UEDA GROUP
Department of Physics, The University of Tokyo
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2018年度 / Academic Year 2018

研究室メンバーによるレギュラー・セミナー(下記要領)、 および、他研究室・機関からのゲスト・スピーカーのセミナーを開催しています。 ご興味ある方の参加、歓迎します。事前の連絡なく参加頂けますが、 夏学期の論文紹介ゼミについては発表者に連絡頂けましたら発表資料を必要分印刷します。
Access map:理学部1号館 (Faculty of Science Bldg. 1) / 理学部4号館 (Faculty of Science Bldg. 4)

夏学期 / Summer semester (April-July 2018)

<Regular seminars (review of seminal papers)>
    木曜日13時から理学部1号館447号室で行います(普段と場所or時間の異なる場合は赤字で示します)。
    Each seminar starts from 13:00, Thursday @ #447, Faculty of Science Bldg. 1 (unless otherwise indicated).
<Seminars by guest speakers>
    May 1 (Tue.) Pau Gomez (ICFO) 14:00- @#447
    July 2 (Mon.) Prof. Minoru Yamashita (ISSP) 14:00- @1320
    July 12 (Wed.) 安藤 陽一 氏 (ケルン大学物理学科教授) 10:00- @1320
    July 19 (Wed.) Prof. Ryuichi Shindou (Peking Univ.) 13:00- @1320
    August 3 (Fri.) Prof. Nir Navon (Yale Univ.) 13:00- @447

April 19 (Thu) Takumi Yoshino
    "Experimental realization of the topological Haldane model with ultracold fermions"
    G. Jotzu, M. Messer, R. Desbuquois, M. Lebrat, T. Uehlinger, D. Greif, and T. Esslinger, Nature 515, 237 (2014)

April 26 (Thu) Zongping Gong
    "Random transverse field Ising spin chains"
    D. S. Fisher, Phys. Rev. Lett. 69, 534 (1992)
    "Random antiferromagnetic quantum spin chains"
    D. S. Fisher, Phys. Rev. B 50, 3799 (1994)
    "Many-Body Localization in One Dimension as a Dynamical Renormalization Group Fixed Point"
    R. Vosk and E. Altman, Phys. Rev. Lett. 110, 067204 (2013)

2018/5/1 (Tue.) 14:00- @ #447, Faculty of Science Bldg. 1
speaker Pau Gomez (Prof. Mitchell's group, ICFO)
title A Spinor BEC Co-Magnetometer for Studies of Magnetic Symmetry Breaking
abstract The "co-magnetometer" is a technology developed for rotation sensing and searches for physics beyond the standard model, consisting of two different magnetically-sensitive systems operating in the same volume and thus experiencing the same magnetic field. A differential measurement can then reject true magnetic influences (which typically are strong and noisy), while sensitively detecting small signals that differently affect the two components. Here we report a spinor BEC (SBEC) co-magnetometer, with the two components being the F=1 and F=2 ground state populations, independently detected using Faraday rotation probing [1], an extension of our recently-reported single-domain magnetic SBEC [2]. We study spin oscillation and spin amplification in F=2, using the F=1 component as a reference. This novel scheme gives accurate information on both the amplitude and phase of the F=2 SBEC as it rotates in a magnetic field, allowing tomographic study of spontaneous symmetry breaking, spin squeezing, and quantum entropy generation in a magnetically-polarized system.
[1] M. Koschorreck et al., "Sub-projection-noise sensitivity in broadband atomic magnetometry", Phys. Rev. Lett. 104, 093602 (2010).
[2] S. Palacios et al., "Multi-second magnetic coherence in a single domain spinor Bose-Einstein condensate", preprint arXiv:1707.09607 (2017).


