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1068 イベント
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セミナー
Hydrodynamic theory of electron and spin transport
2022年3月30日(水) 13:30 - 15:00
多々良 源 (理化学研究所 創発物性科学研究センター (CEMS) スピン物性理論研究チーム チームリーダー)
Electron and spin transports in metals are theoretically studied from a hydrodynamic viewpoint by calculating momentum flux density as a linear response to an applied electric field. Dissipative (ohmic) fluid regime is considered. An angular momentum generation in chiral (Weyl) system and spin motive force (voltage generation) by magnetization-vorticity coupling in anomalous Hall system are discussed. The spin Hall effect is argued from the viewpoint of a spin-vorticity coupling.
会場: via Zoom
イベント公式言語: 英語
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Local and global topology for Dirac points with multi-helicoid surface states
2022年3月24日(木) 17:00 - 18:15
Tiantian Zhang (東京工業大学 理学院 特任助教)
Though topological invariants defined for topological semimetals are usually local ones, they also have a global nature. For example, the Z type local monopole charge C for Weyl points, has a global nature, telling us its influence to the rest of the Brillouin zone, giving rise to bulk-surface correspondence associated with helical surface states. In Dirac systems, helical surface states are not guaranteed due to C=0. However, a new bulk-surface correspondence associated with double/quad-helicoid surface states (DHSSs/QHSSs) can be obtained for Dirac points with the protection of a Z2 type monopole charge Q, which is defined in terms of the time-reversal (T)-glide (G) symmetry (TG)2= -1. Here we study the topology of Q for Z2 Dirac points and establish its bulk-surface correspondence with strict proofs. We find that Q is equivalent to the G-protected Z2 invariant v mathematically and physically in Z2 Dirac systems. This result is counterintuitive, since v is always trivial in T-preserving gapped systems, and was thought to be ill-defined in gapless systems. We offer a gauge-invariant formula for Q, which is associated with DHSSs in both the spinless and spinful systems with single G. Q is formulated in a simpler form in spinless systems with two vertical G, associated with QHSSs, which is also entangled with filling-enforced topological band insulators in three space groups when a T-breaking perturbation is introduced. Since Q is ill-defined in spinful systems with two vertical G, QHSSs will not be held. Material candidate Li2B4O7 together with a list of possible space groups preserving QHSSs are also proposed for demonstration on our theory and further studies. *Detailed information about the seminar refer to the email.
会場: via Zoom
イベント公式言語: 英語
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Criticality in stochastic SIR model for infectious diseases based on path-integral approach
2022年3月24日(木) 10:00 - 11:00
安井 繁宏 (高知大学医学部 医療情報科学センター 助教)
The susceptible-infected-removed (SIR) model provides us with a basic scheme for the analysis of the epidemic infectious diseases such as the COVID-19. In this presentation, we focus on the stochastic SIR model which describes the stochastic time-evolutions of the population sizes for the susceptible, infected, and removed individuals. We consider the master equation (Kolmogorov forward equation) for the infection transmission and recovery processes (SI->II and I->R), and transform it into the Hamiltonian formalism with the Fock space a la quantum physics. According to the Doi-Peliti prescription, furthermore, we introduce the path-integral formalism similar to the quantum field theory, and perform the perturbative and non-perturbative calculations for the time-evolution of the susceptible, infected, and removed populations. We find that the critical value Rc of the basic reproduction number, which determines the spreading or the convergence of the infectious diseases, can be modified by the stochastic effects in comparison to the Rc in the conventional deterministic SIR model.
