Wydział Fizyki UW > Badania > Seminaria i konwersatoria > Seminarium "Modeling of Complex Systems"

Seminarium "Modeling of Complex Systems"

2018/2019 | 2019/2020 | 2020/2021 | 2021/2022 | 2022/2023 | 2023/2024

Zapraszamy do sali 2.22, ul. Pasteura 5 o godzinie 15:15

Anna Dawid (IFT UW)

Can a learning machine teach us physics?

Machine learning not only succeeds in everyday tasks like character and voice recognition problems, ﬁngerprint identiﬁcation, e-mail spam ﬁltering, autonomously driving cars, credit card fraud detection, health care, ﬁnancial modeling, and many more, but also has been already applied to quantum chemistry and material science problems. In physics, main contributions concerned the improved variational ansatz for many-body problems, wave-function reconstruction, but above all – detection of phases from synthetic or experimental data. Can this approach teach us something new about phase classification? So far it has only enabled the recovery of known phase diagrams or the location of phase transitions with qualitative agreement with more conventional approaches (but at much lower computational cost). Can we be even sure that the machine learns anything related to order parameter? Not really. These two questions should be addressed in order to fully legimitize the use of machine learning methods at least in this type of physical problems.The seminar aims to give a friendly introduction to the topic of machine learning and its use in quantum physics, and to present influence functions, being an interpretation tool developed in machine learning community. Preliminary results from studies on this method will be shown, and their potentially fruitful application on the border between machine learning and phase classification problems will be discussed.

Zapraszamy do sali 2.22, ul. Pasteura 5 o godzinie 15:15

Piotrr Szymczak (IFT UW)

Pulling knotted proteins through the pores: topological traps and how to avoid them

If we tie a knot on a piece of a rope and then pull it through a narrow hole, the knot tightens, and can block the opening. We argue that a similar phenomenon can take place in microworld, during thetransport of knotted proteins through the pores in cellular membranes. The radius of gyration of the tight knot is about 7-8 Angstrom for a trefoil, whereas the radius of the narrowest constriction of e.g. the mitochondrial pores are about 6 Angstrom, which means that the knot is a shade too large to squeeze through the pore opening. We show how such topological traps might be prevented by using a pulling protocol of a repetitive, on-off character. During the off-force period some stored length is inserted into the knotted core, and the knot loosens, thus escaping the tightened configuration. Subsequently, during the next on-force period the protein makes another attempt at the translocation. Since the probability of getting trapped in each of n successive tries rapidly decreases with n, repetitive trying always leads to a final success. Importantly, such a repetitive pulling is biologically relevant, since molecular import motors are ATP-hydrolysis driven and thus cyclic in character. Finally, we analyze the dependence of the translocation rate on force period and magnitude and show that there exists an optimum range of these parameters which lead to the most efficient translocation.

Zapraszamy do sali 2.22, ul. Pasteura 5 o godzinie 15:15

Agnieszka Jamróz (IFT UW)

Multiscale modeling of graphene-like two-dimensional C-B-N alloys: morphology, ordering, and electronic structure

Lateral alloys of graphene and hexagonal boron nitride exhibit properties ranging from wide insulating, semi-metallic to metallic, which is mostly dependent on the concentration and ordering of the constituent atoms. This makes them promising for numerous applications in electronics, optoelectronics, sensorics, light harvesting, etc.We develop Valence Force Field – Monte Carlo scheme and combine it with ab initio and tight binding simulations to obtain the most complete picture of stable phases and properties of these monolayer materials. This approach allows for examining the influence of growth conditions and defects on the ordering and electronic structure in the wide range of possible concentrations of C, B and N atoms.

Zapraszamy do sali 2.22, ul. Pasteura 5 o godzinie 15:15

Marta Sroczyńska (IFT UW)

Trap-induced shape resonances in an ultracold few-body system of an atom and static impurities

Hybrid systems of ultracold atoms and trapped ions or Rydberg atoms can be useful for quantum simulationpurposes. By tuning the geometric arrangement of the impurities, it is possible to mimic solid-state and molecularsystems. Here, we study a single trapped atom interacting with a set of arbitrarily arranged static impurities andshow that the problem admits an analytical solution. We analyze in detail the case of two impurities, findingmultiple trap-induced resonances which can be used for entanglement generation. Our results serve as a buildingblock for the studies of quantum dynamics of complex systems.

Zapraszamy do sali 2.22, ul. Pasteura 5 o godzinie 15:15

dr Tomasz Gubiec (Faculty of Physics, UW)

What are we doing @complex_warsaw?

We are a research group working in the field of complex systems withinthe Institute of Experimental Physics UW. The group was founded byprof. Ryszard Kutner with econophysics as our primary research topic(application of methods of physics in economics). Over time ourinterests have expanded.In the talk, I will present the team and share a few of our latestresults. As we are applying stochastic processes, complexnetworks theory and agent-based modeling in fields like stock marketdynamics, the stability of banking system, dissemination of languageand culture it will be a trip through very exotic applications ofphysics.

Zapraszamy do sali 2.22, ul. Pasteura 5 o godzinie 15:15

dr Magdalena Birowska-Popielska (IFT UW)

Is it possible to obtain non-degraded Black Phosphorus in air?

