Stability and control of topological nature of quasiparticles emerging in quantum spin liquids

• Project Area: Frontiers of materials science spun from topology
• Project/Area Number: 18H04223
• Research Category: Grant-in-Aid for Scientific Research on Innovative Areas (Research in a proposed research area)
• Allocation Type: Single-year Grants
• Research Institution: Yokohama National University
• Principal Investigator: 那須 譲治 横浜国立大学, 大学院工学研究院, 准教授 (40610639)
• Project Period (FY): 2018-04-01 – 2020-03-31
• Project Status: Completed (Fiscal Year 2019)
• Budget Amount: ¥3,900,000 (Direct Cost: ¥3,000,000、Indirect Cost: ¥900,000); Fiscal Year 2019: ¥1,950,000 (Direct Cost: ¥1,500,000、Indirect Cost: ¥450,000); Fiscal Year 2018: ¥1,950,000 (Direct Cost: ¥1,500,000、Indirect Cost: ¥450,000)
• Keywords: 量子スピン液体 / トポロジカル量子物性 / トポロジカル物性

Outline of Annual Research Achievements

本年度では、量子スピン液体において現れる創発準粒子を制御する方法を理論提案することを目的に研究を進めてきた。それを達成するため具体的にはキタエフ量子スピン液体を考える。そこで現れる創発準粒子は磁場で制御できると考えられるため、磁場効果を詳しく調べた。低磁場領域では、マヨラナ準粒子を素励起に持つ量子スピン液体が実現するが、磁場を強くしていくと、スピン波であるマグノンを素励起に持つ強制強磁性状態へと変化する。これらがどのように繋がるかを解析するために、量子モンテカルロ法を主に用いて、厳密対角化法、スピン波近似法などを併用することで、素励起のダイナミクスを計算した。
さらに、創発準粒子を実験的に捉えるために、実験の研究グループとの共同研究も行った。例えば、ラマン散乱を用いることで、ストークス及びアンチストークス散乱スペクトルからからマグノン励起とは全く異なる散乱強度のエネルギー及び温度依存性が現れることがわかった。この結果は、キタエフ量子スピン液体を実現する候補物質である塩化ルテニウムにおいてマヨラナ準粒子の存在を示唆するものと考えられる。
また、核磁気共鳴実験においても、塩化ルテニウムにおいて緩和率に2種類のギャップの存在を示唆する結果が得られており、この結果は、スピンが2種類の準粒子に分数化していることを反映していることが期待される。
これらの研究を通して、創発準粒子を観測する方法に関しては明らかになりつつあるが、制御する方法については、まだ研究途上であるといわざるを得ないので、ここで得られた成果をもとに、準粒子を制御する実現可能な方法の確立を目指して研究を継続していく。

