Seminar

Introduction to Topological Insulators: The Ten-fold Classification of Topological Insulators and Superconductors Part.1

May 24 (Tue) at 14:00 - 15:30, 2022

Ching-Kai Chiu (Senior Research Scientist, iTHEMS)

Venue: via Zoom

Event Official Language: English

A mathematical formulation of two-dimensional conformal field theory

May 23 (Mon) at 14:00 - 16:30, 2022

Yuto Moriwaki (Special Postdoctoral Researcher, iTHEMS)

The mathematical construction of non-trivial quantum field theory in four dimensions, known as the "Yang-Mills existence and mass gap problem", is a very important issue in mathematical sciences. There are many examples of rigorous quantum field theories in two dimensions, although the four dimensions have not yet been solved. In particular, two-dimensional conformal field theory, which is a quantum field theory with conformal symmetry, has good properties and can be formulated mathematically using algebraic structures formed by "products of a field and a field" (operator product expansion). In this talk, this algebraic formulation (full vertex algebra) will be explained. Various construction methods and concrete examples (construction using codes, construction from quantum groups, and construction by deformation) will then be discussed. All the talk here is mathematical, but I will try to speak in a way that is motivated by physics as much as possible throughout the talk. I hope to receive various comments from the viewpoints of other fields.

Venue: Hybrid Format (Common Room 246-248 and Zoom)

Event Official Language: English

The Hunt for Extraterrestrial Neutrino Counterparts

May 20 (Fri) at 16:00 - 17:00, 2022

Yannis Liodakis (Postdoctoral Researcher, University of Turku, Finland)

The origin of high-energy neutrinos is fundamental to our understanding of the Universe. Apart from the technical challenges of operating detectors deep below ice, oceans, and lakes, the phenomenological challenges are even greater. The sources are unknown, unpredictable, and we lack clear signatures. Neutrino astronomy therefore represents the greatest challenge faced by the astronomy and physics communities thus far. The possible neutrino sources range from accretion disks and tidal disruption events, through relativistic jets to galaxy clusters with blazar TXS 0506+056 the most compelling association thus far. Since then, immense effort has been put into associating AGN-jets with high-energy neutrinos, but to no avail. I will discuss our current efforts in understanding the multimessenger processes in the Universe, and once and for all proving or disproving if AGN-jets are neutrino emitters.

Venue: via Zoom

Event Official Language: English

Deviations from the Standard Model Predictions and New Physics Interpretations

May 20 (Fri) at 13:30 - 15:00, 2022

Teppei Kitahara (Assistant Professor, Kobayashi-Maskawa Institute for the Origin of Particles and the Universe (KMI), Nagoya University)

Continuous development of experiments in recent years has revealed a large number of experimental anomalies which the Standard Model cannot explain. It is statistically obvious that as the number of experiments increases, one encounters a new anomaly due to the statistical fluctuation. But interestingly, some of the anomalies have been cross-checked by different experiments. These would be hints for physics beyond the Standard model. In this talk, I will review the flavor anomalies (also known as lepton flavor universality violation), the muon g-2 anomaly, and recently measured the W boson mass anomaly. I will also discuss these implications for the new physics, and introduce several of our works.

Venue: Hybrid Format (Common Room 246-248 and Zoom)

Event Official Language: English

More Data, More Problems: Big Data in Correlative Ecology

May 19 (Thu) at 16:00 - 17:00, 2022

Dan Warren (Staff Scientist, Biodiversity and Biocomplexity Unit, Okinawa Institute of Science and Technology Graduate University (OIST))

The rapidly expanding pool of large data sets on species distributions, community composition, and environmental factors has been accompanied by an increasing number of methodological approaches to analyze this data. If done correctly, this represents an unprecedented opportunity for understanding ecological processes at large scales. However, it also represents an opportunity to be wrong about those same processes at a scale that was previously not possible. In this talk, I will use examples from ecology and other fields to discuss some of the issues that arise when we take big data approaches to ecological questions.

Venue: via Zoom

Event Official Language: English

Recent Progress in the Swampland Program

May 19 (Thu) at 14:00 - 15:30, 2022

Toshifumi Noumi (Associate Professor, Institute of Cosmophysics, Department of Physics, Graduate School of Science, Kobe University)

In the past years, it has become increasingly clear that there exist nontrivial consistency conditions on symmetries in quantum gravity, that are invisible in classical gravity. The Swampland program aims at identifying such quantum gravity constraints and their implications for particle physics and cosmology, toward quantum gravity phenomenology. In this talk, I will review recent progress in this program, including my own works.

Venue: Hybrid Format (Common Room 246-248 and Zoom)

Event Official Language: English

Khovanov homology theory - an introduction to categorification

May 13 (Fri) at 14:00 - 16:30, 2022

Taketo Sano (Special Postdoctoral Researcher, iTHEMS)

Jones polynomial is a knot invariant discovered by V. F. R. Jones in 1984. Not only that it is a useful mathematical tool, the discovery led to opening up a new research area, quantum topology, which connects quantum mechanics and low-dimensional topology. In 2000, M. Khovanov introduced a “categorification of the Jones polynomial”, which is now called Khovanov homology, and made categorification one of the fundamental concept in knot theory. Now what does categorification mean, and what is it good for? In this talk, assuming that many of the audience are not familiar with abstract category theory, I will start from easy examples of categories and categorifications, for example categorification of natural numbers, and explain why they are something natural to think of. In the latter part, I will briefly explain the construction of Khovanov homology, and introduce several related topics.

