KEIO RESEARCHERS INFORMATION SYSTEM

Career 【 Display / hide 】

Career 【 Display / hide 】

• 2007.04

  -

  2009.08

  RIKEN, Condensed Matter Theory Laboratory, Special Postdoctoral Researcher

• 2009.09

  -

  2011.06

  University of Toronto, Department of Physics, Postdoctoral Fellow

• 2011.07

  -

  2012.08

  University of Tokyo, Department of Physics, Postdoctoral Researcher

• 2012.08

  -

  2019.03

  University of Tokyo, Department of Physics, Assistant Professor

• 2019.04

  -

  Present

  Keio University, Department of Physics, Assistant Professor

Academic Background 【 Display / hide 】

Academic Background 【 Display / hide 】

• 1999.04

  -

  2002.03

  Tokyo Institute of Technology, School of Science, Department of Physics

  University, Skipped grade(s)

• 2002.04

  -

  2004.03

  Tokyo Institute of Technology, Graduate School of Science, Department of Physics

  Graduate School, Completed, Master's course

• 2004.04

  -

  2007.03

  Tokyo Institute of Technology, Graduate School of Science, Department of Physics

  Graduate School, Completed, Doctoral course

Academic Degrees 【 Display / hide 】

Academic Degrees 【 Display / hide 】

• 博士（理学）, Tokyo Institute of Technology, Coursework, 2007.03

  Characterization of Exotic Orders in Quantum Spin Systems

Research Areas 【 Display / hide 】

Research Areas 【 Display / hide 】

• Natural Science / Mathematical physics and fundamental theory of condensed matter physics

• Natural Science / Semiconductors, optical properties of condensed matter and atomic physics

Research Keywords 【 Display / hide 】

Research Keywords 【 Display / hide 】

• topological states of matter

• ultracold atomic systems

• magnetism

Research Themes 【 Display / hide 】

Research Themes 【 Display / hide 】

• Competition of topological and ordered phases in spin ladder systems, 

  2022.07

  -

  Present

• Vortex lattices in Bose-Einstein condensates in synthetic gauge fields, 

  2017.01

  -

  2022.05

• Quantum critical phenomena in non-Hermitian quantum many-body systems, 

  2015.12

  -

  Present

• Theory of frustrated magnetism of volborthite based on a coupled-trimer model, 

  2015.03

  -

  Present

• Quantum Hall physics of ultracold atomic systems in synthetic gauge fields, 

  2011.07

  -

  Present

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Papers 【 Display / hide 】

Papers 【 Display / hide 】

• Ground-state phase diagram of a spin- 12 frustrated XXZ ladder

  Ogino T., Kaneko R., Morita S., Furukawa S.

  Physical Review B （Physical Review B)  106 （ 15 ）  2022.10

  ISSN  24699950

  　View Summary

  We study the ground-state phase diagram of a spin-12 frustrated XXZ ladder, in which two antiferromagnetic chains are coupled by competing rung and diagonal interactions, J⊥ and J×. Previous studies on the isotropic model have revealed that a fluctuation-induced effective dimer attraction between the legs stabilizes the columnar dimer (CD) phase in the highly frustrated regime J⊥≈2J×, especially for ferromagnetic J⊥,×<0. By means of effective field theory and numerical analyses, we extend this analysis to the XXZ model, and we obtain a rich phase diagram. The diagram includes four gapped featureless phases with no symmetry breaking: the rung singlet (RS) and Haldane phases as well as their twisted variants, the RS∗ and Haldane∗ phases, which are all distinct in the presence of certain symmetries. Significantly, the Haldane-CD transition point in the isotropic model turns out to be a crossing point of two transition lines in the XXZ model, and the stripe Néel and RS∗ phases appear between these lines. This indicates a nontrivial interplay between the effective dimer attraction and the exchange anisotropy. In the easy-plane regime, the four featureless phases and two critical phases are found to compete in a complex manner depending on the signs of J⊥,×.