Research Progress Status

令和元年度が最終年度であるため、記入しない。

Strategy for Future Research Activity

令和元年度が最終年度であるため、記入しない。

Research Products

• [Int'l Joint Research] University of Tennessee/Oak Ridge National Laboratory/Boston College(米国)
• [Int'l Joint Research] Imperial College London(英国)
• [Journal Article] Hunting Majorana Fermions in Kitaev Magnets (2020)
  • Author(s): Motome Yukitoshi、Nasu Joji
  • Journal Title: Journal of the Physical Society of Japan Volume: 89 Issue: 1 Pages: 012002-012002
  • DOI: 10.7566/jpsj.89.012002
  • NAID: 210000157849
  • Peer Reviewed / Open Access
• [Journal Article] Two-step gap opening across the quantum critical point in the Kitaev honeycomb magnet α?RuCl3 (2020)
  • Author(s): Nagai Yuya、Jinno Takaaki、Yoshitake Junki、Nasu Joji、Motome Yukitoshi、Itoh Masayuki、Shimizu Yasuhiro
  • Journal Title: Physical Review B Volume: 101 Issue: 2 Pages: 020414-020414
  • DOI: 10.1103/physrevb.101.020414
  • Peer Reviewed / Int'l Joint Research
• [Journal Article] Spin transport in the Quantum Spin Liquid State in the S=1 Kitaev model: role of the fractionalized quasiparticles (2020)
  • Author(s): A. Koga, T. Minakawa, Y. Murakami, and J. Nasu
  • Journal Title: J. Phys. Soc. Jpn. Volume: 89 Issue: 3 Pages: 33701-33701
  • DOI: 10.7566/jpsj.89.033701
  • NAID: 40022185595
  • Peer Reviewed
• [Journal Article] The range of non-Kitaev terms and fractional particles in α-RuCl3 (2020)
  • Author(s): Wang Yiping、Osterhoudt Gavin B.、Tian Yao、Lampen-Kelley Paige、Banerjee Arnab、Goldstein Thomas、Yan Jun、Knolle Johannes、Ji Huiwen、Cava Robert J.、Nasu Joji、Motome Yukitoshi、Nagler Stephen E.、Mandrus David、Burch Kenneth S.
  • Journal Title: npj Quantum Materials Volume: 5 Issue: 1 Pages: 14-14
  • DOI: 10.1038/s41535-020-0216-6
  • Peer Reviewed / Open Access / Int'l Joint Research
• [Journal Article] Majorana-magnon crossover by a magnetic field in the Kitaev model: Continuous-time quantum Monte Carlo study (2020)
  • Author(s): Yoshitake Junki、Nasu Joji、Kato Yasuyuki、Motome Yukitoshi
  • Journal Title: Physical Review B Volume: 101 Issue: 10 Pages: 100408-100408
  • DOI: 10.1103/physrevb.101.100408
  • Peer Reviewed
• [Journal Article] Low temperature properties in the Bilayer Kitaev model (2019)
  • Author(s): H. Tomishige, J. Nasu, and A. Koga
  • Journal Title: Phys. Rev. B Volume: 99 Issue: 17 Pages: 1744241-5
  • DOI: 10.1103/physrevb.99.174424
  • Peer Reviewed
• [Journal Article] Ferromagnetic Instability for single-band Hubbard model in the strong-coupling regime (2019)
  • Author(s): Y. Kamogawa, J. Nasu, and A. Koga
  • Journal Title: Phys. Rev. B Volume: 99 Issue: 23 Pages: 2351071-6
  • DOI: 10.1103/physrevb.99.235107
  • Peer Reviewed
• [Journal Article] Residual entropy and spin fractionalizations in the mixed-spin Kitaev model (2019)
  • Author(s): A. Koga and J. Nasu
  • Journal Title: Phys. Rev. B Volume: 100 Issue: 10 Pages: 1004041-5
  • DOI: 10.1103/physrevb.100.100404
  • Peer Reviewed
• [Journal Article] Nonequilibrium Majorana dynamics by quenching a magnetic field in Kitaev spin liquids (2019)
  • Author(s): Nasu Joji、Motome Yukitoshi
  • Journal Title: Physical Review Research Volume: 1 Issue: 3 Pages: 033007-033007
  • DOI: 10.1103/physrevresearch.1.033007
  • Peer Reviewed / Open Access
• [Journal Article] Symmetry breaking states in the half-filled two-orbital Hubbard model with crystalline electric field (2019)
  • Author(s): K. Ishigaki, J. Nasu, and A. Koga
  • Journal Title: J. Phys. Soc. Jpn. Volume: 88 Issue: 2 Pages: 0247021-5
  • DOI: 10.7566/jpsj.88.024702
  • NAID: 210000135099
  • Peer Reviewed
• [Journal Article] Staggered ordered phases in the three-orbital Hubbard model (2019)
  • Author(s): K. Ishigaki, J. Nasu, A. Koga, S. Hoshino, and P. Werner
  • Journal Title: Phys. Rev. B Volume: 99 Issue: 8 Pages: 0851311-11
  • DOI: 10.1103/physrevb.99.085131
  • Peer Reviewed / Int'l Joint Research
• [Journal Article] Quantum and classical behavior of spin-S Kitaev models in the anisotropic limit (2019)
