Office address: A310, Institute for Solid State Physics, University of Tokyo, Chiba, 277-8581, Japan
Email: rokuma@issp.u-tokyo.ac.jp
Current position: Research Associate in Prof. Yoshihiko Okamoto's group
D.Sci. Apr. 2016 - Mar. 2019, University of Tokyo, Tokyo, Japan (advisor: Prof. Zenji Hiroi)
M.Sci. Apr. 2014 - Mar. 2016, Univeristy of Tokyo, Tokyo, Japan (graduated with honor, advisor: Prof. Zenji Hiroi)
B.A. Apr. 2010 - Mar. 2014, University of Tokyo, Tokyo, Japan
Apr. 2023 - present, Research Associate, Institute for Solid State Physics, University of Tokyo, Chiba, Japan
Oct. 2020 - Mar. 2023, Postdoctoral Research Assistant, University of Oxford, Oxford, UK
Apr. 2019 - Sep. 2020, Postdoctoral Scholar, Okinawa Institute of Science and Technology, Okinawa, Japan
2014-2019 Materials Education program for the future leaders in Research, Industry, and Technology, \10800000 JPY
2023-2024 Grant-in-Aid for Research Activity (KAKENHI) Start-up " Topological magnetism realized by excitonic instability" \2200,000 JPY
Best Poster Award in the 16th International Conference on Megagauss Magnetic Field Generation and Related Topics, Kashiwa, Japan, September 2018
Poster Award in FYR03 QLC meeting, online, February 2022
Leonie Woodland, Ryutaro Okuma, J. Ross Stewart, Christian Balz, Radu Coldea "From continuum excitations to sharp magnons via transverse magnetic field in the spin-1/2 Ising-like triangular lattice antiferromagnet Na2BaCo(PO4)2." arXiv:2505.06398 (accepted in PRB available from https://doi.org/10.1103/1pvl-kzjm )
R. Okuma, K. Yamagami, Y. Fujisawa, C. H. Hsu, Y. Obata, N. Tomoda, M. Dronova, K. Kuroda, H. Ishikawa, K. Kawaguchi, K. Aido, K. Kindo, Y. H. Chan, H. Lin, Y. Ihara, T. Kondo, Y. Okada "Emergent topological magnetism in Hund's excitonic insulator." arXiv:2405.16781
Ryutaro Okuma, Kylie MacFarquharson, Roger D. Johnson, David Voneshen, Pascal Manuel & Radu Coldea "Compass-model physics on the hyperhoneycomb lattice in the extreme spin-orbit regime." Nature Communications, 15, 10615 (2024).
Ryutaro Okuma, Daisuke Nakamura, Tsuyoshi Okubo, Atsushi Miyake, Akira Matsuo, Koichi Kindo, Masashi Tokunaga, Naoki Kawashima, Shojiro Takeyama, & Zenji Hiroi, “A series of magnon crystals appearing under ultrahigh magnetic fields in a kagomé antiferromagnet.” Nature Communications 10, 1229 (2019).
Ryutaro Okuma, Maiko Kofu, Shinichiro Asai, Maxim Avdeev, Akihiro Koda, Hirotaka Okabe, Masatoshi Hiraishi, Soshi Takeshita, Kenji M Kojima, Ryosuke Kadono, Takatsugu Masuda, Kenji Nakajima, & Zenji Hiroi, “Dimensional reduction by geometrical frustration in a cubic antiferromagnet composed of tetrahedral clusters.” Nature Communications 12, 4382 (2021).
Ryutaro Okuma, Takeshi Yajima, Daisuke Nishio-Hamane, Tsuyoshi Okubo, & Zenji Hiroi, “Weak ferromagnetic order breaking the threefold rotational symmetry of the underlying kagome lattice in CdCu3(OH)6(NO3)2•H2O.”Physical Review B 95, 094427 (2017).
Ryutaro Okuma, Takeshi Yajima, Tatsuo Fujii, Mikio Takano, & Zenji Hiroi, “Frustrated magnetism of pharmacosiderite comprising tetrahedral clusters arranged in the primitive cubic lattice.” Journal of the Physical Society of Japan 87, 093702 (2018).
