# Coffee Meeting Log

2023-03-24

## The introduce of twisted bilayer graphene

Congcong Le (Postdoctoral Researcher, iTHEMS)

Graphene, a two-dimensional material possessing Dirac points, is recognized as a topological semimetal. Twisted bilayer graphene, a configuration produced by rotating two layers of graphene relative to each other, can host the energy flat bands contributed by Dirac points. These flat bands serve as a fruitful playground for investigating strong correlations and topological phases, and hence the study of twisted bilayer graphene has brought excitement and impacts to the condensed matter physics community. In this coffee meeting, I will present the introduction of twisted bilayer gaphene.

2023-03-10

## Theoretical analysis of supernova neutrino data

Akira Harada (Special Postdoctoral Researcher, iTHEMS)

A supernova explosion is the explosive death of a massive star. Neutrino observation is important to examine the mechanism. However, a theoretical framework to analyze the observed neutrinos has not been established. In order to make a tractable theory, we constructed an analytic neutrino signal model based on the Lane-Emden equation and thermal neutrino emission. Besides, to bridge the theory and observation, I developed a code, "SPECIAL BLEND," to estimate the mass and radius of the central proto-neutron star by fitting the analytic model to the supernova neutrino observation with the Bayesian approach. Using SPECIAL BLEND, I analyzed the neutrinos from the supernova SN1987A and estimated the mass and radius of the central neutron star.

2023-03-03

## Gravitational methods for condensed matter and a curious puzzle for superfluid effective field theory

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

At the beginning of my PhD in theoretical gravitational physics, my advisor gave me a copy of Kittel's book "Introduction to solid state theory" to study. I was confused, very confused! After almost ten years of research, I have fully embraced the power of the gauge-gravity duality as a tool to investigate open questions in different fields such as condensed matter or nuclear theory. After briefly reviewing the most salient features and successes of this program, I will share with you an interesting puzzle for the effective field theory of superfluids that I recently became aware of. Maybe, we can find a solution to it together!

2023-02-24

## Physical and local observables in gravitational theory

Christophe Goeller (Humboldt Fellow, Ludwig-Maximilians-Universität München, Germany)

I will discuss a general formalism for the construction of dynamical reference frames and local and physical observables in gauge and gravitational theories and their interactions with the gauge charges and their associated field transformations.

2023-02-17

## Can you hear the shape of a drum?

Christopher Bourne (Visiting Scientist, iTHEMS / Assistant Professor, Advanced Institute for Materials Research (AIMR), Tohoku University)

I will review what it means to 'hear the shape of a drum' and how it offers a glimpse into the interesting ways that analysis and geometry interact.

2023-02-10

## Generative models and stochastic differential equations

Akinori Tanaka (Senior Research Scientist, iTHEMS / Senior Research Scientist, RIKEN Center for Advanced Intelligence Project (AIP))

Generative models, a.k.a statistical models, are widely accepted frameworks for generating “new” samples from given dataset. Recently, a class of generative models, called diffusion models, made great impact on not only researcher’s community but communities on SNS and industries also. I would like to introduce these developments within 15 min. on the blackboard.

2023-02-03

## Quantum decoherence and the Caldeira-Leggett model

Takeru Yokota (Special Postdoctoral Researcher, iTHEMS / Postdoctoral Researcher, Institute for Solid State Physics, The University of Tokyo)

Quantum decoherence is one of the fundamental phenomena of quantum systems caused by coupling to an environment. Such a phenomenon is theoretically analyzed by, for example, the Caldeira-Leggett model, which describes a quantum particle coupled to an environment represented as a collection of harmonic oscillators, and is also investigated by various experiments including cavity QED systems and quantum circuits. In this talk, I will briefly introduce these topics and may refer to our recent related work [Takeru Yokota, Kanta Masuki, Yuto Ashida, arXiv:2208.14107].

2023-01-27

## Quantum many-body physics with cold atoms

Yuta Sekino (Postdoctoral Researcher, iTHEMS / Postdoctoral Researcher, Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research (CPR))

Physical systems consisting of many quantum particles are ubiquitous in diverse fields of physics. Such systems called quantum many-body systems include solids surrounding us, superconductors used for quantum computations, and neutron matter in neutron stars. Understanding of such quantum many-body systems is, however, often challenging. Among various systems, cold atoms, which are atomic gases below 10^-6 kelvin, are ideal platforms for investigating quantum many-body physics because of their simplicity and high controllability. In this talk, I explain how studies of cold atoms have contributed to deepen our understanding of quantum many-body systems.

2023-01-20

## complexity in non-unitary dynamics of photons

Ken Mochizuki (Hamazaki Hakubi Lab.)

