UEDA GROUP

Department of Physics, The University of Tokyo

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、 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。 レギュラー・セミナーは下記要領で行います。 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)

Every seminar

Oct. 4 Yuto Ashida / Naoto Tsuji

Oct. 11 Sho Higashikawa / Masaya Nakagawa

Oct. 18 Dr. Rashi Sachdeva (OIST) / Takumi Yoshino

Oct. 25 Prof. Arnab Sen (IACS, Kolkatta) / Ryusuke Hamazaki

Nov. 1 Kohei Kawabata / Emi Yukawa @ #1320, Faculty of Science Bldg. 4

Nov. 8 (No seminar - Monday lecture day)

Nov. 15 Naoto Kura / Kazuya Fujimoto

Nov. 22 Zhikang Wang / Shunsuke Furukawa

Nov. 30 - Dec. 1 (非平衡系・非エルミート系の新奇量子現象 @ YITP)

Dec. 6 Zongping Gong / Takashi Mori

Dec. 13 Norifumi Matsumoto / Dr. Taiki Haga (Kyoto Univ.)

Dec. 20 (No seminar)

Jan. 10 (tentative) Kangqiao Liu / Ziyin Liu

Jan. 17 (tentative) Yusuke Sato / Kazuki Sone

2018/10/4(Thu.)

speaker | Yuto Ashida |

title | Thermalization and heating dynamics in open generic many-body systems |

abstract |
The last decade has witnessed remarkable progress in understanding of thermalization in isolated quantum systems. In this work [1], we combine the ideas of the eigenstate thermalization hypothesis and quantum measurement theory to extend the framework of quantum thermalization to open many-body systems. As for physical measurement processes, a generic many-body system subject to continuous observation is shown to thermalize at a single trajectory level for a typical realization of measurement outcomes. The nonunitary nature of quantum measurement leads to several unique thermalization mechanisms that are unseen in isolated systems. We present numerical evidence of our ﬁndings by applying our theory to nonintegrable hard-core bosons under local and global measurements, which can be experimentally realized by quantum gas microscopy and in atom-cavity systems. Our theory also provides a general way to determine an effective temperature of many-body systems subject to the Lindblad master equation and thus should be applicable to noisy dynamics or dissipative systems coupled to nonthermal environments as well as continuously monitored systems. [1] YA, K. Saito and M. Ueda, arXiv:1807.00019 |

speaker | Naoto Tsuji |

title | Quantum-state fluctuation theorem |

abstract | Fluctuation theorem has played a central role in the understanding of irreversibility in nonequilibrium processes. A conventional approach to the fluctuation theorem for isolated quantum systems is based on a two-point measurement of work, for which one obtains limited information on the quantum state at the end of the process. Here I show that a collection of the whole quantum-state data satisfies an infinite series of exact relations. This includes the work fluctuation theorem as a special case, and further gives a new generalization. If one applies the relation to near equilibrium, one can reproduce the out-of-time-order fluctuation-dissipation theorem. I also discuss that the fluctuation of the quantum-state data is closely related to quantum chaos. |

2018/10/11 (Thu.)

speaker | Sho Higashikawa |

title | Floquet engineering of classical systems |

abstract |
Floquet engineering, the control of quantum systems by a periodic drive, is a powerful tool to control non-equilibrium systems and realize exotic states of matter. In this work [1], we generalize the Floquet methodology to classical systems, developing a systematic high-frequency expansion for nonlinear (stochastic) equations that is applicable to both closed and open classical systems. We apply this formalism to a laser-driven magnet coupled with an environment and compare the time-dependent equation of motion and its high-frequency expansion. Interestingly, the latter well approximates the former up to the non-equilibrium steady state. This agreement is in stark contrast to closed quantum systems, where the high-frequency expansion fail to capture eventual heating to infinite temperature. [1] S. Higashikawa, H. Fujita, and M. Sato, arXiv: 1810:01103 |

speaker | Masaya Nakagawa |

title | Non-Hermitian quantum many-body physics of fermions in one dimension |

abstract | Ultracold atoms have offered an ideal platform for studies of quantum many-body physics. However, atomic gases are sometimes suffered from loss of atoms due to inelastic scattering between atoms. Such inelastic scattering can be formulated in the framework of open quantum systems, leading to emergence of an effective non-Hermitian Hamiltonian where the interaction strength becomes complex-valued. In this seminar, I will discuss quantum many-body physics that arises from the complex-valued non-Hermitian interactions, using two paradigmatic examples of strongly correlated quantum systems in one dimension: (i) Hubbard model with attractive or repulsive interaction, (ii) quantum impurity immersed into a Tomonaga-Luttinger liquid. |