Masatoshi Akazawa, Masaaki Shimozawa, Shunichiro Kittaka, Toshiro Sakakibara, Ryutaro Okuma, Zenji Hiroi, Hyun-Yong Lee, Naoki Kawashima, Jung Hoon Han, & Minoru Yamashita, “Thermal hall effects of spins and phonons in kagome antiferromagnet cd-kapellasite.” Physical Review X 10, 041059 (2020).
Ryutaro Okuma, Daisuke Nakamura, & Shojiro Takeyama, “Magnetization plateau observed by ultrahigh-field Faraday rotation in the kagome antiferromagnet herbertsmithite.” Physical Review B 102, 104429 (2020).
R. Okuma, D. Ueta, S. Kuniyoshi, Y. Fujisawa, B. Smith, C. H. Hsu, Y. Inagaki, W. Si, T. Kawae, H. Lin, F. C. Chuang, T. Masuda, R. Kobayashi, & Y. Okada, “Fermionic order by disorder in a van der Waals antiferromagnet.” Scientific Reports 10, 15311 (2020).
Ryutaro Okuma, Clemens Ritter, Gøran J. Nilsen, & Yoshinori Okada “Magnetic frustration in a van der Waals metal CeSiI.” Physical Review Materials 5, L121401 (2021).
Gøran J. Nilsen, Virginie Simonet, Claire V. Colin, Ryutaro Okuma, Yoshihiko Okamoto, Masashi Tokunaga, Thomas C. Hansen, Dmitry D. Khalyavin, & Zenji Hiroi, “Phase diagram of multiferroic KCu3As2O7(OD)3.” Physical Review B 95, 214415 (2017).
D. Nishio–Hamane, K. Momma, M. Ohnishi, N. Shimobayashi, R. Miyawaki, N. Tomita, R. Okuma, A. R. Kampf, & T. Minakawa, “Iyoite, MnCuCl(OH)3 and misakiite, Cu3Mn(OH)6Cl2: new members of the atacamite family from Sadamisaki Peninsula, Ehime Prefecture, Japan.” Mineralogical Magazine 81, 485-498 (2017).
Yoshihiko Ihara, Ryoga Hiyoshi, Masakazu Shimohasi, Kaoru Hayashi, Ryutaro Okuma, Fabio Orlandi, Gøran Nilsen, & Zenji Hiroi, “Negative-chirality order in S = 1/2 kagome antiferromagnet CdCu3(OH)6(NO3)2•H2O.” Physical Review B 106, 024401 (2022). (Editor’s suggestion)
H. Murayama, T. Tominaga, T. Asaba, A. de Oliveira Silva, Y. Sato, H. Suzuki, Y. Ukai, S. Suetsugu, Y. Kasahara, R. Okuma, I. Kimchi, & Y. Matsuda, “Universal scaling of specific heat in S = 1/2 quantum kagome antiferromagnet herbertsmithite.” Physical Review B 106, 174406 (2022).
Haruka Matsumoto, Youichi Yamakawa, Ryutaro Okuma, Daisuke Nishio-Hamane, Yoshihiko Okamoto, "Superconductivity in Ternary Zirconium Telluride Zr6MTe2 with 3d Transition Metals." Journal of Physical Society of Japan 93, 023705 (2024).
Ryutaro Okuma, Kylie MacFarquharson, and Radu Coldea "Selective Synthesis and Crystal Chemistry of Candidate Rare-Earth Kitaev Materials: Honeycomb and Hyperhoneycomb Na2PrO3." Inorganic Chemistry, 63, 15941–15950 (2024).
Toshiya Ikenobe, Takahiro Yamada, Jun-ichi Yamaura, Tamio Oguchi, Ryutaro Okuma, Daigorou Hirai, Hajime Sagayama, Yoshihiko Okamoto, Zenji Hiroi, “Topological Semimetal KAlGe with Novel Electronic Instability.” Chemistry of Materials 37, 189–197 (2025).
Kosuke Yuchi, Daisuke Nishio-Hamane, Keita Kojima, Kodai Moriyama, Ryutaro Okuma, and Yoshihiko Okamoto," Superconductivity in Hexagonal Zr6CoAl2-Type Zr6RuBi2 and Zr6FeBi2." Journal of Physical Society of Japan 94, 013701 (2025).