Sampling probability distributions of photons in optical networks can be hard for classical computers. Therefore, the boson sampling problem has been extensively explored from the viewpoint of the quantum supremacy. However, sampling photon distributions is not always hard and physical situations determine the hardness, which suggests that the computational complexity can be used to characterize quantum dynamics and quantum phases. I discuss the distinction between unitary and non-unitary dynamics from such a point of view, where the former and later respectively correspond to isolated and open quantum systems.

2023-01-13

## How can we describe the hierarchical structure of our Universe?

Nagisa Hiroshima (Visiting Scientist, iTHEMS / Assistant Professor, Department of Physics, Faculty of Science, University of Toyama)

Our Universe exhibits a highly hierarchical structure. Halos, which are gravitationally bounded objects of dark matter, are building blocks of such structures. Halos are distributed in more than 20 orders of magnitudes in the mass scale. For example, our Milky Way resides in a halo of which mass is about a trillion solar masses. Indications about the nature of dark matter could be obtained by studying the hierarchical structures of dark matter halos while it needs considerations about the schemes to cover a wide mass range. In this talk, I will introduce a simple framework to describe the structure based on analytical descriptions.

2022-12-23

## Physics Through the Looking Glass

Puttarak Jai-akson (Postdoctoral Researcher, iTHEMS)

The terminology Carrollian has recently shown up in many fields of theoretical physics. In this short talk, I will explain what Carrollian means and why some physicists find it interesting.

2022-12-09

## Supernova signatures of neutrino mass ordering

Maria Manuela Saez (Postdoctoral Researcher, iTHEMS)

Supernovae events are one the most powerful cosmic sources of neutrinos with energies of several MeV. The emission of neutrinos and antineutrinos of all flavors carries away the gravitational binding energy of the compact remnant and drives its evolution from the hot initial to the cold final state. I will briefly describe how to detect these neutrinos from Earth, and how to use these data to address the neutrino mass ordering problem.

2022-12-02

## Quantum many body treatment of collective neutrino flavor oscillations

Ermal Rrapaj (Postdoctoral Researcher, iTHEMS)

Collective flavor oscillations occur in extreme environments like supernovae and binary neutron star mergers where many neutrinos are produced in a short period of time. I will describe the quantum many body treatment and how it effects the evolution.

2022-11-25

## Counting by Drawing

Michel Van Garrel (Assistant Professor, School of Mathematics, University of Birmingham, UK)

How many other circles can you find that meet each of the 3 circles in exactly 1 point? I will talk about fun questions like this one that are solved by drawing, and have been with mathematics all along.

2022-11-18

## Knot theory and its interactions with other fields

Masaki Taniguchi (Special Postdoctoral Researcher, iTHEMS)

Knot theory is one of subjects in the field of topology. I'll exlain what are purposes of knot theory and its interactions with other fields.

2022-11-11

## New approach to spontaneous symmetry breaking by gradient flow

Kengo Kikuchi (Special Postdoctoral Researcher, iTHEMS)

The spontaneous symmetry breaking (SSB) is one of the most important concept in the elementary particle physics. In this coffee meeting, I explain the fundamental content of the SSB briefly, and after that, I talk about our recently research, new approach to analyze the phase structure of SSB using the gradient flow method.

2022-11-04

## Waveform analysis of biological oscillatory models

Shingo Gibo (Postdoctoral Researcher, iTHEMS)

In biological systems, many oscillatory phenomena emerge. For example, our sleep-wake rhythms are regulated by gene activity oscillation with a period of 24 hours. Time series of these biological oscillations are of various shapes. In this talk, I would like to talk about the effect of the waveform on period stability and synchronization.

2022-10-28

## A new B.O.A.T. in astrophysics

Don Warren (Research Scientist, iTHEMS / Research Scientist, Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research (CPR))

On October 9 of this year, an extremely bright gamma-ray burst was detected — it has been called the "brightest of all time". I will briefly describe some of the ways this burst was so extraordinary, and what science we can do with such an unusual opportunity.

2022-10-21

## Geometirical chracteristics of a polymer chain: twist and writhe

Hiroshi Yokota (Postdoctoral Researcher, iTHEMS)

Polymer is a string-like molecules composed of molecular units. Many physical phenomena on polymers are described by using consecutive beads connected by springs (bead-spring model). Although this model is widely used, sometimes this model is not sufficient when the twist or writhe structures are considered. In this talk, I would like to introduce the kink structure (twist and writhe) and its mathematical and physical description. And then, I would like to talk about the computational treatment of twist and writhe.

2022-10-14

## Evolution and our daily rhythms

Gen Kurosawa (Senior Research Scientist, iTHEMS)

Imagine that you are in a room without information of time. The room is in a cave so that temperature and light-intensity are constant over time. Can you wake up tomorrow or day after tomorrow? In fact, most humans can wake up tomorrow and day after tomorrow almost regularly. It is because we have daily rhythms in our body. Biological experiments have shown that not only humans but also other many species on the Earth have these daily rhythms. In this talk, unsolved problems about the rhythms, and some approaches from the point of view of dynamical system will be introduced.