Details of the Researcher

PHOTO

Sota Yamamoto
Section
Graduate School of Engineering
Job title
Specially Appointed Assistant Professor(Research)
Degree

  • 博士（工学）（北海道大学）

  • 修士（工学）（北海道大学）

e-Rad No.
20991706

Research History 5

  • 2023/08 - Present
    Tohoku University　Graduate School of Engineering

  • 2022/07 - 2023/07
    TOME R&D Inc.

  • 2022/05 - 2022/06
    Hokkaido University　School of Engineering Department of Applied Science and Engineering

  • 2021/04 - 2022/03
    Japan Society for the Promotion of Science

  • 2020/04 - 2021/03
    Japan Society for the Promotion of Science

Education 3

  • Hokkaido University　Graduate School of Engineering　Division of Applied Physics

    2018/04 - 2021/03

  • Hokkaido University　Graduate School of Engineering　Division of Applied Physics

    2016/04 - 2018/03

  • Hokkaido University　School of Engineering　Department of Applied Science and Engineering

    2012/04 - 2016/03

Professional Memberships 1

  • 応用物理学会

Research Interests 2

  • 超微細相互作用

  • 半導体スピン物性

Research Areas 3

  • Nanotechnology/Materials / Applied materials /

  • Nanotechnology/Materials / Nanostructure physics /

  • Natural sciences / Semiconductors, optical and atomic physics /

Papers 12

  1. Effect of Oblique Magnetic Field on Dynamic Nuclear Polarization in Single Self‐Assembled Quantum Dots Governed by Nuclear Quadrupole Interaction

    Sota Yamamoto, Reina Kaji, Hirotaka Sasakura, Satoru Adachi

    physica status solidi (b)　2025/01/19

    Publisher: Wiley

    DOI： 10.1002/pssb.202400535 　

    ISSN： 0370-1972

    eISSN： 1521-3951

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    The dynamic nuclear spin polarization dominated by the nuclear quadrupole interaction has been studied in a single self‐assembled quantum dot. Spin transfer from an optically injected spin‐polarized electron to the lattice nuclear spin ensemble results in the nuclear spin polarization, which is probed by standard magnetophotoluminescence measurements under off‐resonant excitation. In an oblique magnetic field configuration, the observed electron spin depolarization curve, the Hanle curve, has a significantly different behavior compared to the previously reported anomalous Hanle curves obtained in a transverse magnetic field. In particular, an asymmetry in the Hanle curve depending on the sign of the external magnetic field indicates that the inclination of the magnetic field largely modifies the spin configurations during the dynamic nuclear spin polarization process. Numerical simulations based on a model in which the electron and nuclear spin states are described by the Bloch equation and the rate equation for the occupancy of nuclear spin eigenstates, respectively, reproduce the observations. This comparison supports the importance of the in‐plane component of the principal axis of the nuclear quadrupole interaction in generating the large in‐plane nuclear field, known as the anomalous Hanle effect.

  2. Reproduction of single-to-double hysteresis transition of nuclear spin polarization in a single InAlAs quantum dot Peer-reviewed

    Z.-R. Li, S. Yamamoto, S. Adachi, R. Kaji

    Journal of Applied Physics　136　(8)　2024/08/28

    Publisher: AIP Publishing

    DOI： 10.1063/5.0223272 　

    ISSN： 0021-8979

    eISSN： 1089-7550

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    We have observed a transition from single to double hysteresis of nuclear spin polarization (NSP) with increasing magnetic field in a single InAlAs self-assembled quantum dot, which means the birth of a third stable branch that depends on the field strength. This NSP transition behavior could not be reproduced by standard model calculations using a fixed electron–nuclear spin correlation time τc under different external field conditions. By introducing the field dependence of τc, we were able to successfully reproduce the observed NSP transition. This finding on the phenomenological description of the magnetic field dependence will be a future highlight of τc.

