# Seminar

## Geometry of hyperkahler 4 manifolds

October 22 at 13:00 - 15:00, 2021

Prof. Song Sun (Associate Professor, Department of Mathematics, University of California, Berkeley, USA)

An n dimensional Riemannian metric g defines a holonomy group, which is a subgroup of SO(n) given by parallel transport along all contractible loops (with respect to the Levi-Civita connection). According to the Berger classification we know that if a complete Riemannian metric is not locally symmetric and not locally reducible then its holonomy group is either the entire SO(n) (generic case), or U(n) (Kahler), or is special and belongs to a small list. Riemannian metrics with special holonomy are very interesting geometric objects to study, with many connections to analysis and physics. The simplest model is given by a 4 dimensional hyperkahler metric. We will explain the general background and discuss recent progress on understanding the geometry of hyperkahler 4 manifolds. *Please contact Keita Mikami's mail address to get access to the Zoom meeting room.

Venue: via Zoom

Event Official Language: English

## Evaluation of origin of driving force for loop formation in a chromatin fiber

October 21 at 10:00 - 11:00, 2021

Dr. Hiroshi Yokota (Postdoctoral Researcher, iTHEMS)

During cell division, chromatin fiber is condensed into the rod-like shape which is called chromosome. The rod-like shape of the chromosome is constructed by consecutive chromatin loop structures which are formed by the protein complex named condensin. In this talk, by calculating the driving force for the loop formation, we discuss the mechanism of loop formation which is the one of the controversial issues on chromosome condensation. The driving force is evaluated based on the free energy of the chromatin loop formation by constructing the polymer model. Based on the free energy, the loop growth length in the unit time is also evaluated. These evaluations also lead to the time evolution of the loop length and the mechanism of the loop formation.

Venue: via Zoom

Event Official Language: English

## Nonlinear response in strongly correlated systems

October 20 at 17:00 - 18:15, 2021

Dr. Robert Peters (Lecturer, Department of Physics, Graduate School of Science, Kyoto University)

Nonlinear responses in condensed matter are intensively studied because they provide rich information about materials and hold the possibility of being applied in diodes or high-frequency optical devices [1-4]. While nonlinear responses in noninteracting models have been explored widely, the effect of strong correlations on the nonlinear response is still poorly understood. This talk will introduce a Green's function method to calculate nonlinear conductivities in strongly correlated materials [5-6]. Correlation effects are thereby included by the self-energy of the material. I will then use this method to study the nonlinear conductivities in noncentrosymmetric f-electron systems. The first system is a heavy Fermion system, where a nonreciprocal conductivity appears in the ferromagnetic phase. The nonreciprocal conductivity thereby always occurs perpendicular to the magnetization of the system and has a strong spin dependence, which might be advantageous for spintronic applications. The second system is a model corresponding to the Weyl-Kondo semimetal Ce3Bi4Pd3, in which a giant spontaneous Hall effect without time-reversal symmetry breaking has been observed [7]. This Hall effect can be explained as a nonlinear Hall effect in an inversion-symmetry broken Weyl-semimetal. It has been shown that the nonlinear Hall effect is related to the Berry curvature dipole [4]. Our study shows that the magnitude of the experimentally observed nonlinear Hall effect can be explained by the strong correlations inherent in this f-electron material [8]. *Detailed information about the seminar refer to the email.

Venue: via Zoom

Event Official Language: English

## Floquet vacuum engineering: laser-driven chiral soliton lattice in the QCD vacuum

October 20 at 13:30 - 15:00, 2021

Mr. Akihiro Yamada (Master's Student, School of Fundamental Science and Technology, Graduate School of Science and Technology, Keio University)

What happens to the QCD vacuum when a time-periodic circularly polarized laser field with a sufficiently large intensity and frequency is applied? Based on the Floquet formalism for periodically driven systems and the systematic low-energy effective theory of QCD, we show that for a sufficiently large frequency and above a critical intensity, the QCD vacuum is unstable against the chiral soliton lattice of pions, a crystalline structure of topological solitons that spontaneously breaks parity and continuous translational symmetries. In the chiral limit, in particular, the QCD vacuum is found unstable by the laser with an arbitrary small intensity. Our work would pave the way for novel “Floquet vacuum engineering.”

