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63 件
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The Electron-Ion Collider: the Ultimate Electron Microscope
2023年2月20日(月) 15:00 - 16:30
ゴードン・ベイム (Professor Emeritus, University of Illinois, USA)
How does the nucleon get its mass? Certainly not from the Higgs -- the rest masses of the quarks it contains add up to only one percent of the nucleon mass. Rather the remaining 99% comes from the zero-point energy of the quarks, antiquarks and gluons localized in the nucleon. How do nuclei differ from being a simple collection of nucleons? How are the gluons, for example, distributed in nuclei? Do they stick out, or are they clumped towards the center of the nucleon? Gluons, like dark matter unseen but playing the crucial role in gluing matter together, are strongly interacting. Do such gluons form new emergent quantum states in nuclei, as in condensed matter physics? And how is the spin of the proton -- the key to NMR imaging -- put together from the spin and orbital motion of the quarks and gluons in the proton?
会場: 大河内記念ホール (メイン会場) / via Zoom
イベント公式言語: 英語
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Entanglement in non-Hermitian quantum systems and non-unitary conformal field theories
2023年2月9日(木) 17:00 - 18:15
Chang Po-Yao (Assistant Professor, Department of Physics, National Tsing Hua University, Taiwan)
Time: 5pm ~ 6:15pm (JST); 9am ~ 10:15am (CET); 4pm ~ 5:15pm (Taiwan) Entanglement is a powerful tool to diagnose many-body quantum systems. One example is the critical system where the low energy property can be described by conformal field theories (CFTs), and the central charge which uniquely characterizes the CFT can be perfectly extracted from the entanglement entropy. However, the entanglement properties for non-unitary CFTs are not well understood. Moreover, the entanglement properties in many-body microscopic models which can be described by non-unitary CFTs have not been explored. In this talk, I would like to demonstrate several non-Hermitian systems which can be described by non-unitary CFTs, and show their entanglement properties can be correctly obtained by the proposed generic entanglement entropy. Field: Condensed Matter Physics Keywords: non-Hermitian systems, conformal field theory, many-body systems, entanglement entropy
会場: via Webex
イベント公式言語: 英語
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Ultra-Light Axion Dark Matter: Bose-Einstein condensates and superfluids in the sky
2022年10月11日(火) 13:30 - 15:00
エリザ・フェレイラ (東京大学 カブリ数物連携宇宙研究機構 (Kavli IPMU) 特任助教)
The nature of dark matter (DM) is one of the biggest mysteries in cosmology. There are many different models to explain the nature of this elusive component. In this talk I will present a class of dark matter models: ultra-light dark matter (ULDM) or ultra-light axions (ULA). I will show the different models present in the literature and classify them according to the way they behave on small scales. One of the most interesting features of this class of DM models is that it might condense in the interior of the halos of galaxies forming a Bose-Einstein condensate (BEC) or superfluid. This interesting quantum phenomena on macroscopic scales, and the wave nature of ULDM leads to different and interesting astrophysical consequences that can be probed on small scales. I will quickly review first the fuzzy dark matter model, one of the most well studied ULA models, where I will present its description, predictions and current bounds. Then I will introduce the DM superfluid model, where, upon condensation in the interior of galaxies, DM dynamics represents that of MOdified Newtonian Dynamics (MOND) on galactic scales. This behaviour can address some of the curiosities of the behaviour of DM on small scales. I plan to show the theoretical description of this model and its interesting phenomenology.
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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Isometric tensor networks in two dimension
2022年10月11日(火) 10:30 - 12:00
ヤンタオ・ウー (数理創造プログラム 特別研究員)
In this talk, I would like to explain the ansatz of isometric tensor network states (isoTNS) as candidate wavefunctions in two-dimensional condensed matter systems. I will explain how the isometric structure in 2D helps generalize many 1D tensor network algorithms, like the density matrix renormalization group and the time-evolution block decimation methods, to two dimensions. Both bosons and fermions; ground states and dynamics will be discussed. I will also explain why it is a friendly trial wavefunction in the context of variational Monte Carlo, where the sampling correlation time vanishes. I will also explain its relation to quantum error correction and how it provides an interesting playground of quantum information. If time permits, I would like to discuss some open questions about its representability of topological phases.