Venue: Hybrid Format (Common Room 246-248 and Zoom)

Event Official Language: English

Classical and Quantum Chaos

May 12 (Thu) at 16:00 - 17:00, 2022

Akira Shudo (Professor, Department of Physics, Graduate School of Science, Tokyo Metropolitan University)

Classical and quantum mechanics in multi-dimensions are qualitatively different from those in one-dimension since they are no more integrable in general and chaos appears in the dynamics. This brings a great deal of complexity or even richness both in classical and quantum dynamics. Especially in generic nonintegrable systems which are neither completely integrable nor fully chaotic, phase space becomes a mixture of regular and chaotic components. Such an aspect is a source of inexhaustible questions not only in the past but in the future. We here overview classical and quantum chaos in Hamiltonian systems.

Venue: via Zoom

Event Official Language: English

Introduction to Topological Insulators: Topological Superconductors and Quantum Computing

May 9 (Mon) at 14:00 - 15:30, 2022

Ching-Kai Chiu (Senior Research Scientist, iTHEMS)

Venue: via Zoom

Event Official Language: English

Diversity of Asgardarchaota and Theoretical verification of the endosymbiotic theory

April 28 (Thu) at 10:00 - 11:00, 2022

Daiki Kumakura (Ph.D. Student, Graduate School of Life Science, Hokkaido University)

How did intracellular symbiosis occur and give rise to eukaryotic ancestor? This question has been considered to the two theories as three-domain theory and eocyte theory. Here I present asgard archaea, the archaeon closest to eukaryotes. Asgard archaea is an archaeon found at a deep-sea sampling site called Loki's castle at between Greenland and Norway. So all the closely related species are named after Norse mythology (Loki-, Thor-, Odin-, Heimdall-, etc.). Unlike other archaea, asgard archaea has many eukaryotic-specific proteins and is considered to be the closest to eukaryotes. In 2020, one of the asgard archaea species was finally successfully cultured. This archaeon was cultured and found to take on a branch-like structure. It is then hypothesized that intracellular symbiosis between this archaeon and the ancestor of mitochondria resulted in the ancestor of today's eukaryotic cells. In this talk, I would like to discuss with you the explanation of how we arrived at this hypothesis and how to construct a mathematical model.

Venue: via Zoom

Event Official Language: English

Recurrence theorems for topological Markov chains

April 22 (Fri) at 17:00 - 19:00, 2022

Cédric Ho Thanh (Postdoctoral Researcher, Prediction Science Laboratory, RIKEN Cluster for Pioneering Research (CPR))

Recurrence theorems place conditions under which probabilistic systems, specifically Markov chains, are expected to visit certain states infinitely often. For example, a printer with its many moving parts and the random requests it receives, may be described as a probabilistic system, and recurrence of the "ready to print" state is desirable. Recurrence theorems in the case of finite Markov chains are widely known. In this talk, we are interested in generalization to the infinitary setting. As it turns out, some care has to be put in the definition of infinite Markov chains. Rather than simply infinite, the introduct topological Markov chains, and show how standard constructions can be naturally extended to thisframework: path spaces, cylinder sets, as well as the semantic of LTL and PCTL. With all these tools in hand, we finally state our recurrence theorems. This is work in progress in collaboration with Natsuki Urabe and Ichiro Hasuo. This seminar is hold in a hybrid style. If you want attend the seminar onsite, please contact to Keita Mikami.

Venue: Hybrid Format (Common Room 246-248 and Zoom) (Main Venue)

Event Official Language: English

Coherent emission from 3D relativistic shocks

April 22 (Fri) at 14:00 - 15:00, 2022

Masanori Iwamoto (Kyushu University)

The origin of fast radio bursts (FRBs; Lorimer et al. 2007) is one of the unsolved problems in astrophysics. Many observations of FRBs indicate that FRBs must be coherent emission in the sense that coherently moving electrons radiate electromagnetic waves. In relativistic shocks, it is well known that coherent electromagnetic waves are excited by synchrotron maser instability (SMI) in the shock transition (Hoshino & Arons 1991). The SMI is also known as the emission mechanism of coherent radio sources such as auroral kilometric radiation at Earth and Jovian decametric radiation. Recently, some models of fast radio burst based on the coherent emission from relativistic shock via the SMI have been proposed (e.g., Lyubarsky 2014; Beloborodov 2017; Plotnikov & Sironi 2019; Metzger et al. 2019) and the SMI in the context of relativistic shocks attracts more attention from astrophysics. In this study, by performing the world’s first three-dimensional (3D) particle-in-cell (PIC) simulation of relativistic shocks, we will demonstrate that large-amplitude electromagnetic waves are indeed excited by the SMI even in 3D and that the wave amplitude is significantly amplified and comparable to that in pair plasmas due to a positive feedback process associated with ion-electron coupling. Based on the simulation results, we will discuss the applicability of the SMI for FRBs in this talk.