  • Author(s): Minakawa Tetsuya、Nasu Joji、Koga Akihisa
  • Journal Title: Physical Review B Volume: 99 Issue: 10 Pages: 104408-104408
  • DOI: 10.1103/physrevb.99.104408
  • Peer Reviewed
• [Journal Article] Ground-state and thermodynamic properties of an S=1 Kitaev model (2018)
  • Author(s): A. Koga, H. Tomishige, and J. Nasu
  • Journal Title: J. Phys. Soc. Jpn. Volume: 87 Issue: 6 Pages: 0637031-4
  • DOI: 10.7566/jpsj.87.063703
  • NAID: 210000134846
  • Peer Reviewed
• [Journal Article] Unusual thermal Hall effect in a Kitaev spin liquid candidate α-RuCl3 (2018)
  • Author(s): Y. Kasahara, K. Sugii, T. Ohnishi, M. Shimozawa, M. Yamashita, N. Kurita, H. Tanaka, J. Nasu, Y. Motome, T. Shibauchi, and Y. Matsuda
  • Journal Title: Phys. Rev. Lett. Volume: 120 Issue: 21 Pages: 217205-217205
  • DOI: 10.1103/physrevlett.120.217205
  • Peer Reviewed
• [Journal Article] Magnetoelectric Effect in a Spin-State Transition System (2018)
  • Author(s): Naka Makoto、Mizoguchi Eriko、Nasu Joji、Ishihara Sumio
  • Journal Title: Journal of the Physical Society of Japan Volume: 87 Issue: 6 Pages: 063709-063709
  • DOI: 10.7566/jpsj.87.063709
  • NAID: 210000134852
  • Peer Reviewed
• [Journal Article] Majorana quantization and half-integer thermal quantum Hall effect in a Kitaev spin liquid (2018)
  • Author(s): Kasahara Y.、Ohnishi T.、Mizukami Y.、Tanaka O.、Ma Sixiao、Sugii K.、Kurita N.、Tanaka H.、Nasu J.、Motome Y.、Shibauchi T.、Matsuda Y.
  • Journal Title: Nature Volume: 559 Issue: 7713 Pages: 227-231
  • DOI: 10.1038/s41586-018-0274-0
  • NAID: 120006497340
  • Peer Reviewed
• [Journal Article] Successive Majorana topological transitions driven by a magnetic field in the Kitaev model (2018)
  • Author(s): Nasu Joji、Kato Yasuyuki、Kamiya Yoshitomo、Motome Yukitoshi
  • Journal Title: Physical Review B Volume: 98 Issue: 6 Pages: 060416-060416
  • DOI: 10.1103/physrevb.98.060416
  • Peer Reviewed
• [Journal Article] Spontaneously orbital-selective superconductivity in a three-orbital Hubbard model (2018)
  • Author(s): K. Ishigaki, J. Nasu, A. Koga, S. Hoshino, and P. Werner
  • Journal Title: Phys. Rev. B Volume: 98 Issue: 23 Pages: 2351201-7
  • DOI: 10.1103/physrevb.98.235120
  • Peer Reviewed / Int'l Joint Research
• [Presentation] Nonequilibrium dynamics induced by magnetic-field quench in Kitaev spin liquids with fractional Majorana excitations (2019)
  • Author(s): J. Nasu and Y. Motome
  • Organizer: International Conference on Strongly Correlated Electron Systems 2019
  • Int'l Joint Research
• [Presentation] Magnetic-field effect and transport properties in Kitaev spin liquids (2019)
  • Author(s): J. Nasu
  • Organizer: 3rd EPiQS-TMS alliance workshop on Topological Phenomena in Quantum Materials (TPQM2019)
  • Int'l Joint Research / Invited
• [Presentation] Nonequilibrium dynamics in Kitaev spin liquids with fractional Majorana quasiparticles (2019)
  • Author(s): J. Nasu
  • Organizer: The 3rd Asia-Pacific workshop on Quantum Magnetism
  • Int'l Joint Research / Invited
• [Presentation] Nonequilibrium dynamics of Kitaev spin liquids with fractional Majorana excitations (2019)
  • Author(s): J. Nasu
  • Organizer: International Conference on Topological Material Science 2019
  • Int'l Joint Research
• [Presentation] Non-equilibrium dynamics by field quench in Kitaev model (2019)
  • Author(s): J. Nasu and Y. Motome
  • Organizer: Symposium: Topological Phases and Functionality of Correlated Electron Systems (2019)
  • Int'l Joint Research
• [Presentation] キタエフ模型における磁場効果 (2019)
  • Author(s): 那須譲治
  • Organizer: 第4回TMS領域研究会
  • Invited
• [Presentation] キタエフ量子スピン液体におけるマヨラナ準粒子 (2019)
  • Author(s): 那須譲治
  • Organizer: 量子多体系の素核・物性クロスオーバー
  • Invited
• [Presentation] Majorana topological transition driven by a magnetic field in the Kitaev model (2018)
  • Author(s): J. Nasu
  • Organizer: TMS-PKU 2nd Alliance Workshop
  • Int'l Joint Research / Invited
• [Presentation] Kitaevスピン液体の有限温度ダイナミクスと輸送現象 (2018)
  • Author(s): 那須譲治
  • Organizer: 基研研究会「スピン系物理の最前線」
  • Invited