会場: コモンルーム (246-248号室) (メイン会場) / via Zoom
イベント公式言語: 英語
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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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Phantom Bethe excitations and spin helices in integrable spin chains
2022年9月15日(木) 17:00 - 18:15
Vladislav Popkov (University Wuppertal, Germany)
We demonstrate the existence of a special chiral “phantom” mode with some analogy to a Goldstone mode in the anisotropic quantum XXZ Heisenberg spin chain. The phantom excitations contribute zero energy to the eigenstate, but a finite fixed quantum of momentum. The mode exists not due to symmetry principles, but results from nontrivial scattering properties of magnons with momentum k given by the anisotropy via cos (k) = Jz/Jx. The mode originates from special string-type solutions of the Bethe ansatz equations with unbounded rapidities, the phantom Bethe roots. All such Bethe states are chiral (the simplest representative being factorized state with helicoidal magnetization profile) and exist in both periodic and open XXZ spin chain under fine-tuning. I show how phantom Bethe states can be generated dissipatively, by setting a polarization gradient via coupling the ends of the open spin chain to suitable dissipative baths. Spin helix eigenstates were observed and used in recent cold atom experiments, and led to further surprising findings.
会場: via Zoom
イベント公式言語: 英語
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Adiabatic pumps in quantum spin systems
2022年7月12日(火) 16:00 - 17:15
塩崎 謙 (京都大学 基礎物理学研究所 助教)
The Thouless pump is a one-parameter cycle of 1-dimensional gapped quantum systems with U(1) symmetry, which is classified by integers. In this talk, I introduce a generalization of the Thouless pump to quantum spin systems in any dimension with any finite group onsite symmetry. I show a simple model with Z_2 onsite symmetry, and how it is nontrivial via boundary degrees of freedom. Using the framework of the injective matrix product state, one can construct the topological invariant in a way similar to the Berry phase. If time allows, I will briefly introduce a group cohomology model by Roy and Harper for generic space dimensions and discuss its properties.
会場: via Zoom
イベント公式言語: 英語
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Topological quantum effects in low-dimensional spin systems - The power of the boundary
2022年6月30日(木) 17:00 - 18:15
Thore Posske (Group Leader, I. Institute for Theoretical Physics, University of Hamburg, Germany)
Manipulating the boundary of low-dimensional magnetic structures could grant control about topological magnetic quantum sates. I will discuss the creation of one- and two-dimensional topological quantum magnets by manipulating the boundary magnetization, address their stability against external perturbations, and discuss their possible application to quantum information processing.
会場: via Zoom
イベント公式言語: 英語
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Topological aspects of non-Hermitian physics
2022年6月21日(火) 16:00 - 17:15
大熊 信之 (京都大学 基礎物理学研究所 助教)
The past decades have witnessed an explosion of interest in topological materials, and a lot of mathematical concepts have been introduced in condensed matter physics. Among them, the bulk-boundary correspondence is the central topic in topological physics, which has inspired researchers to focus on boundary physics. Recently, the concepts of topological phases have been extended to non-Hermitian Hamiltonians, whose eigenvalues can be complex. Besides the topology, non-Hermiticity can also cause a boundary phenomenon called the non-Hermitian skin effect, which is an extreme sensitivity of the spectrum to the boundary condition. In this talk, I will explain recent developments in non-Hermitian topological physics by focusing mainly on the boundary problem. As well as the competition between non-Hermitian and topological boundary phenomena, I will discuss the topological nature inherent in non-Hermiticity itself. Field: condensed matter physics Keywords: topological materials, non-Hermitian systems, skin effect, bulk-boundary correspondence
会場: via Zoom
イベント公式言語: 英語
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Introduction to Topological Insulators: The Ten-fold Classification of Topological Insulators and Superconductors Part.2
2022年6月13日(月) 14:00 - 15:30
邱 靖凱 (数理創造プログラム 上級研究員)
会場: via Zoom
イベント公式言語: 英語
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Introduction to Topological Insulators: The Ten-fold Classification of Topological Insulators and Superconductors Part.1
2022年5月24日(火) 14:00 - 15:30
邱 靖凱 (数理創造プログラム 上級研究員)
会場: via Zoom
イベント公式言語: 英語
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Introduction to Topological Insulators: Topological Superconductors and Quantum Computing
2022年5月9日(月) 14:00 - 15:30
邱 靖凱 (数理創造プログラム 上級研究員)
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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Local and global topology for Dirac points with multi-helicoid surface states