Venue: via Zoom

Event Official Language: English

## Distinctive signals of boosted dark matter from semi-annihilations

October 20 at 10:00 - 11:30, 2021

Prof. Takashi Toma (Assistant Professor, Institute of Liberal Arts and Science, Kanazawa University)

The recent dark matter direct detection experiments impose the stringent upper bound on the elastic scattering cross section with nucleons. This implies that the cross section is suppressed by small dark matter velocity. However such dark matter can be probed if it is boosted by some mechanism. In this talk, we show that the specific semi-annihilation channel where two dark matter particles annihilate into a pair of anti-dark matter and neutrino indicates signals distinctive from the other semi-annihilation and standard dark matter annihilation processes. Since the boosted dark matter produced by this semi-annihilation is regarded as a high energy neutrino, the total flux of the dark matter and the accompanying neutrino yields double peaks at the energy close to the dark matter mass. Both of the particles can be detectable at large volume neutrino detectors.

Venue: via Zoom

Event Official Language: English

## Recent progress on the r-process in the era of gravitational-wave astronomy

October 15 at 16:00 - 18:00, 2021

Dr. Nobuya Nishimura (Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research (CPR))

The r-process, the rapid neutron-capture process, is a major origin of heavy nuclei beyond iron in the universe, occurring in explosive astrophysical phenomena with very neutron-rich environments. In the studies of r-process nucleosynthesis, there are several unsolved problems in nuclear physics and astrophysics. In this talk, I will briefly summarize recent progress on the studies of the r-process, mainly focusing on neutron star mergers. We will see that the scenario of neutron star mergers is consistent with several observations, e.g., GW170817 with a kilonova, chemical evolution of r-process elements. In addition, nevertheless, there are several remaining (or newly realized) problems on the origin of r-process elements in the universe. Focusing on our own research, I will introduce attempts to address these issues.

Venue: via Zoom

Event Official Language: English

## The branched deformations of special Lagrangian submanifolds

October 15 at 10:00 - 12:00, 2021

Prof. Siqi He (Research Assistant Professor, Simons Center for Geometry and Physics, Stony Brook University, USA)

Special Lagrangian submanifolds are a distinguished class of real calibrated submanifolds defined in a Calabi-Yau manifold, which are calibrated by the real part of the holomorphic volume form. Given a compact, smooth special Lagrangian submanifold, Mclean proved that the space of nearby special Lagrangian submanifolds of it could be parametrized by the harmonic 1-forms. In this talk, we will discuss some recent progress on generalizing Mclean’s result to the branched deformations. We will describe how to use multi-valued harmonic functions to construct branched nearby deformations. In the first one hour, we will introduce the background of special Lagrangian submanifold and explain the motivations to study this problem. In this second one hour, we will discuss the technical details and interesting new phenomenon in this branching deformation problem. *Please contact Keita Mikami's mail address to get access to the Zoom meeting room.

Venue: via Zoom

Event Official Language: English

## Understanding the effect of defective, interfering influenza virus

October 14 at 10:00 - 11:00, 2021

Prof. Catherine Beauchemin (Deputy Program Director, iTHEMS)

Defective interfering virus particles (DIPs) are viruses that are defective in a very specific way that allows them to out-compete standard, non-defective virus. It is difficult to count DIPs because they can look too similar to standard virus. So instead, people are counting them based on their effect on suppressing the standard virus population. In this talk, I will explain the basic biology of virus replication, what are DIPs, and how they compete with standard virus. I will present our mathematical model (ordinary differential equation) that describes co-infection competition with DIPs and standard virus. I will use the mathematical model to show how experiments to count DIPs can give incorrect results, and I will propose some solutions.

Venue: via Zoom

Event Official Language: English

## Symmetry-based analysis for unconventional superconductors: Diagnosis of topological and nodal superconductivity

October 12 at 16:00 - 17:15, 2021

Mr. Seishiro Ono (Department of Applied Physics, School of Engineering, The University of Tokyo)

The physics of unconventional superconductors has gained a new dimension in the past decade, thanks to the bloom in the understanding of topological quantum materials. Keeping in mind the success of the symmetry-based diagnosis in the large-scale discovery of topological insulator and semimetal candidates [1], it is natural to ask whether the approach can be generalized to superconducting systems. In this talk, I provide a unified way to diagnose topology and superconducting nodes in unconventional superconductors. First, I review symmetry-indicator theory for the topological insulators [2]. Also, I also discuss how to generalize the theory to superconductors [3,4,5]. Next, I show that the symmetry-based approach can extensively classify superconducting nodes pinned to high-symmetric momenta [6]. Finally, I show that these results enable us to derive the comprehensive correspondences between pairing symmetries and topological/nodal superconducting nature for each material [7]. *Detailed information about the seminar refer to the email.