会場: via Zoom
イベント公式言語: 英語
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Explore the possibility to control hurricanes
2022年3月18日(金) 16:00 - 18:00
Lin Li (理化学研究所 開拓研究本部 (CPR) 三好予測科学研究室 特別研究員)
Hurricanes, also known as tropical cyclones and typhoons, are the biggest and the most devastating storms on Earth. In this seminar, I will talk about the possibility to control hurricanes with existing human capability. Energetically speaking, controlling hurricanes is a very challenging task due to a large gap: hurricanes are gigantic heat engines with a power of around 1014 Watt, while the most powerful manmade engines have the power of only 108 Watt. This six-order-magnitude gap is the major obstacle toward using existing engines to control hurricanes. To fill in this gap, we propose to utilize the chaotic nature of hurricanes, namely, the sensitivity of a chaotic system to its initial condition, to control hurricanes. In this presentation, I will first review the basics of hurricanes and existing chaos control methods, and then present my thoughts on hurricane control and preliminary results I acquired since joining Prediction Science Laboratory. Future directions on using reinforcement learning to control hurricanes will also be discussed. Since it is a very challenging task, I welcome any discussions, questions, and comments. I hope we can make the hurricane-risk-free future come earlier.
会場: via Zoom
イベント公式言語: 英語
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Phylogenomics revealed one of the problems for phylogeny –The monophyly of Archaeplastida including land plant-
2022年3月17日(木) 10:00 - 11:00
矢﨑 裕規 (理化学研究所 数理創造プログラム (iTHEMS) 特別研究員)
There are many problems between large eukaryotic lineages. One of these is the monophyly of Archaeplastida to which land plants and other photosynthetic organisms belong. Although it has been believed that the Archaeplastida are monophyletic because they share common chloroplast structures, several large-scale molecular phylogenetic analyses have failed to reproduce this phylogenetic relationship. In this study, by enhancing the taxon sampling of the data set, the monophyly of Archaeplastida was successfully reconstructed, showing that the taxa critical for the reconstruction are present. Through detailed molecular phylogenetic and statistical analyses, it was estimated that the lack of monophyly ofArchaeplastida is due to the specific evolutionary signals of certain taxa.
会場: via Zoom
イベント公式言語: 英語
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Extracting rules from trained machine learning models with applications in Bioinformatics
2022年3月11日(金) 16:00 - 18:00
Pengyu Liu (理化学研究所 情報統合本部 (R-IH) 医療データ数理推論チーム 特別研究員)
Recently, Machine learning methods have achieved great success in various areas. However, some machine learning-based models are not explainable (e.g., Artificial Neural Networks), which may affect the massive applications in medical fields. In this talk, we first introduce two approaches that extract rules from trained neural networks. The first one leads to an algorithm that extracts rules in the form of Boolean functions. The second one extracts probabilistic rules representing relations between inputs and the output. We demonstrate the effectiveness of these two approaches by computational experiments. Then we consider applying an explainable machine learning model to predict human Dicer cleavage sites. Human Dicer is an enzyme that cleaves pre-miRNAs into miRNAs. We develop an accurate and explainable predictor for the human Dicer cleavage site -- ReCGBM. Computational experiments show that ReCGBM achieves the best performance compared with several existing methods. Further, we find that features close to the center of pre-miRNA are more important for the prediction.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Toward modeling complete supernova neutrino emissions
2022年3月11日(金) 16:00 - 17:00
諏訪 雄大 (東京大学 大学院総合文化研究科 宇宙地球部会 准教授 / 京都大学 基礎物理学研究所 基研特任准教授)
Neutrinos are guaranteed observable from the next Galactic supernova (SN). Optical lights and gravitational waves are also observable but can be difficult to observe if SN location in the galaxy and the explosion details are unsuitable. The key to the next coming SN observation will be understanding various physical quantities using neutrinos first and then connecting them to other signals. In particular, understanding neutrinos in the late time (> 1 sec after the onset of explosion) is essential, since physics in this time scale has much smaller uncertainties than that of the early time. We should construct a simple and understandable neutrino model based on the late-time emissions. It allows us to tackle the physics in the early phase like the explosion mechanism. In this talk, I will discuss the following topics: 1) how to model the complete neutrino emissions from the very early phase up to the last observable event. 2) what physical quantities (e.g., mass and radius of neutron stars) can be extracted from observations using large statistical neutrinos as physics probes. 3) how to use these extracted physical quantities to link with the explosion mechanism of SN and multi-messenger observations.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Nonperturbative cavity/waveguide quantum electrodynamics and dissipative quantum phase transition
2022年3月10日(木) 13:30 - 15:00
蘆田 祐人 (東京大学 大学院理学系研究科 准教授)
Strong coupling between matter and quantized electromagnetic modes in cavity or waveguide may offer yet another approach of controlling equilibrium phases or dynamics of many-body systems. Recent developments have realized such strong light-matter interaction in genuinely quantum and nonperturbative regimes, where conventional approximate theoretical methods cannot be applied in general. I will talk about how one can analyze strongly coupled quantum light-matter systems at arbitrary interaction strengths on the basis of an asymptotically disentangling unitary transformation [1,2]. I discuss its application to construction of tight-binding Hamiltonians, dynamics of bound states in the continuum, and revisiting dissipative quantum phase transition in resistively shunted Josephson junctions [3].