  • Access to Document (DOI)

• Vortex lattices in binary Bose–Einstein condensates: collective modes, quantum fluctuations, and intercomponent entanglement

  Takumi Yoshino and Shunsuke Furukawa and Masahito Ueda

  Journal of Physics B: Atomic, Molecular and Optical Physics （IOP Publishing)  55 （ 10 ）  2022.05

  ISSN  0953-4075

  　View Summary

  We study binary Bose-Einstein condensates subject to synthetic magnetic fields in mutually parallel or antiparallel directions. Within the mean-field theory, the two types of fields have been shown to give the same vortex-lattice phase diagram. We develop an improved effective field theory to study properties of collective modes and ground-state intercomponent entanglement. Here, we point out the need to introduce renormalized coupling constants for coarse-grained densities. We show that the low-energy excitation spectra for the two types of fields are related to each other by suitable rescaling with the renormalized coupling constants. By calculating the entanglement entropy, we find that for an intercomponent repulsion (attraction), the two components are more strongly entangled in the case of parallel (antiparallel) fields, in qualitative agreement with recent studies for a quantum (spin) Hall regime. We also find that the entanglement spectrum exhibits an anomalous square-root dispersion relation, which leads to a subleading logarithmic term in the entanglement entropy. All of these are confirmed by numerical calculations based on the Bogoliubov theory with the lowest-Landau-level approximation. Finally, we investigate the effects of quantum fluctuations on the phase diagrams by calculating the correction to the ground-state energy due to zero-point fluctuations in the Bogoliubov theory. We find that the boundaries between rhombic-, square-, and rectangular-lattice phases shift appreciably with a decrease in the filling factor.

  • Access to Document (DOI)

• Symmetry-protected topological phases and competing orders in a spin- 12 XXZ ladder with a four-spin interaction

  Ogino Takuhiro, Furukawa Shunsuke, Kaneko Ryui, Morita Satoshi, Kawashima Naoki

  Physical Review B （Physical Review B)  104 （ 7 ）  2021.08

  ISSN  2469-9950

  　View Summary

  We study a spin-12 XXZ model with a four-spin interaction on a two-leg ladder. By means of effective field theory and matrix product state calculations, we obtain rich ground-state phase diagrams that consist of eight distinct gapped phases. Four of them exhibit spontaneous symmetry breaking with either a magnetic or valence-bond-solid (VBS) long-range order. The other four are featureless, i.e., the bulk ground state is unique and does not break any symmetry. The featureless phases include the rung singlet (RS) and Haldane phases as well as their variants, the RS∗ and Haldane∗ phases, in which twisted singlet pairs (|↑↓)+|↓↑))/2 are formed between the two legs. We argue and demonstrate that Gaussian transitions with the central charge c=1 occur between the featureless phases and between the ordered phases while Ising transitions with c=1/2 occur between the featureless and ordered phases. The two types of transition lines cross at the SU(2)-symmetric point, where the criticality is described by the SU(2)2 Wess-Zumino-Witten theory with c=3/2. The RS-Haldane∗ and RS∗-Haldane transitions give examples of topological phase transitions. Interestingly, the RS∗ and Haldane∗ phases, which have highly anisotropic nature, appear even in the vicinity of the isotropic case. We demonstrate that all the four featureless phases are distinguished by topological indices in the presence of certain symmetries.

  • Access to Document (DOI)

• Intercomponent entanglement entropy and spectrum in binary Bose-Einstein condensates

  Yoshino T., Furukawa S., Ueda M.

  Physical Review A （Physical Review A)  103 （ 4 ）  2021.04

  ISSN  2469-9926

  　View Summary

  We study the entanglement entropy and spectrum between components in binary Bose-Einstein condensates in d spatial dimensions. We employ effective field theory to show that the entanglement spectrum exhibits an anomalous square-root dispersion relation in the presence of intercomponent tunneling (Rabi coupling) and a gapped dispersion relation in its absence. These spectral features are associated with the emergence of long-range interactions in terms of the superfluid velocity and the particle density in the entanglement Hamiltonian. Our results demonstrate that unusual long-range interactions can be emulated in a subsystem of multicomponent BECs that have only short-range interactions. We also find that for a finite Rabi coupling the entanglement entropy exhibits a volume-law scaling with subleading logarithmic corrections originating from the Nambu-Goldstone mode and the symmetry restoration for a finite volume.