2018/10/18 (Thu.)

speaker | Dr. Rashi Sachdeva (Quantum Systems Unit, OIST) |

title | Bose-Einstein condensates in optical lattices with artificial magnetic fields |

abstract |

speaker | Takumi Yoshino |

title | Collective modes of vortex lattices in two-component Bose-Einstein condensates under synthetic gauge fields |

abstract |
There has been an ever-growing interest in artificially created gauge fields in ultracold atomic gases, which are induced by either rotating gases or optically dressing atoms.
When the gas is composed of two components, the former (latter) method induces mutually parallel (antiparallel) synthetic magnetic fields in the two components.
Two-component Bose-Einstein condensates (BECs) in parallel and antiparallel fields are known to show the same vortex-lattice phase diagram with five different lattice structures determined by the ratio of the intercomponent interaction $g_{¥uparrow¥downarrow}$ to the intracomponent one $g$ within the mean-field theory.
Therefore it is interesting to ask whether and in what way the difference between the cases of parallel and antiparallel fields occurs. Here we study collective modes of vortex lattices in two-component BECs by means of the Bogoliubov theory and an effective field theory as a consequence of quantum fluctuations.
We find that two modes with linear and quadratic dispersion relations appear in both the types of synthetic fields.
We also analyze the anisotropy of the Bogoliubov excitation spectra at low energies and find that it is well described by the effective theory for all vortex lattice phases.
While the excitation spectra are significantly different between the two types of synthetic fields, their low-energy parts are found to be related to each other by simple rescaling for intercomponent attraction $g_{¥uparrow¥downarrow}<0$.
However, contrary to the effective theory prediction, this relation is violated for intercomponent repulsion $g_{¥uparrow¥downarrow}>0$ with a greater degree of violation for larger repulsion.
Apart from the low-energy features, the excitation spectra exhibit the band touching at high-symmetry points and along lines in the 1st Brillouin zone. We find that particular band touching can be understood from "fractional" translational symmetry. However, we can not explain some features of the spectra at high-symmetry points from a symmetry viewpoint. We explain their origins on the basis of numerical data of the Bogoliubov Hamiltonian matrix. [1] T. Yoshino et al., arXiv:1807.11666 |

2018/10/25 (Thu.)

speaker | Prof. Arnab Sen (Indian Association for the Cultivation of Science, Kolkata) |

title | Aperiodically driven Integrable Systems and their emergent steady states |

abstract |
Does a closed quantum many-body system that is continually driven with a time-dependent Hamiltonian finally reach a steady state? This question has only recently been answered for driving protocols that are periodic in time, where the long-time behavior of the local properties synchronizes with the drive and can be described by an appropriate periodic ensemble. Here, we explore the consequences of breaking the time-periodic structure of the drive with additional aperiodic noise in a class of integrable systems. We show that the resulting unitary dynamics leads to new emergent steady states in at least two cases. While any typical realization of random noise causes eventual heating to an infinite-temperature ensemble for all local properties in spite of the system being integrable, noise that is self-similar in time leads to an entirely different steady state (which we dub the “geometric generalized Gibbs ensemble”) that emerges only after an astronomically large time scale. To understand the approach to the steady state, we study the temporal behavior of certain coarse-grained quantities in momentum space that fully determine the reduced density matrix for a subsystem with size much smaller than the total system. Such quantities provide a concise description for any drive protocol in integrable systems that are reducible to a free-fermion representation. Collaborators: Sourav Nandy (IACS, Kolkata) and Diptiman Sen (IISc, Bangalore) Reference: Phys. Rev. X 7, 031034 (2017) |

speaker | Ryusuke Hamazaki |

title | Operator noncommutativity and irreversibility in quantum chaos |

abstract |
Irreversibility under time-reversal test and the growth of noncommutativity of two unequal-time operators are two seemingly different probes of quantum chaos. In this work [1], we argue that these two probes are quantitatively equivalent for initially localized states. After discussing the general scenario that motivates our argument, we verify our theory for two prototypical models of quantum chaos: a quantum kicked rotor and nonintegrable many-body systems. Our results also indicate the dominance of three-rather than four-point out-of-time-ordered correlators for the growth of the squared commutator. [1] RH, K.Fujimoto and M.Ueda, arXiv:1807.02360 |

