Einstein condensate (BEC). In the last three decades, such cold atomic gases have been used with great success as a highly controllable platform for studying diverse many-body phenomena. I will give a brief introduction to the field and to our experiments with ultracold atoms in optical box traps, and later discuss some of our recent studies focusing in particular on 1) far-from-equilibrium dynamics of Bose gases and 2) physics of impurities immersed in a Bose-Einstein condensate.
Due to the exponential progress in laser technology, ultra-intense lasers are quickly becoming an essential tool to probe quantum electrodynamics phenomena. These include e.g. radiation of accelerated charged particles, particle-antiparticle pair production, and polarization of vacuum. After a brief overview of these effects, we will discuss “Flying Focus” (FF) laser pulses which enable co-propagation of particle beams (e.g. ultra-relativistic electrons or hard photons) with the laser focus, so that they stay in the region of peak field intensity for prolonged interaction times. We will introduce FF generation methods and analytical description of FF pulses with arbitrary focal velocities and discuss experimental configurations in which the long laser-particle interaction time aids high-intensity applications. Since signatures of strong field effects in laser-particle interactions accumulate with interaction time, the FF regime enables experimental access at orders of magnitude lower laser powers and intensities than in conventional fixed-focus setups. Even more importantly, in the quantum regime of the laser-electron interaction the energy loss and photon yield scale more favorably with the interaction time than the laser intensity, giving FF an outright advantage over fixed-focus pulses.
Our Sun is a single star, which is somewhat of an exception in the Universe as most stars are found in binary and multiple systems. For a wide range of initial conditions at stellar birth, the two stars in a binary will interact by exchanging mass, exploding, or merging to a single object. Computer simulations of binary systems are indispensable in revealing the fundamental physical processes governing the binary's structure and evolution, yet they are complicated by a lack of any useful symmetry. In this talk I will review results of radiation / magneto-hydrodynamical simulations of a certain evolutionary phase studied with the help of ERC StG and describe plans for a new set of simulations with discontinuous Galerkin method to be done with the support of ERC CoG.
Inelastic electron tunneling spectroscopy is a well-established technique used for investigation of vibrational spectra and, more recently, also for characterization of spin excitations in magnetic nanosystems probed in scanning tunneling microscopes. In cases of tunneling through a single magnetic center (a single partially filled atomic shell), the spectra are successfully modeled by means of an anisotropic spin model assuming that the tunneling electron is exchange-coupled to the spin of the nanosystem. I will discuss a more general theory, in which the magnetic nanosystem is represented by a cluster Hubbard model (one site for each partially filled shell in the nanosystem) and in which the tunneling current is calculated by means of a perturbation theory, utilizing the Kramers-Heisenberg formula. I will show that this model reproduces the predictions of the spin model in simple systems, and then illustrate how it fares in more complex settings.
The Higgs boson is one of the key elements in the Standard Model that describe the elementary particles and their interactions in nature. The theoretical motivation will be summarized and the history of the experimental searches for the Higgs boson will be briefly reminded. The Higgs boson discovery by the ATLAS and CMS collaborations at the Large Hadron Collider in 2012 will be described together with the recent status of the measurements including the searches for beyond Standard Model phenomena in the Higgs sector. Finally, the outlook for the High Luminosity LHC measurements will be discussed.
The application of mass spectrometry for asteroid exploration has recently become a hot topic. Mass spectrometry can be used both in orbit and on the asteroid’s surface for the analysis of space dust, micrometeorites, and particles from larger objects. The HANKA (Hmotnostný ANalyzér pre Kozmické Aplikácie) space instrument is a high-resolution mass spectrometer based on an electrostatic ion trap, which is a principal component of commercial instruments established in biology and medicine research, the so-called Orbitrap, and the space CosmOrbitrap prototype. HANKA will bring this new technology into space to combine a small CubeSat space version of this high-mass resolution ion trap analyzer, with a velocity/charge detector and a hypervelocity impact ionization source. Based on the results obtained on the laboratory prototype, a miniature version of HANKA will be constructed.
Neural signals evoked by sound travel in our brain at speeds that are slower than the speed of sound conduction in bone, water, or air. This almost anecdotal statement evokes many questions. Cochlea (inner ear) acts as a transducer of mechanical sound energy to electrical signals in neurons. How does it work? How do we perceive the location of the sound source? Can we measure the extent of hearing loss without using subjective responses of a non-cooperating subject (small children or patients simulating deafness)?
(seminář v češtině)
Fyzikální akustika je v současnosti tematicky velmi rozsáhlý obor. S ohledem na tuto skutečnost je přednáška zaměřena na dvě dílčí oblasti. První téma přednášky se věnuje nelineární akustice v tekutinách a poukazuje na jevy, které v rámci lineární akustiky nelze pozorovat. Součástí je také seznámení s aplikačním potenciálem velmi intenzivních akustických polí, zejména v kontextu potlačení saturačních jevů a prolomení difrakčního limitu. Druhé téma spadá do oblasti lineární akustiky v nehomogenních prostředích a zaměřuje se na využití Heunových funkcí při řešení příslušných modelových rovnic.
The lecture will focus on aerodynamic simulations and typical practical applications in both external and internal aerodynamics. Topics will include flow simulations, turbulence modeling, aerodynamic design, internal flow analysis in turbomachinery and various interdisciplinary applications. The discussion will also address the computational complexity of these simulations and the role of high-performance computing (HPC).
Active matter is a 30-year-old subfield of statistical physics that aims to apply its tools to study the origins and universal traits of structure formation in large groups of nonequilibrium self-propelling agents, such as flocks of birds, swarms of insects or robots, or bacterial colonies. I will first review the scope of the field and present its main results and models. In the second part of the talk, I will focus on our ongoing research in the field, namely the effects of time delay and prediction in the interactions of individual active agents.
The discovery of a large number of extrasolar planets has demonstrated that our own system is not "typical". Exo-planetary systems can be very different from our own, and diverse from each other. Understanding this diversity is a major goal of modern planetary science. The formation of planetary systems is not fully understood, but major advances have been obtained in the last 10 years. New concepts have been proposed, such as the streaming instability for the formation of planetesimals and pebble accretion for the formation of protoplanets. It is also now clear that planets forming in the proto-planetary disks have to migrate during their accretion, if their mass exceeds a few times the mass of Mars. Accretion and dynamical evolution are therefore very coupled processes. This leads to complex evolutions, very sensitive to initial conditions and fortuitous events, that are the key to understand the observed diversity of planetary systems. The early formation of Jupiter and its limited migration due to the formation of Saturn are two fundamental ingredients that determined the basic structure of the Solar System. There is also evidence that the vast majority of planetary systems become unstable after the removal of the protoplanetary disk. The effects of this instability are very different depending on the masses of the planets involved. Our Solar System also experienced a global instability, but fortuitously our giant planets did not develop large orbital eccentricities.
Jiří Horáček David Heyrovský