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

Seminar
Tricritical phenomena in holographic chiral transitions
November 29 (Tue) at 13:30  15:00, 2022
Dr. Masataka Matsumoto (Postdoctoral Researcher, Department of Physics, Shanghai University, China)
Tricritical point (TCP) is the endpoint of a line of threephase coexistence (a triple line) at which three coexisting phases simultaneously become identical. A TCP can be observed in various systems, for example, the QCD phase diagram with the chiral limit and a metamagnet such as a FeCl2 crystal. In the AdS/CFT correspondence, a TCP associated with a chiral phase transition has been found in the D3/D7 model [1]. In this talk, I will discuss the recent study [2] of critical phenomena at a tricritical point which emerges in the D3/D7 model in the presence of a finite baryon number density and an external magnetic field. We found all the critical exponents defined in this paper take the meanfield values. I will also compare the results with our previous works about the critical phenomena at the TCP that emerges in the steady state [3,4].
Venue: Hybrid Format (Common Room 246248 and Zoom)
Event Official Language: English

Seminar
Quantum nucleation of topological solitons
October 20 (Thu) at 13:30  15:00, 2022
Dr. Minoru Eto (Professor, Faculty of Science, Yamagata University)
The chiral soliton lattice is an array of topological solitons realized as ground states of QCD at finite density under strong magnetic fields or rapid rotation, and chiral magnets with an easyplane anisotropy. In such cases, topological solitons have negative energy due to topological terms originating from the chiral magnetic or vortical effect and the DzyaloshinskiiMoriya interaction, respectively. We study quantum nucleation of topological solitons in the vacuum through quantum tunneling in 2+1 and 3+1 dimensions, by using a complex ϕ4 (or the axion) model with a topological term proportional to an external field, which is a simplification of lowenergy theories of the above systems. In 2+1 dimensions, a pair of a vortex and an antivortex is connected by a linear soliton, while in 3+1 dimensions, a vortex is stringlike, a soliton is walllike, and a disk of a soliton wall is bounded by a string loop. Since the tension of solitons can be effectively negative due to the topological term, such a composite configuration of a finite size is created by quantum tunneling and subsequently grows rapidly. We estimate the nucleation probability analytically in the thindefect approximation and fully calculate it numerically using the relaxation (gradient flow) method. The nucleation probability is maximized when the direction of the soliton is perpendicular to the external field.
Venue: via Zoom
Event Official Language: English

Seminar
Transport coefficients of a Bose gas in one dimension
October 5 (Wed) at 13:30  15:00, 2022
Dr. Yusuke Nishida (Associate Professor, Department of Physics, Tokyo Institute of Technology)
I will present two of our recent studies on transport coefficients of a Bose gas in one dimension. The first part is on the thermal conductivity [1], which is typically divergent for quantum integrable systems in one dimension. However, it is found to be finite and dominated by an effective threebody interaction that inevitably arises by confining bosons into a tight matter waveguide. The second part is on the bulk viscosity [2], which is computed perturbatively in the hightemperature, weakcoupling, and strongcoupling limits. In particular, the strongcoupling limit is accessible thanks to the BoseFermi duality, which is shown for the dynamic bulk viscosity provided by the contactcontact response function.
Venue: via Zoom
Event Official Language: English

Seminar
Superconductinglike heat current: Effective cancellation of currentdissipation tradeoff by quantum coherence
July 25 (Mon) at 13:30  15:00, 2022
Dr. Tajima Hiroyasu (Assistant Professor, Graduate School of Informatics and Engineering, The University of ElectroCommunications)
Recent developments in statistical mechanics have revealed a tradeoff between heat current and dissipation [1,2]. In various situations, this currentdissipation tradeoff represents a relationship between thermal energy flow and entropy increase, similar to Joule’s law W=RI^2. On the other hand, the coherence effect on the currentdissipation tradeoff has not been thoroughly analyzed. Here, we systematically analyze how coherence affects the currentdissipation tradeoff [3]. The results can be summarized in the following three rules:
Venue: via Zoom
Event Official Language: English

Seminar
NonAbelian vortices in twoflavor dense QCD
June 15 (Wed) at 13:30  15:00, 2022
Dr. Yuki Fujimoto (Postdoctoral Scholars, Department of Physics, University of Washington, USA)
Recently, the phase of the twoflavor quark matter with the new pattern of color superconductivity was proposed so that the continuous crossover from the hadronic to the quark phase is realized [1]; it is in consonance with the recent observation of neutron stars. In this talk, I will show the classification of the topological vortices in this phase. We found that the stable vortices are what we call the "nonAbelian Alice strings" [2]. They are superfluid vortices carrying 1/3 quantized circulation and color magnetic fluxes. I will discuss their properties in comparison to the wellestablished CFL vortices in threeflavor symmetric setup, by putting some emphasis on their peculiarity: the nonAbelian generalization of the Alice property. I will then discuss in detail the possibility that these vortices are confined as well as how the vortices in the quark phase can be connected to those in the hadronic phase [3]. [1] Y. Fujimoto, K. Fukushima, W. Weise, PRD 101, 094009 (2020) [1908.09360]. [2] Y. Fujimoto, M. Nitta, PRD 103, 054002 (2021) [2011.09947]; JHEP 09 (2021) 192 [2103.15185]. [3] Y. Fujimoto, M. Nitta, PRD 103, 114003 (2021) [2102.12928].
Venue: via Zoom
Event Official Language: English