  3. Enhanced interlayer electron transfer by surface treatments in mixed-dimensional van der Waals semiconductor heterostructures Peer-reviewed

    Takeshi Odagawa, Sota Yamamoto, Chaoliang Zhang, Kazuki Koyama, Jun Ishihara, Giacomo Mariani, Yoji Kunihashi, Haruki Sanada, Junsaku Nitta, Makoto Kohda

    APL Materials　12　(6)　061120-1-061120-8　2024/06/01

    Publisher: AIP Publishing

    DOI： 10.1063/5.0214718 　

    eISSN： 2166-532X

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    We investigate the excitonic species in WS2 monolayers transferred onto III–V semiconductor substrates with different surface treatments. When the III–V substrates were covered with amorphous native oxides, negatively charged excitons dominated the spectral weight in low-temperature near-resonance photoluminescence (PL) measurements. However, when the native oxides of the III–V substrates were reduced, neutral excitons began to dominate the spectral weight, indicating a reduction in the electron density in the WS2 monolayers. The removal of the native oxides enhanced the electron transfer from the WS2 monolayer to the III–V substrate. In addition, an additional shoulder-like PL feature appeared ∼50 meV below the emission of neutral excitons, which can be attributed to the emission of localized excitons. When the III–V substrate surface was passivated by sulfur after the reduction of the native oxides, neutral excitons still dominated the spectral weight. However, the low-energy PL shoulder disappeared again, suggesting the effective delocalization of excitons through substrate surface passivation. Surface engineering of the semiconductor substrates for two-dimensional (2D) materials can provide a novel approach to control the carrier density of the 2D materials, implement deterministic carrier localization or delocalization for the 2D materials, and facilitate the interlayer transfer of charge, spin, and valley currents. These findings open the avenue for novel device concepts and phenomena in mixed-dimensional semiconductor heterostructures.

  4. Anomalous Hanle effect considered in time-resolved measurements and numerical simulations Peer-reviewed

    S. Yamamoto, R. Kaji, S. Adachi

    Physical Review B　108　(5)　054422/1-054422/11　2023/08/16

    Publisher: American Physical Society (APS)

    DOI： 10.1103/physrevb.108.054422 　

    ISSN： 2469-9950

    eISSN： 2469-9969

  5. Double nuclear spin switching in single quantum dots Peer-reviewed

    S. Yamamoto, R. Kaji, H. Sasakura, S. Adachi

    Japanese Journal of Applied Physics　60　(SB)　SBBH07-SBBH07　2021/01/15

    Publisher: IOP Publishing

    DOI： 10.35848/1347-4065/abd709 　

    ISSN： 0021-4922

    eISSN： 1347-4065

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    Abstract Hyperfine interaction in a quantum dot produces a spin-coupled system between an injected electron and a few tens of thousands of nuclei in the electron wavefunction. We report here that the double bistable phenomenon of nuclear spin polarization (NSP) can be observed by sweeping three control parameters. All the observations indicate that the NSP has three stable branches under a longitudinal magnetic field, whereas the number of the stable branches of NSP has been believed to be two at most. The incorporation of the electron spin relaxation due to the nuclear spin fluctuation enables reproduction of the observed features of the NSP and the electron spin polarization.

  6. Third stable branch and tristability of nuclear spin polarizations in a single quantum dot system Peer-reviewed

    S. Yamamoto, R. Kaji, H. Sasakura, S. Adachi

    Physical Review B　101　(24)　245424/1-245424/8　2020/06/17

    Publisher: American Physical Society (APS)

    DOI： 10.1103/physrevb.101.245424 　

    ISSN： 2469-9950

    eISSN： 2469-9969

  7. Influence of a Longitudinal Field on the Large In‐Plane Nuclear Field Formation in Single Quantum Dots Peer-reviewed

    Sota Yamamoto, Takuya Arakawa, Ryosuke Matsusaki, Reina Kaji, Satoru Adachi

    physica status solidi (b)　257　(2)　1900381-1900381　2019/10/29

    Publisher: Wiley

    DOI： 10.1002/pssb.201900381 　

    ISSN： 0370-1972

    eISSN： 1521-3951

  8. Anomalous Hanle curves induced by in-plane nuclear field in single self-assembled InAlAs and InAs nanostructures Peer-reviewed

