NEW WG Seminar

We organize external speaker events (regular seminar, workshop, intensive lecture) on nonequilibrium physics. For more information, visit NEW WG website.

Seminar

NEW WG Seminar

A simple XY model for cascade transfer

January 20 at 13:30 - 15:00, 2022

Mr. Tomohiro Tanogami (Graduate School of Science, Kyoto University)

Cascade transfer is the phenomenon that an inviscid conserved quantity, such as energy or enstrophy, is transferred conservatively from large (small) to small (large) scales. As a consequence of this cascade transfer, the distribution of the transferred quantity obeys a universal scaling law independent of the details of large (small) scales. For example, in the energy cascade in fluid turbulence, the energy spectrum follows Kolmogorov's power law [1]. Such behavior is observed even in systems different from ordinary fluids, such as quantum fluid, elastic body, and spin systems. Here, we aim to establish the concept of a universality class for cascade transfer. As a first step toward this end, we propose a simple model representing one universality class [2]. In doing so, we regard cascade transfer as a cooperative phenomenon of unidirectional transport across scales and ask how it emerges from spatially local interactions. The constructed model is a modified XY model with amplitude fluctuations, in which the spin is regarded as the “velocity” of a turbulent field in d dimensions. We show that the model exhibits an inverse energy cascade with the non-Kolmogorov energy spectrum. We also discuss the relation to spin turbulence [3,4] and atmospheric turbulence [5].

Venue: via Zoom

Event Official Language: English

Seminar

NEW WG Seminar

Quantum metric of topological and non-topological insulators in AMO and other systems

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

Dr. Tomoki Ozawa (Associate Professor, Advanced Institute for Materials Research (AIMR), Tohoku University)

Recently, the concept of quantum geometry is attracting great interests in various areas of condensed matter and AMO physics. Quantum geometry tells how much the quantum states "change" as one moves in a parameter space, and is closely related to the topology of the quantum states. Quantum geometric tensor is often used to characterize the geometry, whose real part is the quantum metric and the imaginary part is the Berry curvature. Although Berry curvature is rather well-studied in the context of topological insulators and superconductors, less has been known about the quantum metric. However, experiments detecting the quantum metric have appeared in the past couple of years and interest in quantum metric is indeed growing. In this talk, I first explain basics of quantum metric and its recent experimental observations. I then discuss various aspects of quantum metric, including its relation to localization, topology, and the Kähler geometry.

Venue: via Zoom

Event Official Language: English

Seminar

NEW WG Seminar

Cosmological particle production as Stokes phenomena

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

Dr. Yusuke Yamada (JSPS Research Fellowship for Young Scientists, Research Center for the Early Universe (RESCEU), The University of Tokyo)

Particle production from “vacuum” takes place in time-dependent backgrounds. In very early universe, particularly just after inflation, expanding metric as well as oscillating scalar fields play the role of such backgrounds. Mathematically, “particle production from vacuum” can be understood as “Stokes phenomena”, and such understanding enables us to estimate the amount of produced particles in a systematic way. In this talk, I will review the relation between Stokes phenomena and particle production. Then, from the Stokes phenomena viewpoint, I will (re)consider particle production associated with expanding universe, an oscillating scalar field, or both of them. I will also discuss the time evolution of particle number, and its relation to the ambiguity of “vacuum states”.

Venue: via Zoom

Event Official Language: English

Seminar

NEW WG Seminar

Imaging Theory of Optical Microscopy: Basic to Super Resolution

November 25 at 13:30 - 15:00, 2021

Prof. Ryosuke Oketani (Assistant Professor, Physical Chemistry, Department of Chemistry, Faculty of Sciences, Kyushu University)

Optical microscopy is one of the sophisticated techniques to manipulate light based on well-established theories, as well as a powerful tool to observe living micro-organisms. The developments are still ongoing to overcome their limitations in observation. Recently, the invention of several super-resolution techniques has overcome the limit in spatial resolution caused by the wave nature of light. In this presentation, I discuss the theories behind optical microscopy. My talk starts with basic wave optics to explain how a lens forms and magnifies an image in a conventional microscope. Then, I introduce laser scanning microscopy as an alternative form to the microscope. At last, as a recent development, I discuss several super-resolution techniques, which utilize interesting theory to improve spatial resolution.

Venue: via Zoom

Event Official Language: English

Seminar

NEW WG Seminar

Geometry in optical responses of quantum materials

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

Prof. Naoto Nagaosa (Deputy Director, Group Director, Strong Correlation Theory Research Group, RIKEN Center for Emergent Matter Science (CEMS) / Professor, Department of Applied Physics, School of Engineering, The University of Tokyo)

Studies on optical responses of solids have the long history, and has been considered to be well established. However, a new development has been on-going recently, which explores the geometric nature of the electronic states in solids and its crucial role in optical processes. In this talk, I discuss the geometry and topology in the optical responses both in linear and nonlinear regimes, which includes (i) optical responses in clean superconductors, (ii) shift current in noncentrosymmetric quantum materials driven by Berry phases, and (iii) Riemannian geometry in nonlinear optical responses.

Venue: via Zoom

Event Official Language: English

Seminar

NEW WG Seminar

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

Seminar

NEW WG Seminar

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

Seminar

NEW WG Seminar

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