2022年3月24日(木) 17:00 - 18:15
Tiantian Zhang (東京工業大学 理学院 特任助教)
Though topological invariants defined for topological semimetals are usually local ones, they also have a global nature. For example, the Z type local monopole charge C for Weyl points, has a global nature, telling us its influence to the rest of the Brillouin zone, giving rise to bulk-surface correspondence associated with helical surface states. In Dirac systems, helical surface states are not guaranteed due to C=0. However, a new bulk-surface correspondence associated with double/quad-helicoid surface states (DHSSs/QHSSs) can be obtained for Dirac points with the protection of a Z2 type monopole charge Q, which is defined in terms of the time-reversal (T)-glide (G) symmetry (TG)2= -1. Here we study the topology of Q for Z2 Dirac points and establish its bulk-surface correspondence with strict proofs. We find that Q is equivalent to the G-protected Z2 invariant v mathematically and physically in Z2 Dirac systems. This result is counterintuitive, since v is always trivial in T-preserving gapped systems, and was thought to be ill-defined in gapless systems. We offer a gauge-invariant formula for Q, which is associated with DHSSs in both the spinless and spinful systems with single G. Q is formulated in a simpler form in spinless systems with two vertical G, associated with QHSSs, which is also entangled with filling-enforced topological band insulators in three space groups when a T-breaking perturbation is introduced. Since Q is ill-defined in spinful systems with two vertical G, QHSSs will not be held. Material candidate Li2B4O7 together with a list of possible space groups preserving QHSSs are also proposed for demonstration on our theory and further studies. *Detailed information about the seminar refer to the email.
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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How is turbulence born: Spatiotemporal complexity and phase transition of transitional fluids
2022年2月24日(木) 17:00 - 18:15
Hong-Yan Shih (Assistant Research Fellow, Institute of Physics, Academia Sinica, Taiwan)
How a laminar flow becomes turbulence has been an unsolved problem for more than a century and is important in various industrial applications. Recently precise measurements in pipe flow experiments showed non-trivial spatiotemporal complexity at the onset of turbulence. Based on numerical evidence from the hydrodynamics equations, we discovered the surprising fact that the fluid behavior at the transition is governed by the emergent predator-prey dynamics of the important long-wavelength mode, leading to the mathematical prediction that the laminar-turbulent transition is analogous to an ecosystem on the edge of extinction. This prediction demonstrates that the laminar-turbulent transition is a non-equilibrium phase transition in the directed percolation universality class, and provides a unified picture of transition to turbulence emerging in systems ranging from turbulent convection to magnetohydrodynamics. *Detailed information about the seminar refer to the email.
会場: via Zoom
イベント公式言語: 英語
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Spin transport in ultracold atomic gases
2022年2月18日(金) 14:00 - 15:00
関野 裕太 (理化学研究所 開拓研究本部 (CPR) 長瀧天体ビッグバン研究室 特別研究員)
In condensed matter physics, transport measurement has played crucial roles in understanding fascinating phenomena such as superconductivity and quantum Hall and Kondo effects. In this talk, we discuss the usefulness of spin transport as a probe for many-body properties in ultracold atoms. In the first part, we focus on the conductivity of alternating spin current, which includes information on superfluid gap, pseudogap, and topological phase transition. In the latter part, we consider mesoscopic spin transport between two Fermi gases weakly connected with each other. Our analysis suggests that the spin current is sensitive to whether the gases have pseudogaps, which are gap-like structures in densities of states just above the superfluid transition temperature. In this talk, we also mention similarities of ultracold atoms to neutron star matter.
会場: via Zoom
イベント公式言語: 英語
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Bethe ansatz and quantum computing
2022年1月26日(水) 22:00 - 23:15
Rafael I. Nepomechie (Professor, Physics Department, University of Miami, Florida, USA)
We begin with a brief review of the Heisenberg quantum spin chain and its remarkable solution found by Bethe. We then review a probabilistic algorithm for preparing exact eigenstates of this model on a quantum computer. An exact formula for the success probability is presented, and the computation of correlation functions is discussed. A generalization of the algorithm to open chains with boundaries is also noted.
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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