Ryutaro Okuma, “Dimensional reduction by geometrical frustration in a cubic antiferromagnet composed of tetrahedral clusters.” 11th International Conference on Highly Frustrated Magnetism 2022 (HFM22), Paris, June 2022. (Invited)
Ryutaro Okuma, “Discovery of frustrated van der Waals metals CeSiI and GdGaI.” The 7th QLC Young Colloquium, online, May 2022.
Ryutaro Okuma, “Magnon crystallization in kagome antiferromagnets.” waiting for the conference on Highly Frustrated Magnetism (wHFM21), online, January 2021.
Ryutaro Okuma, Daisuke Nakamura, Tsuyoshi Okubo, Atsushi Miyake, Akira Matsuo, Koichi Kindo, Masashi Tokunaga, Naoki Kawashima, Shojiro Takeyama, & Zenji Hiroi, ”A series of magnon crystals appearing under ultrahigh magnetic fields in a kagome antiferromagnet.” American Physical Society March Meeting 2021, online, March 2021.
Ryutaro Okuma, Kylie J. MacFarquharson, Roger D. Johnson, David J. Voneshen, Pascal Manuel, Radu Coldea "Noncollinear magnetism from frustrated bond-dependent anisotropic exchange on the f-electron hyperhoneycomb lattice in β-Na2PrO3." American Physical Society March Meeting 2024, Minneapolis, US, March 2024.
Ryutaro Okuma, "Itinerant frustration in van der Waals antiferromagnets" New Frontiers in Advanced Magnetism (NFAM), Sapporo, Japan, August 2024 (Invited).
Ryutaro Okuma, "Compass model physics on the hyperhoneycomb lattice antiferromagnet β-Na2PrO3." Correlated Quantum Materials + beyond: Symposium, Kashiwa, Japan, November 2024 (Invited).
Ryutaro Okuma, Kylie MacFarquharson, Roger Johnson, David Voneshen, Pascal Manuel, Radu Coldea, "Compass-model physics on the hyperhoneycomb lattice in the extreme spin-orbit regime." International Conference on Highly Frustrated Magnetism, Toronto, Canada, May 2025 (Invited).
Ryutaro Okuma, Atsushi Miyake, Daisuke Nakamura, Yoshimitsu Kohama, Tsuyoshi Okubo, Masashi Tokunaga, Matsuo Akira, Koichi Kindo, Shojiro Takeyama, and Zenji Hiroi, ”Negative" vector chirality order breaking the threefold rotation symmetry of the kagomé lattice in CdCu3(OH)6(NO3)2•H2O.” 8th International Conference on Highly Frustrated Magnetism, 0606, Taiwan, June 2016.
Ryutaro Okuma, Maiko Kofu, Shinichiro Asai, Seiko Ohira-Nakamura, Takatsugu Masuda, Kenji Nakajima, and Zenji Hiroi ”Frustrated magnetism of pharmacosiderite comprising tetrahedral clusters arranged in the primitive cubic lattice.” Polarised Neutrons for Condensed-Matter Investigations, UK, July 2018.
Ryutaro Okuma, Roger D. Johnson, Pascal Manuel, David Voneshen, and Radu Coldea, "Neutron scattering studies of the candidate Kitaev material Na2PrO3." International Conference on Quantum Liquid Crystals 2023, Japan, August 2023.
R. Okuma, K. Yamagami, Y. Fujisawa, Y. Ihara, Y. Okada "Tetrahedral magnetic order by excitonic instability in a van der Waals coupled semiconductor GdGaI." HFM 2024 Chennai, India, January 2024.
Various single crystal growth of inorganic materials by low/high temperature solution and chemical transport method
Physicial property measurement of extremely air-sensitive materials
Powder and single crystal X-ray and neutron diffraction for crystalline materials
Element analysis by SEM-EDX and ICP-AES
Physical property measurement by MPMS and PPMS (magnetization, torque magnetometry, heat capacity, and resistivity)
(Magnetic) Rietveld refinement from X-ray and neutron diffraction data by Fullprof
Single crystal structure analysis by Shelx and Fullprof
Spin wave analysis by Spinw software