Coffee Meeting Log

2021-07-09

Von Neumann algebras and their projections

Michiya Mori (Special Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

The theory of von Neumann algebras began in a work by von Neumann in 1929. A von Neumann algebra is widely regarded as a noncommutative measure space. I am particular interested in the structure of projections of a von Neumann algebra, which play the role of measurable sets in measure theory. In this talk I will invite the audience to the mysterious world of the geometry of noncommutative measurable sets.

2021-07-02

Exact WKB and its applications to physics

Hidetoshi Taya (Special Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

I explain the exact WKB method, which is a method to solve ordinary differential equations and has been developed (mainly) in mathematics since 1980's. I also discuss its applications to physics, including nonperturbative particle production by strong fields, and mention open problems in this field.

2021-06-25

The wave operator and the Oppenheim conjecture

Kazuki Kannaka (Special Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

I am a new member of iTHEMS and working on spectral analysis of a Lorentzian manifold in mathematics.In this talk, we will discuss the eigenvalue distribution of a wave operator on flat tori as the simplest case. Through the Oppenheim conjecture about the distribution of values at integer points of indefinite quadratic forms, I would like to explain mathematically interesting aspects of spectral analysis of a wave operator. I don't know the physical meaning of this study, but I hope this talk will be an opportunity to discuss it.

2021-06-18

Phase transtion' in Kahler geometry

Eiji Inoue (Special Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

I am a new member of RIKEN iTHEMS and working on Kahler geometry in mathematics. As I cannot overview my study in this short time, I would like to explain a strange phenomenon now I am facing in Kahler geometry with a specific example. It is reminiscent of `phase transition' or `spontaneous symmetry breaking'. I hope it makes a chance of discussion with researchers in other areas.

2021-06-11

The death of a star: the theory of the supernova explosion

Akira Harada (Special Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

At the end of the stellar evolution, the star explodes as bright as a galaxy and leaves a neutron star. This explosion is known as the supernova. Understanding the supernova explosion mechanism is crucial because it plays a vital role in the stellar and cosmic matter evolutions. The explosion mechanism includes a variety of physical processes, and hence it is a challenging problem. In this coffee talk, I would like to introduce the basic features of supernovae and the current understanding of the explosion mechanism.

2021-06-04

Canonical form of tensor networks in one and two dimensions

Yantao Wu (Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

I will be a new member of RIKEN iTHEMS and work on computational study in condensed matter physics. In this short coffee meeting, I would like to discuss the basics of tensor networks. I will be pedagogical and not assume knowledge of quantum mechanics. I will assume basic linear algebra at the level of singular value decomposition. I will introduce the graphical notation of a tensor network, and the notion of a canonical form in one dimension. If time permits, I will also discuss the canonical form in two dimensions.

2021-05-28

The basics of topological band theory

Congcong Le (Post-doctoral research scientist, Max Planck Institute for Chemical Physics of Solids, Germany)

I will be a new member of RIKEN iTHEMS and work on theoretical study in condensed matter.  In this short coffee meeting, I would like to disuss the topology properties of photocurrent. Firstly, based on band theory, I will discuss the topological classification of crystalline materials. Secondly,  Similar to direct current formular  I=U/R, the photocurrents will be discussed. Finally, I will discuss the topology properties of photocurrent in the various topological semimetals.

2021-05-21

Cosmology 101 - expanding universe and inflationary cosmology

Ryo Namba (Senior Research Scientist, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

I am a new member of RIKEN iTHEMS and working on several aspects of theoretical cosmology. As a partial self-introduction and introduction to the field, I would like to discuss some basics of cosmology, making it accessible by the iTHEMS members in different areas of expertise. In this short coffee meeting talk, my main focus is the evolution of the universe and the physics during its earliest period.

2021-05-14

Reducing complexity and pursuing reality in computational biophysics

Yuji Sugita (Senior Research Scientist, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS) / Chief Scientist, Theoretical Molecular Science Laboratory, RIKEN Cluster for Pioneering Research (CPR))

The basic unit of all the living matters is a cell, where huge number of biomolecules exist and express their biological functions. When we aim to study biological systems in computational biophysics, we always encounter many problems originated from the complexity of the systems. Reducing complexity has been one of the most essential issues in computational biophysics, while too much simplifications may lose the reality of biological phenomena. I would like to discuss several key mathematical concepts that have been used in computational biophysics for reducing complexity and pursuing reality of biology in the cell.

2021-04-30

Computational phases of quantum matter

Shunji Matsuura (Visiting Scientist, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS) / Fundamental Researcher, Quantum Simulation Division, 1QBit, Canada)

I will talked about a connection between measurement-based quantum computing and quantum phases in physics.

2021-04-23

Building something from nothing: a mathematical framework for aperiodic/amorphous sets

Christopher Bourne (Visiting Scientist, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS) / Assistant Professor, Advanced Institute for Materials Research (AIMR), Tohoku University)

I will introduce a mathematical construction due to Bellissard that allows us to study disordered materials via Delone sets.

2021-04-16

Density Functional Theory from Functional Renormalization Group

Takeru Yokota (Special Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS) / Postdoctoral Researcher, The Institute for Solid State Physics (ISSP), The University of Tokyo)

In this talk, I will give a brief introduction of density functional theory. I will also talk about our recent attempt to improve density functional theory for electron systems by use of the functional renormalization group.

2021-04-02

Spin transport in ultracold atomic superfluids

Yuta Sekino (Visiting Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

I will briefly introduce transport phenomena of spin in ultracold atomic gases in a superfluid phase

2021-03-26

Hunting the standard candles in their own nest

Maria Giovanna Dainotti (NAOJ/Space Science Institute)

I will introduce briefly the concept of Standard candles and how Gamma-Ray Bursts can be used as standardized candles through a new three-dimensional relation among important features of GRBs and how the kilonovae events associated with GRBs form a plane can naturally form a different plane with a much smaller dispersion around the plane.

2021-03-19

Sensitivity and robustness of chemical reaction network systems

Takashi Okada (Senior Research Scientist, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

In living cellls, many reactions occur and form complex networks, such as metabolic networks. I will discuss how biological functions can emerge from the complex network dynamics, from network topology perspectives.

2021-03-12

Recent progress in black hole information paradox

Kanato Goto (Special Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

2021-02-26

Fermionic transport under gravity

Kazuya Mameda (QHP)

I will briefly talk about transport phenomena induced by a background gravitational field.

2021-02-19

information scrambling is growth of wormwhole

Masahiro Nozaki (Special Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

I will explain what is the information scrambling, why it is important, and how it is related to the growth of wormhole.

2021-02-12

Active and Passive Phase Separation in a Lattice Model

Kyosuke Adachi (Special Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS) / Special Postdoctoral Researcher, Nonequilibrium Physics of Living Matter RIKEN Hakubi Research Team, RIKEN Center for Biosystems Dynamics Research (BDR))

Phase separation is spontaneous segregation into high-density and low-density phases, observed in a variety of systems, e.g., alloy, polymer solution, and intracellular cytoplasm. In this talk, I will review the phase separation phenomena and introduce a model that connects the equilibrium phase separation to another class of separation driven by motility.

2021-02-05

Toward the standard model of elementary particles -- a brief history in 15 min. --

Tetsuo Hatsuda (Program Director, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

I will make a very brief overview on the path toward the modern understanding on the law(s) of fundamental particles.