  • Access to Document (DOI)

• Continuous phase transition between Néel and valence bond solid phases in a J−Q -like spin ladder system

  Ogino Takuhiro, Kaneko Ryui, Morita Satoshi, Furukawa Shunsuke, Kawashima Naoki

  Physical Review B （Physical Review B)  103 （ 8 ）  2021.02

  ISSN  2469-9950

  　View Summary

  We investigate a quantum phase transition between a Néel phase and a valence bond solid (VBS) phase, each of which breaks a different Z2 symmetry, in a spin-1/2 two-leg XXZ ladder with a four-spin interaction. The model can be viewed as a one-dimensional variant of the celebrated J-Q model on a square lattice. By means of variational uniform matrix product state calculations and an effective field theory, we determine the phase diagram of the model and present evidence that the Néel-VBS transition is continuous and belongs to the Gaussian universality class with the central charge c=1. In particular, the critical exponents β,η, and, ν are found to satisfy the constraints expected for a Gaussian transition within numerical accuracy. These exponents do not detectably change along the phase boundary while they are in general allowed to do so for the Gaussian class.

  • Access to Document (DOI)

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Reviews, Commentaries, etc. 【 Display / hide 】

Reviews, Commentaries, etc. 【 Display / hide 】

• Entanglement entropy and conformal field theory

  S. Furukawa, J. Shiraishi

  BUTSURI 69   541 - 546 2014

  Article, review, commentary, editorial, etc. (scientific journal), Joint Work

  • Access to Document (DOI)

Presentations 【 Display / hide 】

Presentations 【 Display / hide 】

• Magnetic properties of volborthite determined by a coupled-trimer model

  S. Furukawa and T. Momoi

  International Workshop on Microscopic Properties of Quantum Materials (Hokkaido University) , 

  2022.08

  , 

  Oral presentation (invited, special)

• Theory of volborthite based on a coupled trimer model

  Shunsuke Furukawa

  17th Quantum Spin Workshop (沖縄県那覇市ともかぜ振興会館およびオンライン) , 

  2022.04

  , 

  Oral presentation (general)

• 強磁性・反強磁性相互作用の競合する量子磁性体の理論的研究

  古川俊輔

  強磁場コラボラトリーセミナー（強磁場フォーラム フロンティア奨励賞 受賞講演） (東京大学 物性研究所) , 

  2022.03

  , 

  Public lecture, seminar, tutorial, course, or other speech

• Quantum entanglement in topological phases and symmetry-broken phases

  古川俊輔

  Quantum Information Entropy in Physics (京都大学 基礎物理学研究所) , 

  2022.03

  , 

  Oral presentation (invited, special)

  　View Summary

  I will review some applications of entanglement entropy (EE) and entanglement spectra (ES) in quantum many-body systems. In a variety of topological phases, it has been found that the ES has a similar structure as the edge-mode spectrum. The "cut and glue" picture gives a physical explanation of this remarkable entanglement-edge correspondence [1]. I will explain a simple formulation of this picture based on two coupled (chiral) Tomonaga-Luttinger liquids [2]. The obtained ES can be used to derive the so-called topological entropy. In systems with continuous symmetry breaking, the EE shows a subleading logarithmic term whose coefficient is related to the number of Nambu-Goldstone modes [3]. Furthermore, the ES shows a "tower of states" structure reminiscent of the finite-size energy spectrum. I will explain the emergence of analogous features in the intercomponent entanglement in binary Bose-Einstein condensates [4].
  
  [1] X.-L. Qi, H. Katsura, and A.W.W. Ludwig, Phys. Rev. Lett. 108, 196402 (2012).
  [2] R. Lundgren, Y. Fuji, S. Furukawa, and M. Oshikawa, Phys. Rev. B 88, 245137 (2013).
  [3] M. A. Metlitski and T. Grover, arXiv:1112.5166.
  [4] T. Yoshino, S. Furukawa, and M. Ueda, Phys. Rev. A 103, 043321 (2021).

