NEW WGセミナー
21 イベント
非平衡物理に関係したセミナーやワークショップを定期的に企画しています。より詳しい情報はNEWワーキンググループのウェブサイトをご参照ください。
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セミナー
Quantum transport with cold atoms
2023年5月10日(水) 13:30 - 15:00
内野 瞬 (日本原子力研究開発機構 先端基礎科学センター 研究員)
Quantum transport occurring through a mesoscopic conduction region allows us to extract interesting quantum many-body phenomena. For decades, solid-state systems have been the playground of such transport and revealed nontrivial outcomes such as the conductance quantization in a quantum point contact system and the fractional charge measurement in a fractional quantum Hall system. More recently, cold atoms trapped in the vacuum have served as the complementary system to study the quantum transport phenomena. In this seminar, I wish to discuss the recent progresses of quantum transport with cold atoms. The great advantages of those systems are that one can control quantum statistics, inter-particle interactions, dissipation, and dimensions. I try to show that such controllability enables to explore a regime of quantum transport that has yet to be reached with solid-state materials, including transport of bosons, dissipation effect in transport, and transport with synthetic dimensions.
会場: セミナー室 (359号室) (メイン会場) / via Zoom
イベント公式言語: 英語
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セミナー
How to sit Maxwell and Higgs on the boundary of AdS
2023年2月28日(火) 13:30 - 15:00
Matteo Baggioli (Associate Professor, School of Physics and Astronomy, Shanghai Jiao Tong University, China)
Within the holographic correspondence, boundary conditions play a fundamental role in determining the nature of the dual field theory. In this talk, I will show how to exploit mixed boundary conditions to obtain dynamical electromagnetism in the boundary theory. This is necessary to apply AdS-CFT to many real-world applications, e.g., magnetohydrodynamics, plasma physics, superconductors, etc. where dynamical gauge fields and Coulomb interactions are fundamental. As a proof of concept, I will show two emblematic cases. First, I will prove that the results from the 4-dimensional Einstein-Maxwell bulk theory with these deformed boundary conditions are in perfect agreement with relativistic magneto-hydrodynamics in 2+1 dimensions. Second, I will discuss the collective excitations of a bona-fide holographic superconductor and prove the existence of the Anderson-Higgs mechanism therein.
会場: 中央大学後楽園キャンパス 6号館 6209教室 (メイン会場) / via Zoom
イベント公式言語: 英語
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Thermodynamic inequalities: motivation, foundations, and applications
2023年2月7日(火) 13:30 - 15:00
Andreas Dechant (京都大学 理学系研究科 講師)
In this talk, I will introduce the topic of thermodynamic inequalities. One motivation for studying inequalities is that they can provide universal constraints on what can and cannot happen in physical systems. From a more practical point of view, they can be used to estimate physical observables even in situations where no equality is available. I will highlight a few recent examples of thermodynamic inequalities in the form of uncertainty relations and speed limits. In the main part of the talk, I will explain a general technique for deriving new inequalities, by starting from information-theoretic bounds and considering “virtual perturbations” of a physical system. I will show how this method can be used to derive and generalize the so-called “thermodynamic uncertainty relation”. An interesting application of such uncertainty relations is to estimate the dissipation in biological systems such as molecular motors. The second main topic is how to relate inequalities to equalities. When using inequalities to estimate physical quantities, it is crucial to understand the conditions under which the inequality can be tight. One way to achieve this is to “promote” the inequality into an equality via a variational principle. On the one hand, this provides conditions for obtaining a tight bound. On the other hand, variational expressions can also serve as a starting point to derive new inequalities.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Tricritical phenomena in holographic chiral transitions
2022年11月29日(火) 13:30 - 15:00
松本 匡貴 (Postdoctoral Researcher, Department of Physics, Shanghai University, China)
Tricritical point (TCP) is the end-point of a line of three-phase 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 mean-field 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].
会場: コモンルーム 246-248号室 とZoomのハイブリッド開催
イベント公式言語: 英語
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セミナー
Quantum nucleation of topological solitons
2022年10月20日(木) 13:30 - 15:00
衛藤 稔 (山形大学 理学部 教授)
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 easy-plane anisotropy. In such cases, topological solitons have negative energy due to topological terms originating from the chiral magnetic or vortical effect and the Dzyaloshinskii-Moriya 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 low-energy theories of the above systems. In 2+1 dimensions, a pair of a vortex and an anti-vortex is connected by a linear soliton, while in 3+1 dimensions, a vortex is string-like, a soliton is wall-like, 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 thin-defect 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.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Transport coefficients of a Bose gas in one dimension
2022年10月5日(水) 13:30 - 15:00
西田 祐介 (東京工業大学 理学院 物理学系 准教授)
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 three-body 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 high-temperature, weak-coupling, and strong-coupling limits. In particular, the strong-coupling limit is accessible thanks to the Bose-Fermi duality, which is shown for the dynamic bulk viscosity provided by the contact-contact response function.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Superconducting-like heat current: Effective cancellation of current-dissipation trade-off by quantum coherence
2022年7月25日(月) 13:30 - 15:00
田島 裕康 (電気通信大学 大学院情報理工学研究科 助教)
Recent developments in statistical mechanics have revealed a tradeoff between heat current and dissipation [1,2]. In various situations, this current-dissipation 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 current-dissipation tradeoff has not been thoroughly analyzed. Here, we systematically analyze how coherence affects the current-dissipation tradeoff [3]. The results can be summarized in the following three rules:
会場: via Zoom
イベント公式言語: 英語
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セミナー
Non-Abelian vortices in two-flavor dense QCD
2022年6月15日(水) 13:30 - 15:00
藤本 悠輝 (Postdoctoral Scholars, Department of Physics, University of Washington, USA)
Recently, the phase of the two-flavor 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 "non-Abelian 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 well-established CFL vortices in three-flavor symmetric setup, by putting some emphasis on their peculiarity: the non-Abelian 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].