May 3 (Thu) (National Holiday)

May 10 (Thu) Sho Higashikawa
    "Crystal Statistics. I. A Two-Dimensional Model with an Order-Disorder Transition"
    L. Onsager, Phys. Rev. 65, 117 (1944)

May 17 (Thu) Ryusuke Hamazaki
    "Infinite Number of Order Parameters for Spin-Glasses"
    G. Parisi, Phys. Rev. Lett. 43, 1754 (1979)
    "Order Parameter for Spin-Glasses"
    G. Parisi, Phys. Rev. Lett. 50, 1946 (1983)

May 24 (Thu) Naoto Kura
    "Message-passing algorithms for compressed sensing"
    D. L. Donoho, A. Maleki, and A. Montanari, Proc. Nat. Acad. Sci. 106 18914 (2009)

May 31 (Thu) Kohei Kawabata
    "Z2 Topological Order and the Quantum Spin Hall Effect"
    C. L. Kane and E. J. Mele, Phys. Rev. Lett. 95, 146802 (2005)
    "Quantum Spin Hall Effect in Graphene"
    C. L. Kane and E. J. Mele, Phys. Rev. Lett. 95, 226801 (2005)

June 7 (Thu) Yuto Ashida
    "Holographic duality from random tensor networks"
    P. Hayden, S. Nezami, X.-L. Qi, N. Thomas, M. Walter, Z. Yang, J. High Energ. Phys. (2016) 2016: 9.
    "Holographic Derivation of Entanglement Entropy from the anti–de Sitter Space/Conformal Field Theory Correspondence"
    S. Ryu and T. Takayanagi, Phys. Rev. Lett. 96, 181602 (2006)
    "Machine learning spatial geometry from entanglement features"
    Y.-Z. You, Z. Yang, and X.-L. Qi, Phys. Rev. B 97, 045153 (2018)

June 14 (Thu) Zhikang Wang
    "Mastering the game of Go with deep neural networks and tree search"
   D. Silver, A. Huang, C. Maddison et al. Nature 529, 484 (2016)

June 21 (Thu) Norifumi Matsumoto
    "Anyons in an exactly solved model and beyond"
   A. Kitaev, Annals of Physics 321 (2006) 2–111

June 28 (Thu) (No Seminar)

2018/7/2 (Mon.) 14:00- @ #1320, Faculty of Science Bldg. 4
speaker Prof. Minoru Yamashita (ISSP, Univ. of Tokyo)
title Universal thermal Hall conductivity of a kagome antiferromagnet
abstract pdf

July 5 (Thu) Ryosuke Oka
    "An exact mapping between the Variational Renormalization Group and Deep Learning"
    P. Mehta and D. J. Schwab, arXiv:1410.3831 (2014)

2018/7/12 (Thu.) 10:00- @ 理学部4号館3階1320号室
speaker 安藤 陽一 氏 (ケルン大学物理学科教授)
title トポロジカル超伝導体からマヨラナ量子ビットまで
abstract 2016年のノーベル物理学賞は、物質中で実現するトポロジカル相に関する先駆的な理論研究に対して与えられた。その受賞対象になった研究の延長線上に、トポロジカル超伝導体があり、その超伝導状態を記述する波動関数は非自明なトポロジーで特徴付けられる。本セミナーではまずトポロジカル超伝導体と、その低エネルギー励起状態であるマヨラナ準粒子の物理について概説する。このトポロジカル超伝導状態は、スピン軌道相互作用と超伝導近接効果を利用することによって人工的に作り出せると考えられている。そこに現れるマヨラナ準粒子をトポロジカル欠陥に局在させると「マヨラナゼロモード」が得られ、これは非アーベリアン統計に従うことが示される。そのためマヨラナゼロモードをトポロジカルに守られた量子計算に応用することが提案されており、量子コンピュータを実現するための有力なアプローチと考えられている。本セミナーではその実現に向けた実験的取り組みも紹介する。

July 12 (Thu) Masahiro Yamamoto
    "Steady-State Thermodynamics of Langevin Systems"
    T. Hatano and S. Sasa, Phys. Rev. Lett. 86, 3463 (2001)