Venue: via Zoom

Event Official Language: English

## Geography of varieties of general type

October 8 at 16:00 - 18:10, 2021

Prof. Chen Jiang (Associate Professor, Shanghai Math Center, Fudan University, China)

We study birational invariants in order to study birational classifications of varieties. Geography is the study of relations among different invariants. We will focus on two fundamental invariants: canonical volume and geometric genus. For surfaces there are classical results such as Miyaoka-Yau inequality and Noether inequality. I will discuss higher dimensional analogue of them, and introduce our recent work on the optimal Noehter inequality for 3-folds joint with Jungkai Chen and Meng Chen. *Please contact Keita Mikami's mail address to get access to the Zoom meeting room.

Venue: via Zoom

Event Official Language: English

## Wave function geometry and anomalous Landau levels of flat bands

October 7 at 16:00 - 17:15, 2021

Prof. Bohm-Jung Yang (Associate Professor, Department of Physics and Astronomy, Seoul National University, Republic of Korea)

Semiclassical quantization of electronic states under magnetic field describes not only the Landau level spectrum but also the geometric responses of metals under a magnetic field. However, it is unclear whether this semiclassical idea is valid in dispersionless flat-band systems, in which an infinite number of degenerate semiclassical orbits are allowed. In this talk, I am going to show that the semiclassical quantization rule breaks down for a class of flat bands including singular flat bands [1-5] and isolated flat bands [6]. The Landau levels of such a flat band develop in the empty region in which no electronic states exist in the absence of a magnetic field. The total energy spread of the Landau levels of flat bands is determined by the quantum geometry of the relevant Bloch states, which is characterized by their Hilbert–Schmidt quantum distance and fidelity tensors. The results indicate that flat band systems are promising platforms for the direct measurement of the quantum geometry of wavefunctions in condensed matter. *Detailed information about the seminar refer to the email.

Venue: via Zoom

Event Official Language: English

## Donaldson-Thomas invariants, wall-crossing and categorifications

October 1 at 16:00 - 18:10, 2021

Prof. Yukinobu Toda (Professor, Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), The University of Tokyo)

It is an important subject to study algebraic curves inside algebraic varieties, both in classical algebraic geometry and also enumerative geometry inspired by string theory. The Donaldson-Thomas theory is one of curve counting theories on Calabi-Yau 3-folds, and has developed in these 20 years from several aspects of mathematics and mathematical physics. Among them, the wall-crossing in derived category turned out to be a key phenomena in proving deep structures of generating series of Donaldson-Thomas invariants. In the first one hour, I will review the classical aspect of counting curves inside algebraic varieties, and explain how it leads to modern enumerative geometry such as Gromov-Witten invariants, Donaldson-Thomas invariants. In the second one hour, I will explain wall-crossing phenomena in Donaldson-Thomas theory, and its categorification in the case of the resolved conifold. *Please contact Keita Mikami's mail address to get access to the Zoom meeting room.

Venue: via Zoom

Event Official Language: English

## Evolutionary dynamics of seasonal influenza viruses

September 30 at 10:00 - 11:00, 2021

Dr. Takashi Okada (Senior Research Scientist, iTHEMS)

Seasonal influenza viruses undergo rapid evolution, which allows them to escape from human-immune-system responses and infect humans repeatedly. In this talk, I present some counter-intuitive properties observed in time-series data of viral sequence variation and then discuss how these strange properties can be explained by extending the standard framework of population genetics.