会場: via Zoom
イベント公式言語: 英語
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セミナー
Equilibrium or not? Mathematical differences between acute & chronic virus infections
2022年5月25日(水) 13:30 - 15:00
カトゥリン・ボシゥメン (数理創造プログラム 副プログラムディレクター)
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 COVID-19 [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)
会場: via Zoom
イベント公式言語: 英語
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Light-matter control of quantum materials: From light-induced superconductivity to cavity materials
2022年4月20日(水) 15:30 - 17:00
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 light-induced 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 light-induced properties is replaced by quantum fluctuations of light in confined geometries. Ideas and open questions for future work will be outlined.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Hydrodynamic theory of electron and spin transport
2022年3月30日(水) 13:30 - 15:00
多々良 源 (理化学研究所 創発物性科学研究センター (CEMS) スピン物性理論研究チーム チームリーダー)
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 magnetization-vorticity coupling in anomalous Hall system are discussed. The spin Hall effect is argued from the viewpoint of a spin-vorticity coupling.
会場: via Zoom
イベント公式言語: 英語
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Nonperturbative cavity/waveguide quantum electrodynamics and dissipative quantum phase transition
2022年3月10日(木) 13:30 - 15:00
蘆田 祐人 (東京大学 大学院理学系研究科 准教授)
Strong coupling between matter and quantized electromagnetic modes in cavity or waveguide may offer yet another approach of controlling equilibrium phases or dynamics of many-body systems. Recent developments have realized such strong light-matter 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 light-matter systems at arbitrary interaction strengths on the basis of an asymptotically disentangling unitary transformation [1,2]. I discuss its application to construction of tight-binding Hamiltonians, dynamics of bound states in the continuum, and revisiting dissipative quantum phase transition in resistively shunted Josephson junctions [3].
会場: via Zoom
イベント公式言語: 英語
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セミナー
False vacuum decay in the Lorentzian path integral
2022年2月15日(火) 13:30 - 15:00
林 拓未 (東京大学 大学院理学系研究科附属 ビッグバン宇宙国際研究センター (RESCEU) 博士課程)
False vacuum decay is a non-perturbative 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 Picard-Lefschetz 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.
会場: via Zoom
イベント公式言語: 英語
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セミナー
A simple XY model for cascade transfer
2022年1月20日(木) 13:30 - 15:00
田之上 智宏 (京都大学 大学院理学研究科 博士課程／日本学術振興会 特別研究員 DC)
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].
会場: via Zoom
イベント公式言語: 英語
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Quantum metric of topological and non-topological insulators in AMO and other systems
2021年12月20日(月) 13:30 - 15:00
小澤 知己 (東北大学 材料科学高等研究所 (AIMR) 准教授)
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.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Cosmological particle production as Stokes phenomena
2021年12月15日(水) 13:30 - 15:00
山田 悠介 (東京大学 大学院理学系研究科附属 ビッグバン宇宙国際研究センター (RESCEU) 学振特別研究員)
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”.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Imaging Theory of Optical Microscopy: Basic to Super Resolution
2021年11月25日(木) 13:30 - 15:00
桶谷 亮介 (九州大学 大学院理学研究院 化学部門 物理化学講座 助教)
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.
会場: via Zoom
イベント公式言語: 英語
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Geometry in optical responses of quantum materials
2021年11月15日(月) 13:30 - 15:00
永長 直人 (理化学研究所 創発物性科学研究センター (CEMS) 強相関理論研究グループ 副センター長, グループディレクター / 東京大学 大学院工学系研究科 物理工学専攻 教授)
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.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Floquet vacuum engineering: laser-driven chiral soliton lattice in the QCD vacuum
2021年10月20日(水) 13:30 - 15:00
山田 晃弘 (慶應義塾大学 大学院理工学研究科 基礎理工学専攻 修士課程)
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.”
会場: via Zoom
イベント公式言語: 英語
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セミナー
High-harmonic generation in strongly correlated systems
2021年9月15日(水) 13:30 - 15:00
村上 雄太 (東京工業大学 理学院 助教)
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.
会場: via Zoom
イベント公式言語: 英語
21 イベント