Venue: via Zoom

Event Official Language: English

Neurons are potential statisticians

April 21 (Thu) at 10:00 - 11:00, 2022

Takuya Isomura (Unit Leader, Brain Intelligence Theory Unit, RIKEN Center for Brain Science (CBS))

Humans and animals can predict what will happen in the future and act appropriately by inferring how the sensory inputs were generated from underlying hidden causes. The free-energy principle is a theory of the brain that can explain how these processes occur in a unified way. However, how the fundamental units of the brain, such as the neurons and synapses, implement this principle has yet to be fully established. Here, we have mathematically shown that neural networks that minimise a cost function implicitly follow the free-energy principle and actively perform statistical inference. We have reconstructed a biologically plausible cost function for neural networks based on the equation of neural activity and shown that the reconstructed cost function is identical to variational free energy, which is the cost function of the free-energy principle. This equivalence speaks to the free-energy principle as a universal characterisation of neural networks, implying that even at the level of the neurons and synapses, the neural networks can autonomously infer the underlying causes from the observed data, just as a statistician would. The proposed theory will advance our understanding of the neuronal basis of the free-energy principle, leading to future applications in the early diagnosis and treatment of psychiatric disorders, and in the development of brain-inspired artificial intelligence that can learn like humans.

Venue: via Zoom

Event Official Language: English

Light-matter control of quantum materials: From light-induced superconductivity to cavity materials

April 20 (Wed) at 15:30 - 17:00, 2022

Michael Sentef (Emmy Noether Research Group Leader, Max Planck Institute for the Structure and Dynamics of Matter, Germany)

In this talk I will discuss recent progress in controlling and inducing materials properties with light [1]. Specifically I will discuss recent experiments showing light-induced superconductivity through phonon driving in an organic kappa salt [2] and its possible theoretical explanation via dynamical Hubbard U [3]. I will then highlight some recent theoretical and experimental progress in cavity quantum materials [4], where the classical laser as a driving field of light-induced properties is replaced by quantum fluctuations of light in confined geometries. Ideas and open questions for future work will be outlined.

Venue: via Zoom

Event Official Language: English

Coarse-grained molecular dynamics simulation via Langevin simulation

April 14 (Thu) at 10:00 - 11:00, 2022

Hiroshi Yokota (Postdoctoral Researcher, iTHEMS)

In the cell biology or biophysics, many mechanical properties of proteins or DNA are discussed. In order to consider the dynamics, coarse-grained molecular dynamics simulation (Langevin simulation) is useful. In this seminar, I will give you the introductory and methodology talk about the Langevin simulation.

Venue: via Zoom

Event Official Language: English

iTHEMS - Kyoto University Joint Seminar: Single-trajectory map equation

April 1 (Fri) at 8:30 - 10:00, 2022

Tatsuro Kawamoto (Researcher, Artificial Intelligence Research Center, National Institute of Advanced Industrial Science and Technology (AIST))

This seminar is a joint seminar of Blockchain research group in Kyoto University and EcoP WG in iTHEMS.

Venue: via Zoom

Event Official Language: English

Journal Club: Phase separation in a many-component system with random interactions

March 31 (Thu) at 10:00 - 11:00, 2022

Kyosuke Adachi (Special Postdoctoral Researcher, Nonequilibrium Physics of Living Matter RIKEN Hakubi Research Team, RIKEN Center for Biosystems Dynamics Research (BDR))

Several kinds of protein condensates have been observed in living cells, and the liquid-liquid phase separation is regarded as a basic mechanism of the condensate formation. However, given that there are thousands of protein species in a cell, it is not clear how the number and the composition of distinct condensates are controlled. One of the physics approaches to this problem is considering a model of many components with random interactions. In this Journal Club, I will introduce a recent paper [1] that applies random-matrix theory to the phase separation dynamics.

Venue: via Zoom

Event Official Language: English

Hydrodynamic theory of electron and spin transport

March 30 (Wed) at 13:30 - 15:00, 2022

Gen Tatara (Team Leader, Spin Physics Theory Research Team, RIKEN Center for Emergent Matter Science (CEMS))

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.

Venue: via Zoom

Event Official Language: English

Local and global topology for Dirac points with multi-helicoid surface states

March 24 (Thu) at 17:00 - 18:15, 2022

Tiantian Zhang (Specially Appointed Assistant Professor, School of Science, Tokyo Institute of Technology)

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.

Venue: via Zoom

Event Official Language: English

Criticality in stochastic SIR model for infectious diseases based on path-integral approach

March 24 (Thu) at 10:00 - 11:00, 2022

Shigehiro Yasui (Assistant Professor, Center of Medical Information Science, Kochi Medical School)

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.

Venue: via Zoom

Event Official Language: English