• Ground-state phase diagram of a spin-1/2 frustrated XXZ ladder

  T. Ogino, R. Kaneko, S. Morita, S. Furukawa

  日本物理学会2021年秋季大会 (オンライン開催) , 

  2021.09

  , 

  Oral presentation (general)

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Research Projects of Competitive Funds, etc. 【 Display / hide 】

Research Projects of Competitive Funds, etc. 【 Display / hide 】

• Theoretical study of many-body effects in Bose-Einstein condensates in synthetic gauge fields

  2019.04

  -

  2020.03

  Keio Gijuku, Academic Development Funds, Shunsuke Furukawa, Research grant, Principal investigator

• Symmetry-enriched topological phases: theoretical foundation and proposal for realization

  2018.04

  -

  2023.03

  MEXT,JSPS, Grant-in-Aid for Scientific Research, Grant-in-Aid for Scientific Research (C), Principal investigator

• Phase frustration in Bose-Einstein condensates in optical lattices

  2016.11

  -

  2019.03

  Matsuo Foundation, Shunsuke Furukawa, Research grant, Principal investigator

• Exploration of topological orders in strongly correlated ultracold atomic systems and development of their fundamental theory

  2013.04

  -

  2017.03

  The University of Tokyo, FURUKAWA SHUNSUKE, Grant-in-Aid for Young Scientists (B), No Setting

  　View Summary

  Topologically ordered states exhibit a number of intriguing properties such as quantized responses and quasiparticles obeying fractional statistics. In this study, we have theoretically explored novel topologically ordered states that can be realized in highly controlled ultracold atomic systems, and investigated their physical properties. In particular, we have determined detailed ground-state phase diagrams of two-component Bose gases in synthetic gauge fields, and revealed (i) the appearance of a bosonic integer quantum Hall state with unusual edge states and (ii) a nontrivial role of time-reversal symmetry in the interaction effect. We have also shown that Bose-Einstein condensate in optical lattices can host an excitation band with nontrivial topology, and that edge states originating from it can be observed by exploiting a macroscopic quantum interference.

Awards 【 Display / hide 】

Awards 【 Display / hide 】

• The 2nd High Magnetic Field Forum Frontier Incentive Award

  Shunsuke Furukawa, 2020.12, The High Magnetic Field Forum of Japan, Theoretical studies on quantum magnets with competing ferromagnetic and antiferromagnetic interactions

  Type of Award： Award from Japanese society, conference, symposium, etc.

  　View Description

  http://www.issp.u-tokyo.ac.jp/labs/mgsl/himag-forum/frontier.html

• The 7th Young Scientist Award of the Physical Society of Japan

  Shunsuke Furukawa, 2013.03, Physical Society of Japan, Entanglement entropy in quantum many-body systems

  Type of Award： Award from Japanese society, conference, symposium, etc.

  　View Description

  https://www.jps.or.jp/activities/awards/jusyosya/wakate2013.html#r11
  http://www.r11.div.jps.or.jp/wakate07.html
  https://ss.scphys.kyoto-u.ac.jp/TQP/achievements/newsletters.html#no3

Courses Taught 【 Display / hide 】

Courses Taught 【 Display / hide 】

• PHYSICS EXERCISE 1

  2022

• PHYSICS B

  2022

• PHYSICS A

  2022

• MATHEMATICS IN PHYSICS

  2022

• LITERATURE OF PHYSICS

  2022

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Memberships in Academic Societies 【 Display / hide 】

Memberships in Academic Societies 【 Display / hide 】

• Physical Society of Japan, 

  2004.01

  -

  Present

Committee Experiences 【 Display / hide 】

Committee Experiences 【 Display / hide 】

• 2022.04

  -

  2023.03

  新著紹介小委員会 委員長（兼 会誌編集委員）, 日本物理学会

• 2021.04

  -

  2022.03

  新著紹介小委員会 委員, 日本物理学会

• 2020.04

  -

  2021.03

  領域３（磁性）運営委員, 日本物理学会