2018/11/1 (Thu.) @ #1320, Faculty of Science Bldg. 4

speaker | Kohei Kawabata |

title | Symmetry and topology in non-Hermitian physics |

abstract |
Topological phases of matter have been widely explored in equilibrium closed systems, but richer properties appear in nonequilibrium open systems that are effectively described by non-Hermitian Hamiltonians. While several unique properties were uncovered, no research has established a comprehensive theoretical framework for non-Hermitian topological systems. In this seminar, we discuss the topological classification of non-Hermitian insulators and superconductors, as a generalization of the tenfold classification of Hermitian systems. After clarifying symmetry and energy gaps for non-Hermitian Hamiltonians, we provide the periodic table that classifies all the non-Hermitian topological systems in a general manner. Reference: K. Kawabata, K. Shiozaki, M. Ueda, and M. Sato (in preparation). |

speaker | Emi Yukawa |

title | Supercurrent induced by multipoles in nonmagnetic spin-2 Bose-Einstein condensates |

abstract | In spinor Bose-Einstein condensates (BECs), a supercurrent is known to be induced by the magnetic degrees of freedom when the magnitude of the spin vector is finite. On the other hand, when a spinor BEC is nonmagnetic, the supercurrent originating from the magnetic degrees of freedom is absent for a spin-1 BEC; however, it has remained to be clarified if it is essentially zero for any spin degrees of freedom. We show that a supercurrent can be induced by magnetic multipoles for a non-magnetic spin-2 BEC. We analytically derive a superfluid in a nonmagnetic spin-2 BEC that involves components originating from the magnetic degrees of freedom. We also numerically demonstrate that these components can be induced by a spatially dependent magnetic field via the quadratic Zeeman effect. |

2018/11/15 (Thu.)

speaker | Naoto Kura |

title | Quantum Metrology on Functions |

abstract |
Quantum metrology is a field of quantum information theory in which quantum entanglement benefits the estimation efficiency, and the target of the estimation has previously been one or at most finitely many parameters. In this seminar, we extend the analysis of quantum metrology to the estimation of unknown functions, which have infinite degree of freedom. In our metrology on unknown functions, the prior assumption on the smoothness is characterized by the Sobolev–Srobodeckĳ norm, with which we can derive quantum-metrological limits depending on the smoothness. Our theory can be applied to the design of practical quantum metrology such as quantum signal detection of Gaussian processes [1] or possibly the quantum imaging [2]. We also present two methods of metrology; one uses position states as probes and the other uses wavenumber states. Both methods are shown to be asymptotically optimal for not-so-smooth functions, and the position-state method still holds this property for smooth functions. [1] H. Dinani and D. Berry, Physical Review A, 95 063821 (2017)[2] N. Samantaray et al., Light: Science & Applications, 6 e17005 (2017) |

speaker | Kazuya Fujimoto |

title | Non-thermal fixed points in a one-dimensional spinor Bose gas |

abstract |
An ultracold gas has been an excellent playground for studying universal thermalization dynamics. A typical example is critical relaxation phenomena emerging near critical points at second-order phase transitions, where substantial efforts have been devoted to uncover the universal behaviors from the perspective of the Kibble Zurek mechanism. Even far from critical points, a similar type of universal thermalization dynamics is known to appear in coarsening dynamics, decay of turbulence, etc. However, the unified framework to understand them has remained elusive. Against a backdrop of it, a non-thermal fixed point (NTFP) is proposed as a novel universal thermalization scenario, where a strongly quenched initial state attracts to a NTFP and then the correlation function exhibits dynamical scaling transiently [1]. Very recently, this NFTP scenario is experimentally observed in one-dimensional (1D) Bose gases [2,3]. In this work, we study quench dynamics in a 1D spinor Bose gas with an antiferromagnetic interaction, finding emergence of universal thermalization dynamics through a NTFP. We reveal that the dynamics is induced by magnetic solitons and their bound states which we refer to as a Flemish string. This is the first clear example of a soliton-induced NTFP. I will talk about the detail of this results and discuss possible experimental observation for our theoretical prediction. [1] J. Berges, arXiv:1503.02907 (2015). [2] M. Prüfer, P. Kunkel, H. Strobel, S. Lannig, D. Linnemann, C.-M. Schmied, J. Berges, T. Gasenzer, M. K. Oberthaler, arXiv:1805.11881. [3] S. Erne, R. Buecker, T. Gasenzer, J. Berges, J. Schmiedmayer, arXiv:1805.12310. |

2018/11/22 (Thu.)