会場: via Zoom
イベント公式言語: 英語
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セミナー
Independent regulation of multiple checkpoints in cell-cycle network system -Biological function originated in the law of localization-
2022年3月10日(木) 10:00 - 11:00
望月 敦史 (京都大学 ウイルス･再⽣医学研究所 教授)
In cell cycle, G1-S and G2-M checkpoints are regulated by different protein complexes, Cdc2-Cdc13 and Cdc2-Cig2, respectively. For a normal mitosis, activity of two complexes should rise specifically at different timing. However, the complex formations share common species of proteins and activation reactions conform a complicated network. We study how independent regulation of two checkpoints is realized in the network system by “structural sensitivity analysis”, which was previously established by us. The analyses clarified that activities of two complexes are regulated by disjoint sets of reaction parameters in the system. A series of non-trivial behaviors are generated by “buffering structures with an intersection”, which can generally appear in chemical reaction network including complex formation.
会場: via Zoom
イベント公式言語: 英語
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Introduction to stability conditions 2
2022年3月9日(水) 16:00 - 17:30
小関 直記 (Postdoctoral Research Associate, School of Mathematics, University of Edinburgh, UK)
In 2002, Bridgeland defined the notion of stability conditions on a triangulated category, motivated by string theory and mirror symmetry. Since then, Bridgeland stability conditions have been found very useful not only in Mathematical Physics, but also in various areas of Pure Mathematics. In the first part, I will review basic background and open problems in the theory of Bridgeland stability conditions. In the second part, I will explain recent developments of the theory, especially its applications to algebraic geometry.
会場: via Zoom
イベント公式言語: 英語
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Introduction to stability conditions 1
2022年3月2日(水) 16:00 - 17:30
小関 直記 (Postdoctoral Research Associate, School of Mathematics, University of Edinburgh, UK)
In 2002, Bridgeland defined the notion of stability conditions on a triangulated category, motivated by string theory and mirror symmetry. Since then, Bridgeland stability conditions have been found very useful not only in Mathematical Physics, but also in various areas of Pure Mathematics. In the first part, I will review basic background and open problems in the theory of Bridgeland stability conditions. In the second part, I will explain recent developments of the theory, especially its applications to algebraic geometry.
会場: via Zoom
イベント公式言語: 英語
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How to understand Earth science system using data science
2022年2月25日(金) 16:00 - 18:00
Kaman Kong (理化学研究所 計算科学研究センター (R-CCS) 複合系気候科学研究チーム 特別研究員)
Hi everyone, my name is Kaman Kong. After I graduated from Nagoya University last April, I joined the computational climate science research team, R-CCS at Kobe. Although I have still not yet had the important results now, I would like to share my idea and future plan here. In this talk, different from the previous seminar, I would like to highlight how to use data science approaches to understand our Earth system science. In the first 60 minutes, I would like to share my research experiences in ecosystems, dust outbreaks, and atmospheric sciences and try to discuss their limitation in my study. After a 10-minute break, the 30 minutes will be spent discussing the potential methodology to overcome these limitations and new opportunities and challenges in Earth system science. (Part 1) In the first 60 minutes, I would like to talk about the relationships among ecosystems, dust outbreaks, and atmospheric conditions. I used the models of dust and ecosystem to explore seasonal variations of threshold wind speed, an index of soil susceptibility to dust outbreak, and its relations with land surface conditions, such as plant growth and soil moisture and temperature changes, in the Mongolian grasslands. On the other side, I am improving the weather forecast model to accurately predict dust emission and discuss its effects on the Earth system. Meanwhile, I am integrating the dust model into the ecosystem model. During this period, I realized there are many uncertainties of simulation. (Part 2) In the second 30 minutes, I will explain these limitations as I mentioned before and try to discuss how to solve these problems. For example, using deep learning to identify the green and brown plants separately for discussing their different effect on the dust model. And, used data assimilation (e.g., EnKF and Bayesian calibration) to improve the simulated performance of land surface parameters (e.g., soil moisture and vegetation).