Equilibrium or not? Mathematical differences between acute & chronic virus infections
May 25 (Wed) at 13:30  15:00, 2022
Prof. Catherine Beauchemin (Deputy Program Director, iTHEMS)
The widely acclaimed 1995/1996 papers by Ho, Perelson and others [1,2] demonstrated the important insights that come from mathematical modelling of virus infection kinetics within a person. But there are key dynamical differences between chronic and acute infections, namely whether the infection reaches or maintains some equilibrium or not. In this talk, I will introduce the equations used to describe a virus infection within a person. I will show some of the tricks used by mathematical modellers to extract important rate estimates from measurements in patients infected with chronic diseases, like HIV or Hepatitis C virus. I will explain why it is difficult to extract meaningful information from measurements in patients with an acute infection, like influenza or possibly COVID19 [3]. I hope to hear from the audience if they have any thoughts about overcoming the issue to extract better rate information from limited data in patients with acute infections. (This seminar is a joint seminar between Nonequilibrium working group and Biology study group)
Venue: via Zoom
Event Official Language: English

Seminar
Lightmatter control of quantum materials: From lightinduced superconductivity to cavity materials
April 20 (Wed) at 15:30  17:00, 2022
Dr. 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 lightinduced 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 lightinduced 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

Seminar
Hydrodynamic theory of electron and spin transport
March 30 (Wed) at 13:30  15:00, 2022
Dr. 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 magnetizationvorticity coupling in anomalous Hall system are discussed. The spin Hall effect is argued from the viewpoint of a spinvorticity coupling.
Venue: via Zoom
Event Official Language: English

Seminar
Nonperturbative cavity/waveguide quantum electrodynamics and dissipative quantum phase transition
March 10 (Thu) at 13:30  15:00, 2022
Prof. Yuto Ashida (Associate Professor, Graduate School of Science, The University of Tokyo)
Strong coupling between matter and quantized electromagnetic modes in cavity or waveguide may offer yet another approach of controlling equilibrium phases or dynamics of manybody systems. Recent developments have realized such strong lightmatter interaction in genuinely quantum and nonperturbative regimes, where conventional approximate theoretical methods cannot be applied in general. I will talk about how one can analyze strongly coupled quantum lightmatter systems at arbitrary interaction strengths on the basis of an asymptotically disentangling unitary transformation [1,2]. I discuss its application to construction of tightbinding Hamiltonians, dynamics of bound states in the continuum, and revisiting dissipative quantum phase transition in resistively shunted Josephson junctions [3].
Venue: via Zoom
Event Official Language: English

False vacuum decay in the Lorentzian path integral
February 15 (Tue) at 13:30  15:00, 2022
Mr. Takumi Hayashi (Ph.D. Student, Research Center for the Early Universe (RESCEU), The University of Tokyo)
False vacuum decay is a nonperturbative phenomenon in quantum field theory and important quantum process in cosmology. It has relied on the Euclidean formalism developed by Coleman, but there are several subtle issues in cosmological application as a negative mode problem or ambiguity in the definition of the decay rate in the presence of the gravity. Instead of the Euclidean path integral, we directly evaluate the Lorentzian path integral to discuss false vacuum decay and estimate the decay probability. To make the Lorentzian path integral convergent, the deformation of an integral contour is performed on the basis of the PicardLefschetz theory. We show that the nucleation probability of a critical bubble, for which the corresponding bounce action is extremized, has the same exponent as the Euclidean approach. We also extend our computation to the nucleation of a bubble larger or smaller than the critical one to which the Euclidean formalism is not applicable.
Venue: via Zoom
Event Official Language: English

Seminar
A simple XY model for cascade transfer
January 20 (Thu) 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 nonKolmogorov energy spectrum. We also discuss the relation to spin turbulence [3,4] and atmospheric turbulence [5].
Venue: via Zoom
Event Official Language: English

Quantum metric of topological and nontopological insulators in AMO and other systems
December 20 (Mon) 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 wellstudied 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
Cosmological particle production as Stokes phenomena
December 15 (Wed) 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 timedependent 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
Imaging Theory of Optical Microscopy: Basic to Super Resolution
November 25 (Thu) 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 wellestablished theories, as well as a powerful tool to observe living microorganisms. The developments are still ongoing to overcome their limitations in observation. Recently, the invention of several superresolution 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 superresolution techniques, which utilize interesting theory to improve spatial resolution.
Venue: via Zoom
Event Official Language: English

Seminar
Geometry in optical responses of quantum materials
November 15 (Mon) 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 ongoing 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
Floquet vacuum engineering: laserdriven chiral soliton lattice in the QCD vacuum
October 20 (Wed) 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 timeperiodic 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 lowenergy 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
Highharmonic generation in strongly correlated systems
September 15 (Wed) at 13:30  15:00, 2021
Prof. Yuta Murakami (Assistant Professor, School of Science, Tokyo Institute of Technology)
Highharmonic 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 dynamicalmean field theory and the infinite timeevolving 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
Towards a description of amorphous solids and viscoelastic materials using effective field theory and holographic methods
September 10 (Fri) 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
18 events