speaker | Zhikang Wang |

title | Investigation on neural networks with positivity constraint |

abstract | Deep neural networks are known for their large expressivity and universal function approximation ability. It is often believed that the success of deep learning lies in the universality. However, we show that a neural network with all weight parameters constrained to be positive can also learn deep learning tasks, and this network has much smaller expressivity compared with usual neural networks. Especially, it cannot learn logical XOR operation. We show that the positivit-constrained network has properties being different from conventional neural networks, and how this network can by analyzed more easily to know the reason for its decided output. |

speaker | Shunsuke Furukawa |

title | Fractional quantum Hall states with symmetry-enriched edge modes in multicomponent Bose gases |

abstract | While atomic gases are charge neutral, a synthetic magnetic field can be induced in such gases by rotating the system or optically dressing atoms. A sufficiently high synthetic field for ultracold atoms is expected to offer interesting analogues of quantum Hall states with a rich diversity of statistics and spins of constituent particles. For two-component Bose gases, there have been numerical evidences that a bosonic version of an integer quantum Hall state appears at the filling factor 2. This state is an example of a symmetry-protected topological state, featuring counterpropagating charge and spin modes at the edge which are protected by the particle-number conservation. Here, by generalizing this state for n-component Bose gases with n≥3, we construct fractional quantum Hall states at the filling factor n/(n-1). These states show a set of charge modes and n-1 sets of spin modes counterpropagating at the edge when the particle number is conserved. In contrast to the n=2 case, the edge modes have a symmetry-enriched nature in the sense that the n-2 sets of spin modes are robust against the breaking of the symmetry. We present a numerical evidence for the symmetry-enriched edge modes for n=3. |

2018/12/6 (Thu.)

speaker | Zongping Gong |

title | Topological aspects of nonequilibrium quantum dynamics in one spatial dimension |

abstract |
Quench and periodic driving are two common scenarios for coherently bringing a quantum system out of equilibrium. In this seminar, we focus on the topological aspects of far-from-equilibrium quantum dynamics generated by quenches [1] and Floquet unitaries [2] in one spatial dimension. In particular, we present the topological classifications of all the quench dynamics in free-fermion systems with Altland-Zirnbauer symmetries and all the matrix-product unitaries with discrete and unitary on-site symmetries. We show how to visualize the dynamical topology by tracing the time evolution, either continuous or stroboscopic, of the entanglement spectrum. Finally, we briefly discuss a related on-going project concerning the Lieb-Robinson behavior of the entanglement (Schmidt) gap. [1] Z. Gong and M. Ueda, arXiv:1710.05289. [2] Z. Gong, C. Sünderhauf, N. Schuch, and J. I. Cirac, in preparation. |

speaker | Takashi Mori |

title | Gibbs steady state via ETH in driven-dissipative open quantum systems |

abstract | The eigenstate thermalization hypothesis (ETH) has been considered as an important property of a nonintegrable Hamiltonian in the context of thermalization in isolated quantum systems. In this seminar, I discuss an implication of the ETH for the steady state of a many-body quantum system in weak contact with a nonequilibrium environment (such a system is called a driven-dissipative system). I show that the perturbation expansion in a small system-environment coupling is justified in the thermodynamic limit, although its convergence radius shrinks to zero exponentially fast with respect to the system size. The leading-order perturbation theory implies that the nonequilibrium steady state is described by a diagonal density matrix in the energy basis. By combining this fact with the ETH, it is concluded that the steady state is locally indistinguishable from the Gibbs state despite the violation of the detailed balance condition. The seminar will be given on blackboard. |

2018/12/13 (Thu.)

speaker | Dr. Taiki Haga (Kyoto University) |

title | Convergence of the Floquet-Magnus expansion in one-body chaotic systems driven by an oscillating force |

abstract |
Quantum systems driven by an oscillating field have recently
attracted much attention due to their nontrivial dynamics
not observed in equilibrium conditions.
According to the Floquet theory, the stroboscopic evolution
of periodically driven systems is described by a time-independent
effective Hamiltonian, which is called the Floquet Hamiltonian [1].
The Floquet-Magnus (FM) expansion gives a formal series expansion of
the Floquet Hamiltonian with respect to the period of the oscillating field.
It is believed that the divergence of the FM expansion is intimately
related to the onset of chaotic behavior [1].
However, little is known about the radius of convergence of the FM expansion,
except exactly solvable cases such as driven harmonic oscillators. In this study, we consider periodically driven one-body quantum systems which can exhibit chaotic dynamics in the classical limit, for example, driven anharmonic oscillators. One may naively expect that, in a region of the parameter space where the dynamics is integrable (chaotic), the FM expansion converges (diverges). We numerically determine the radius of convergence of the FM expansion and verify this hypothesis. Furthermore, we also discuss how the classical limit of the quantum FM expansion reproduces the dynamical phase diagram of the corresponding classical system. [1] M. Bukov, L. D'Alessio, and A. Polkovnikov, Adv. Phys. 64, 139 (2015) |

Each seminar starts

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 |

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) . |