会場: via Zoom
イベント公式言語: 英語
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セミナー
How is turbulence born: Spatiotemporal complexity and phase transition of transitional fluids
2022年2月24日(木) 17:00 - 18:15
Hong-Yan Shih (Assistant Research Fellow, Institute of Physics, Academia Sinica, Taiwan)
How a laminar flow becomes turbulence has been an unsolved problem for more than a century and is important in various industrial applications. Recently precise measurements in pipe flow experiments showed non-trivial spatiotemporal complexity at the onset of turbulence. Based on numerical evidence from the hydrodynamics equations, we discovered the surprising fact that the fluid behavior at the transition is governed by the emergent predator-prey dynamics of the important long-wavelength mode, leading to the mathematical prediction that the laminar-turbulent transition is analogous to an ecosystem on the edge of extinction. This prediction demonstrates that the laminar-turbulent transition is a non-equilibrium phase transition in the directed percolation universality class, and provides a unified picture of transition to turbulence emerging in systems ranging from turbulent convection to magnetohydrodynamics. *Detailed information about the seminar refer to the email.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Stator dynamics of the bacterial flagellar motor
2022年2月24日(木) 17:00 - 18:00
Ashley Nord (Researcher, Centre de Biologie Structurale, CNRS, France)
Rubén Pérez-Carrasco (Lecturer in Theoretical Systems Biology, Faculty of Natural Sciences, Department of Life Sciences, Imperial College London, UK)The bacterial flagellar motor is the membrane-embedded rotary molecular motor which turns the flagellum that provides thrust to many bacteria for swimming, swarming, and chemotaxis. This large multimeric complex, composed of a few dozen constituent proteins, is a hallmark of dynamic subunit exchange. The stator units are inner-membrane ion channels which dynamically bind to the cell wall and convert electrochemical energy into torque which is applied to the rotor. The dynamic exchange of stator units is a function of the viscous load on the flagellum, allowing the bacterium to adapt to its local environment, though the molecular mechanisms of this mechanosensitivity remain unknown. Previously, we have shown that stator units behave as a catch bond, a counterintuitive bond which becomes stronger under applied tension. Here, by actively perturbing the steady-state stator stoichiometry of individual motors, we reveal a stoichiometry-dependent asymmetry in stator remodeling kinetics. We interrogate the potential effect of next-neighbor interactions and local stator unit depletion and find that neither can explain the observed asymmetry. We then simulate and fit two mechanistically diverse models which recapitulate the asymmetry, finding assembly dynamics to be particularly well described by a two-state catch-bond mechanism.
会場: via Zoom
イベント公式言語: 英語
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iTHEMS - R-CCS(FTRT) Joint Online Seminar: Second order chiral phase transition in three flavor quantum chromodynamics?
2022年2月18日(金) 16:30 - 18:00
ゲルゲイ・フェヨシュ (Assistant Professor, Institute of Physics, Eötvös Loránd University, Hungary)
We calculate the renormalization group flows of all renormalizable interactions in the three dimensional Ginzburg--Landau potential for the chiral phase transition of three flavor quantum chromodynamics [1]. On the contrary to the common belief we find a fixed point in the system that is able to describe a second order phase transition in the infrared. This shows that longstanding assumptions on the transition order might be false. If the transition is indeed of second order, our results can also be interpreted as indirect evidence that the axial anomaly restores at the transition temperature.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Spin transport in ultracold atomic gases
2022年2月18日(金) 14:00 - 15:00
関野 裕太 (理化学研究所 開拓研究本部 (CPR) 長瀧天体ビッグバン研究室 特別研究員)
In condensed matter physics, transport measurement has played crucial roles in understanding fascinating phenomena such as superconductivity and quantum Hall and Kondo effects. In this talk, we discuss the usefulness of spin transport as a probe for many-body properties in ultracold atoms. In the first part, we focus on the conductivity of alternating spin current, which includes information on superfluid gap, pseudogap, and topological phase transition. In the latter part, we consider mesoscopic spin transport between two Fermi gases weakly connected with each other. Our analysis suggests that the spin current is sensitive to whether the gases have pseudogaps, which are gap-like structures in densities of states just above the superfluid transition temperature. In this talk, we also mention similarities of ultracold atoms to neutron star matter.