    S. Yamamoto, R. Matsusaki, R. Kaji, S. Adachi

    Journal of Physics: Conference Series　1220　(1)　012047-012047　2019/05/01

    Publisher: IOP Publishing

    DOI： 10.1088/1742-6596/1220/1/012047 　

    ISSN： 1742-6588

    eISSN： 1742-6596

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    Abstract We studied the formation mechanism of in-plane nuclear field by anomalous Hanle effect measurements in single self-assembled In0.75Al0.25As/Al0.3Ga0.7As quantum dots and InAs/GaAs quantum rings. The observed anomalous Hanle curves indicated quite large width and hysteretic behavior to the externally applied magnetic field. These anomalies appears in both materials whose electron g factors have opposite sign each other, and cannot be reproduced by a traditional spin dynamics model. In this study, we show that a proposed spin dynamics model can explain the formation of a large in-plane nuclear field regardless of the sign of electron g factor. The model is based on the dynamic nuclear spin polarization mechanism including the effects of nuclear quadrupole interaction.

  9. Experimental studies on nuclear field generated orthogonal to electron spin in single self-assembled nanostructures Peer-reviewed

    Reina Kaji, Sota Yamamoto, Ryosuke Matsusaki, Satoru Adachi

    Japanese Journal of Applied Physics　58　(SB)　SBBH10-SBBH10　2019/03/27

    Publisher: IOP Publishing

    DOI： 10.7567/1347-4065/aafe66 　

    ISSN： 0021-4922

    eISSN： 1347-4065

  10. Direct evaluation of nuclear spin fluctuation by using nuclear spin switch in single quantum structures Peer-reviewed

    Reina Kaji, Ryosuke Matsusaki, Sota Yamamoto, Satoru Adachi

    Japanese Journal of Applied Physics　58　(SB)　SBBH05-SBBH05　2019/02/22

    Publisher: IOP Publishing

    DOI： 10.7567/1347-4065/aafd94 　

    ISSN： 0021-4922

    eISSN： 1347-4065

  11. Quadrupolar effect on nuclear spin depolarization in single self-assembled quantum dots Peer-reviewed

    Ryosuke Matsusaki, Reina Kaji, Sota Yamamoto, Hirotaka Sasakura, Satoru Adachi

    Applied Physics Express　11　(8)　085201-085201　2018/07/05

    Publisher: IOP Publishing

    DOI： 10.7567/apex.11.085201 　

    ISSN： 1882-0778

    eISSN： 1882-0786

  12. In-plane nuclear field formation investigated in single self-assembled quantum dots Peer-reviewed

    S. Yamamoto, R. Matsusaki, R. Kaji, S. Adachi

    Physical Review B　97　(7)　075309/1-075309/8　2018/02/26

    Publisher: American Physical Society (APS)

    DOI： 10.1103/physrevb.97.075309 　

    ISSN： 2469-9950

    eISSN： 2469-9969

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Presentations 16

  1. Emergence of the third stable nuclear state due to the reaction of electron spin relaxation via hyperfine interaction

    R. Kaji, S. Yamamoto, Z.-R. Li, H. Sasakura, S. Adachi

    The 22nd International Conference on Electron Dynamics in Semiconductors, Optoelectronics and Nanostructures (EDISON22)　2023/08/15

  2. Impact of nuclear quadrupole interaction in anomalous Hanle effect

    S. Adachi, S. Yamamoto, R. Kaji, H. Sasakura

    The 22nd International Conference on Electron Dynamics in Semiconductors, Optoelectronics and Nanostructures (EDISON22)　2023/08/18

  3. Time-resolved measurements of electron-nuclear spin dynamics via anomalous Hanle effect in a single semiconductor quantum dot

    R. Kaji, S. Yamamoto, S. Adachi

    The 2022 International Conference on Solid State Devices and Materials (SSDM2022)　2022/09/27

  4. Double nuclear spin switching in single self-assembled quantum dots

    S. Yamamoto, R. Kaji, H. Sasakura, S Adachi

    The 2020 International Conference on Solid State Devices and Materials (SSDM2020)　2020/09/28