Venue: via Zoom

Event Official Language: English

## High-harmonic generation in strongly correlated systems

September 15 at 13:30 - 15:00, 2021

Prof. Yuta Murakami (Assistant Professor, School of Science, Tokyo Institute of Technology)

High-harmonic generation (HHG) is an intriguing nonlinear phenomenon induced by a strong electric field. It has been originally observed and studied in atomic and molecular gases, and is used in attosecond laser sources as well as spectroscopies. An observation of HHG in semiconductors expanded the scope of this field to condensed matters [1]. The HHG in condensed matters is attracting interests since it may be used as new laser sources and/or as powerful tools to detect band information such as the Berry curvatures. Recently, further exploration of the HHG in condensed matters are carried out in various other systems than semiconductors. In this talk, we introduce our recent theoretical efforts on the HHG in strongly correlated systems [2,3,4]. In contract to semiconductors, the charge carriers are not normal fermions, which makes HHG in strongly correlated systems unclear. Using the dynamical-mean field theory and the infinite time-evolving block decimation for the Hubbard model, we reveal the HHG features in the Mott insulators. Firstly, we reveal that the origin of the HHG in the Mott insulator is the recombination of doublons (doubly occupied sites) and holons (no electron site). Then, we show that the HHG feature qualitatively changes depending on the field strength due to the change of mobility of charge carriers, and discuss that the HHG directly reflects the dynamics of many body elemental excitations, which the single particle spectrum may miss. These results indicate that the HHG in Mott systems may be used as a spectroscopic tool for many body excitations. We also discuss the effects of spin dynamics on the HHG, which is a unique feature in strongly correlated systems.

Venue: via Zoom

Event Official Language: English

## S-matrix Unitarity toward UV Completion

September 13 at 13:30 - 15:00, 2021

Prof. Keisuke Izumi (Assistant Professor, Kobayashi-Maskawa Institute for the Origin of Particles and the Universe (KMI), Nagoya University)

Einstein gravity is not renormalizable and does not hold perturbative unitarity at high energy. This is the main reason why the construction of quantum gravity is difficult. A conjecture was proposed by Llewellyn Smith, "renormalizablility and tree-unitarity at high energy give the same conditions for theories". This conjecture would be important because it shows that, if a theory is constructed s.t. unitarity is satisfied, renormalizablility holds automatically, and vice versa. Unfortunately, a counterexample was pointed out. If a theory involves higher derivatives, there exists a theory which is renormalizable but does not satisfy tree-unitarity. A candidate of quantum gravity, the quadratic gravity (R_{\mu\nu}^2 gravity), is one of the examples. Therefore, Llewellyn Smith's conjecture would not be useful for the discussion of quantum gravity. Then, we introduce a new conjecture, "renormalizablility and S-matrix unitarity (or often called pseudo-unitarity) at high energy give the same conditions for theories". In this talk, Llewellyn Smith's conjecture and our contribution to it will be explained. Then, our new conjecture will be introduced. Finally, it will be shown that our conjecture works well even in theories with higher derivatives.

Venue: via Zoom

Event Official Language: English

## Towards a description of amorphous solids and viscoelastic materials using effective field theory and holographic methods

September 10 at 13:30 - 15:00, 2021

Prof. Matteo Baggioli (Associate Professor, School of Physics and Astronomy, Shanghai Jiao Tong University, China)

Among the most ubiquitous phases of matter, gases and crystalline solids are definitely the simplest to be described. Their physics is indeed almost entirely textbooks material and it can be summarized within the elegant frameworks of kinetic theory and Debye theory. Liquids and specially viscoelastic systems and amorphous materials (e.g. glasses) exhibit a much richer and complex dynamics with provides a large set of fundamental and unresolved physical questions. Given the tremendous microscopic complexity of these systems, which is manifest in a large landscape of scales and anomalous behaviours, the effective field theory (EFT) paradigm of isolating only a few, but fundamental, information could provide a winning approach. This talk is based on the simple, but indeed extremely difficult, question of whether these phases of matter can be distinguished, classified and understood using emergent and/or fundamental symmetry principles as in their ordered crystal counterpart. More precisely, we will combine EFTs, hydrodynamics and holographic methods to tackle the above question. I will present the most recent developments in this direction and I will discuss with you the most important open questions and avenues to explore in the near future.

Venue: via Zoom

Event Official Language: English

## How to make a dataset for phylogeny and the background of mathematical theory

September 9 at 10:00 - 11:00, 2021

Dr. Euki Yazaki (Postdoctoral Researcher, iTHEMS)

Molecular phylogenetic analysis is a very important method of analysis for understanding the evolution of organisms and so on. The method of molecular phylogenetic analysis itself is often discussed, and you are probably familiar with the background of the analysis. The dataset to be analyzed is just as important as the analysis method. However, it is not well known how the data set is made and what the methodology behind it is. Therefore, I will outline the background to the creation of data sets.