Two-dimensional (2D) materials offer novel physics and potential use in multiple applications. However, many of the layered materials are very sensitive to the local environment and ambient conditions. Black phosphorus (BP) represents an extreme example of sensitivity to moisture and oxygen, which can lead to catastrophic degradation on a time scale of only minutes. Recent studies have shown that encapsulation with hexagonal boron nitride (hBN) protects BP from structural and chemical degradation, while improving its electrical properties making hBN the most commonly used material for encapsulation. In this talk I will examine the influence of hBN encapsulation on the structural and vibrational properties of BP using density functional theory (DFT). I will show that encapsulation strains the BP layer, which has significant impact on the vibrational properties. Both non-encapsulated and encapsulated BP layers, exhibit anomalous evolution of phonon frequencies, which show a redshift with increasing number of layers. The presented theoretical predictions are in good agreement with the results of Raman spectroscopy performed on h-BN encapsulated BP layers.

Zapraszamy do sali 2.22, ul. Pasteura 5 o godzinie 15:15

Alaeksander Ramaniuk (IFT UW)

Coupled Microresonators

Gain and loss are omnipotent in the physical, chemical and biological systems. Their effects can in a convenient way be modelled by effective non-Hermitian Hamiltonians. Imaginary contributions to the potential introduce source and drain terms for the probability amplitude. A special class of non-Hermitian Hamiltonians are those which possess a parity-time symmetry. In spite of their non-Hermiticity these Hamiltonians allow for real energy eigenvalues, i.e. the existence of stationary states in the presence of balanced gain and loss. This effect has been identified theoretically in a large number of quantum systems. Its existence has also been proved experimentally in coupled optical wave guides. In my talk I will provide concise review of these systems including the aspect of physics of energy conversion in nanostructures. Effects described above have very broad context. The dynamics can be very interesting and worth studying even if the parity-time symmetry is not conserved. The list of systems that belong to this class include whispering gallery modes in the micro-resonators, coupled wave-guides, unidirectional reflectionless metamaterial at optical frequencies, polariton condensates and may, many others. In my talk I consider a nanostructure of two coupled ring waveguides with constant linear gain and nonlinear absorption - the system that can be implemented in various settings including polariton condensates, optical waveguides or atomic Bose-Einstein condensates. It was found that, depending on the parameters, this simple configuration allows for observing several complex nonlinear phenomena, which include spontaneous symmetry breaking, modulational instability leading to generation of stable circular flows with various vorticities, stable inhomogeneous states with interesting structure of currents flowing between rings, as well as dynamical regimes having signatures of chaotic behavior.

Zapraszamy do sali 2.22, ul. Pasteura 5 o godzinie 15:15

dr Emil J. Zak (Department of Chemistry, Queen's University, Kingston, ON, Canada)

High-accuracy calculations of rotational-vibrational-electronic spectra of small molecules

High-accuracy spectra of small molecules (2-9 atoms) are used as reference in characterizationof atmospheres of Exoplanets as well as in monitoring concentrations of greenhouse gasses in theEarth's atmosphere. Spectra acquired from experiment often suer from insucient accuracy intransition intensities for the remote sensing purposes. I am going to present a number of theoreticalmethods used to calculate rotational-vibrational and rotational-vibrational-electronic energy levels,wavefunctions and transition intensities for molecules important for astrophysics and atmosphericscience.In variational calculations of rotational-vibrational-electronic energy levels of polyatomicmolecules the total wavefunction is typically represented as linear combination of basis functions.The size of the multidimensional direct-product basis as well as the size of the grid needed to com-pute integrals accurately grows exponentially with the number of atoms. As a result, the memoryrequirements in variational calculations become prohibitive for molecules with more than 4-5 atoms- this is often referred to as the curse of dimensionality.I am going to discuss new developments in the eld, including a method which circumvents theproblem of exponential scaling of the basis set size with the number of atoms. This is achievedthrough a collocation approach which uses a non-direct product basis set and non-direct productgrids. In collocation, the Schroedinger equation is solved at a set of points, which avoids theneed for accurate multidimensional quadratures. In other words, unlike in variational calculations,collocation allows to solve the Schroedinger equation without calculating a single integral.

Zapraszamy do sali 2.22, ul. Pasteura 5 o godzinie 15:15

dr Marcin Gronowski (IFT UW)

Molecules in innterstallar space

Matter in our Galaxy evolves from extremely diluted gas, through molecular clouds, and star-forming regions to stars and their planetary systems. The pre-stellar phase is characterized by a mixture of gas and dust in low density (104 atoms/cm3) and temperature (10-30 K) conditions. Observing atoms and molecules in molecular clouds provides a unique glimpse into an extreme environment whose conditions are difficult to match even in sophisticated laboratories. In course of this lecture, we show a connection between the microscopic processes studied in laboratories on Earth and our understanding of phenomena in different space environments.The bulk of interstellar molecules have been detected by observation of microwave emission from rotationally excited molecules.Theoretical predictions of spectroscopic parameters such as rotational constants and vibrational frequencies by quantum calculations play an important role in guiding spectroscopic and astronomical studies of molecules, as we demonstrate using a few different examples.The main question is: how are these molecules formed? Here, we present the possible cold-chemistry routes leading to selected sulfur-containing molecules of astrophysical significance by modeling of the relevant reaction network using quantum chemical calculations.