2018/7/19 (Thu.) 13:00- @ #1320, Faculty of Science Bldg. 4
speaker Prof. Ryuichi Shindou (Peking Univ.)
title Theories of topological spin-nematic excitonic insulators in graphite under high magnetic field and quantum multicriticality in disordered Weyl semimetal
abstract In the first part of my talk, I will discuss our phenomenological theory for metal-insulator transitions in graphite under high magnetic field [1,2]. Graphite under high magnetic field exhibits consecutive metal-insulator (MI) transitions as well as re-entrant insulator-metal (IM) transition at low temperature. We explain these enigmatic insulator phases as manifestation of topological excitonic insulator phases with spin nematic orderings. We first argue that graphite under high magnetic field (> 20 Tesla) is in the charge neutrality region. Based on this observation, we employ models with electron and hole pocket(s), to construct a bosonized Hamiltonian that comprises of displacement field along the field direction and its conjugate fields. Using a renormalization group argument, we show that there exists a critical interaction strength above which a umklapp term becomes relevant and the system enters excitonic insulator phase with a long-range ordering of spin superfluid phase field, i.e. "spin nematic excitonic insulator (SNEI)". We argue that, when a pair of electron and hole pockets get smaller in size, a quantum fluctuation of the spin superfluid phase becomes larger and destabilizes the excitonic insulator phases, which results in the re-entrant IM transition. We explain field- and temperature-dependences of in-plane resistivity in graphite experiment by surface transports via novel surface states in topological SNEI phases [1,2].
In the second part of my talk, I will discuss our recent theory on multicriticality in disordered Weyl semimetal [3,4]. In electronic band structure of solid state material, two band touching points with linear dispersion (called as `Weyl node') appear in pair in the momentum space (`Nielsen-Ninomiya' theorem). When they annihilate with each other, the system undergoes a quantum phase transition from three-dimensional Weyl semimetal (WSM) phase to a band insulator (BI) phase. The phase transition is described by a new critical theory with a `magnetic dipole' like object in the momentum space. We reveal that the critical theory hosts a novel disorder-driven quantum multicritical point, which is encompassed by three quantum phases, WSM phase, BI phase, and diffusive metal (DM) phase. Based on the renormalization group argument, we first clarify scaling properties around the band touching points at the quantum multicritical point as well as all phase boundaries among these three phases [4]. We argue that a localization-delocalization transition between the BI phase and a WSM phase is controlled by a clean-limit fixed point with spatially anisotropic scale invariance. We show that the anisotropic scale invariance is reflected on unconventional scaling function forms in the quantum phase transition between BI and WSM phases. We verify the proposed scaling function forms in terms of transfer-matrix calculations of conductance and localization length in the tight-binding model [3].
[1] Z. Pan, X.-T. Zhang, and R. Shindou, arXiv:1802.10253.
[2] in preparation
[3] X. Luo, T. Ohtsuki, and R. Shindou, arXiv:1803.09051.
[4] X. Luo, B. Xu, T. Ohtsuki, and R. Shindou, Phys. Rev. B 97, 045129 (2018).

2018/8/3 (Fri.) 13:00- @ #447, Faculty of Science Bldg. 1
speaker Prof. Nir Navon (Yale Univ.)
title Turbulence in a quantum gas
abstract Many turbulent flows form so-called cascades, where excitations injected at large length scales, are transported to gradually smaller scales until they reach a dissipation scale. We initiate a turbulent cascade in a dilute Bose fluid by pumping energy at the container scale of an optical box trap using an oscillating magnetic force [1,2]. In contrast to classical fluids where the dissipation scale is set by the viscosity of the fluid, the turbulent cascade of our quantum gas finishes when the particles kinetic energy exceeds the laser-trap depth. This mechanism allows us to effectively tune the dissipation scale where particles (and energy) are lost, and measure the particle flux in the cascade at the dissipation scale. Our measurements are in very good agreement with simulations of the Gross- Pitaevskii equation including dissipation.
[1] A.L. Gaunt, T.F. Schmidutz, I. Gotlibovych, R.P. Smith, Z. Hadzibabic, Phys. Rev. Lett. 110, 200406 (2013).
[2] N. Navon, A.L. Gaunt, R.P. Smith, Z. Hadzibabic, Nature 539, 72 (2016) .