会場: via Zoom
イベント公式言語: 英語
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Hilbert transform and its applications to biology
2022年2月17日(木) 10:00 - 11:00
儀保 伸吾 (理化学研究所 数理創造プログラム (iTHEMS) 特別研究員)
In chronobiology, the estimation of phase dynamics is important for measuring period and phase shift. However, it is difficult to precisely estimate the phase from time-series data when the frequency and the amplitude are nonstationary. Hilbert transform has been known as a signal processing method for decomposing time-series into the phase and the amplitude dynamics. This method allows us to analyze the phase from nonstationary time-series data. In this talk, I would like to introduce the basic concept of Hilbert transform and a few examples of its applications.
会場: via Zoom
イベント公式言語: 英語
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セミナー
False vacuum decay in the Lorentzian path integral
2022年2月15日(火) 13:30 - 15:00
林 拓未 (東京大学 大学院理学系研究科附属 ビッグバン宇宙国際研究センター (RESCEU) 博士課程)
False vacuum decay is a non-perturbative phenomenon in quantum field theory and important quantum process in cosmology. It has relied on the Euclidean formalism developed by Coleman, but there are several subtle issues in cosmological application as a negative mode problem or ambiguity in the definition of the decay rate in the presence of the gravity. Instead of the Euclidean path integral, we directly evaluate the Lorentzian path integral to discuss false vacuum decay and estimate the decay probability. To make the Lorentzian path integral convergent, the deformation of an integral contour is performed on the basis of the Picard-Lefschetz theory. We show that the nucleation probability of a critical bubble, for which the corresponding bounce action is extremized, has the same exponent as the Euclidean approach. We also extend our computation to the nucleation of a bubble larger or smaller than the critical one to which the Euclidean formalism is not applicable.
会場: via Zoom
イベント公式言語: 英語
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Stochastic operators: properties and applications
2022年2月10日(木) 10:00 - 11:00
ジルベルト・ナカムラ (理化学研究所 数理創造プログラム (iTHEMS) 特別研究員)
Stochastic processes are widely used to model systems in which one or more variables fluctuate randomly. Problems arise when large sets of random variables are allowed to interact with each other, as is often the case with physical and biological systems. Stochastic operators provide a convenient framework for describing the interactions and evolution of the random variables. In this talk, I will discuss techniques and methods typically used in spin systems to deal with stochastic operators and their spectral analysis in the context of random processes. I will briefly review their properties and applications to biological systems. As practical examples, I will present some results of my research in infectious diseases and migration of glioma cells.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Mixed dark matter scenarios consisting of primordial black hole dark matter and WIMPs
2022年1月31日(月) 11:00 - 12:00
門田 健司 (Senior faculty scientist, Hangzhou Institute for Advanced Study at University of Chinese Academy of Sciences (HIAS-UCAS), International Center for Theoretical Physics-Asia Pacific (ICTP-AP) Hangzhou Branch, China)
While the possibility for the primordial black holes (PBHs) to constitute all of the dark matter (DM) is being narrowed by the astrophysical observations such as the gravitational microlensing, the PBH as a partial DM component is still an intriguing possibility. I will discuss the scenarios where the rest of the dark matter consists of the widely discussed weakly interacting massive particles (WIMPs) and show that PBH and WIMP cannot co-exist with an emphasis on the astrophysical probes including the gamma ray, 21cm and CMB observations.
会場: via Zoom
イベント公式言語: 英語
1068 イベント