Venue: via Zoom

Event Official Language: English

## The Polarised ring of the Supermassive Black Hole in M87: EHT observations and theoretical modeling

September 3 at 14:00 - 16:00, 2021

Dr. Yosuke Mizuno (Tsung-Dao Lee Institute, Shanghai Jiao Tong University, China)

The Event Horizon Telescope has mapped the central compact radio source of the elliptical galaxy M87 at 1.3 mm with unprecedented angular resolution. These images show a prominent ring with a diameter of ~40 micro-arcsecond, consistent with the size and shape of the lensed photon orbit encircling the “shadow” of a supermassive black hole. Recently EHT has provided new images of the polarised emission around the central black hole in M87 on event-horizon scale. This polarised synchrotron emission probes the structure of magnetic fields and the plasma properties near the black hole. We found that the net azimuthal linear polarisation pattern may result from organised, poloidal magnetic fields in the emission region. In a quantitative comparison with a large simulated polarimetric image library, we found that magnetically arrested accretion disks are favoured to explain polarimetric EHT observations. In this talk, I also briefly discuss about a new modelling study of M87 jets in the collimation and acceleration region.

Venue: via Zoom

Event Official Language: English

## Journal Club: A quantitative quasispecies theory-based model of virus escape mutation under immune selection

September 2 at 10:00 - 11:00, 2021

Dr. Yingying Xu (Special Postdoctoral Researcher, iTHEMS)

I would like to introduce the paper "a quantitative quasispecies theory-based model of virus escape mutation under immune selection", written by Hyung-June Woo and Jaques Reifman [1]. Paper abstract: Viral infections involve a complex interplay of the immune response and escape mutation of the virus quasispecies inside a single host. Although fundamental aspects of such a balance of mutation and selection pressure have been established by the quasispecies theory decades ago, its implications have largely remained qualitative. Here, we present a quantitative approach to model the virus evolution under cytotoxic T-lymphocyte immune response. The virus quasispecies dynamics are explicitly represented by mutations in the combined sequence space of a set of epitopes within the viral genome. We stochastically simulated the growth of a viral population originating from a single wild-type founder virus and its recognition and clearance by the immune response, as well as the expansion of its genetic diversity. Applied to the immune escape of a simian immunodeficiency virus epitope, model predictions were quantitatively comparable to the experimental data. Within the model parameter space, we found two qualitatively different regimes of infectious disease pathogenesis, each representing alternative fates of the immune response: It can clear the infection in finite time or eventually be overwhelmed by viral growth and escape mutation. The latter regime exhibits the characteristic disease progression pattern of human immunodeficiency virus, while the former is bounded by maximum mutation rates that can be suppressed by the immune response. Our results demonstrate that, by explicitly representing epitope mutations and thus providing a genotype–phenotype map, the quasispecies theory can form the basis of a detailed sequence-specific model of real-world viral pathogens evolving under immune selection. *Please refer to the email to get access to the Zoom meeting room.

Venue: via Zoom

Event Official Language: English

## Extended and interacting bound states in elemental superconductors

September 1 at 16:00 - 17:15, 2021

Dr. Levente Rózsa (Condensed Matter Physics, University of Konstanz, Germany)

Combining magnetism with superconductivity leads to the emergence of localized states, including Majorana bound states predicted to be relevant for topological quantum computation. In this talk, I discuss how these bound states are influenced by the details of the electronic structure. It will be shown how the shape of the Fermi surface leads to a long-ranged anisotropic extension of Yu-Shiba-Rusinov states in the vicinity of magnetic impurities [1]. The same type of Fermi surface will be demonstrated to give rise to topologically trivial Caroli-de Gennes-Matricon bound states in vortex cores [2], with similar spatial profiles to those of topological Majorana bound states. The role of spin-orbit coupling will be discussed in the hybridization of Yu-Shiba-Rusinov bound states of dimers with ferromagnetic and antiferromagnetic spin alignments [3]. The general theoretical concepts will be illustrated by experimental realizations in specific materials. *Detailed information about the seminar refer to the email.

Venue: via Zoom

Event Official Language: English

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