  5. Asymmetrically antibunching behavior of photon correlation between Zeeman splitting states of positive trion in semiconductor quantum dots: Acceleration of electron spin flip process induced by nuclear field

    H. Sasakura, R. Kaji, R. Matsusaki, K. Oomiya, S. Kamono, S. Yamamoto, S. Adachi

    The 38th Electronic Materials Symposium (EMS38)　2019/10/09

  6. Large in-plane nuclear field formation in single quantum dots: Influence of a longitudinal field and role of nuclear quadrupolar interaction

    S. Yamamoto, T. Arakawa, R. Matsusaki, R. Kaji, S. Adachi

    International Conference on Optics of Excitons in Confined Systems (OECS2019)　2019/09/19

  7. A novel method for direct evaluation of nuclear spin fluctuation by using nuclear spin switch in quantum nanostructures

    R. Kaji, R. Matsusaki, S. Yamamoto, S Adachi

    The 50th International Conference on Solid State Devices and Materials (SSDM2018)　2019/09/10

  8. Studies on anomalous Hanle effect observed in single self-assembled quantum dots

    R. Kaji, S. Yamamoto, R. Matsusaki, S Adachi

    The 50th International Conference on Solid State Devices and Materials (SSDM2018)　2018/09/11

  9. Modulation of nuclear quadrupole effect by a longitudinal magnetic field in transverse nuclear field formation

    S. Yamamoto, T. Arakawa, R. Matsusaki, R. Kaji, S. Adachi

    The 46th International Symposium on Compound Semiconductors (ISCS)　2019/05/19

  10. Anomalous Hanle curves induced by in-plane nuclear field in single self-assembled InAlAs and InAs nanostructures

    S. Yamamoto, R. Matsusaki, R. Kaji, S Adachi

    The 12th International Conference on Excitonic and Photonic Processes in Condensed Matter and Nano Materials (EXCON2018)　2018/07/08

  11. Formation of nuclear field orthogonal to electron spin and nuclear quadrupole effects in self-assembled single quantum dots

    S. Yamamoto, R. Matsusaki, R. Kaji, S Adachi

    50 years of optical orientation in semiconductors: From the original discovery to new horizons　2018/06/18

  12. In-plane nuclear field formation in individual InAlAs quantum dots: role of nuclear quadrupole effects

    S. Yamamoto, R. Matsusaki, R. Kaji, S Adachi

    International School and Symposium on Nanoscale Transport and phoTonics (ISNTT2017)　2017/11/13

  13. Evaluations of the electron g-factor anisotropy and fuctuationof the Overhauser field in single quantum dots

    R. Matsusaki, S. Yamamoto, R. Kaji, S Adachi

    International School and Symposium on Nanoscale Transport and phoTonics (ISNTT2017)　2017/11/13

  14. Quadrupolar effect for nuclear spin depolarization in a single InAlAs quantum dot

    R. Kaji, R. Matsusaki, S. Yamamoto, H. Sasakura, S. Adachi

    International Conference on Optics of Excitons in Confined Systems　2017/09/10

  15. Formation mechanism of in-plane Overhauser field in single self-assembled InAlAs quantum dots

    S. Yamamoto, R. Matsusaki, R. Kaji, S Adachi

    International Conference on Optics of Excitons in Confined Systems　2017/09/10

  16. Quadrupolar effect in zero-field dynamic nuclear spin polarization of a single self-assembled InAlAs quantum dot

    R. Matsusaki, S. Yamamoto, R. Kaji, S. Adachi

    The 9th International Conference on Spintronics and Quantum Information Technology　2017/06/04

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Research Projects 1

  1. 単一量子ナノ構造での異常ハンル効果に基づく核スピンの光制御と応用

    山本 壮太

    Offer Organization: 日本学術振興会

    System: 科学研究費助成事業

    Category: 特別研究員奨励費

    Institution: 北海道大学

    2020/04/24 - 2022/03/31

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    単一量子ドットに適用可能なＮＭＲ測定系の構築，電子ー核スピン結合系の数値シミュレーション，核スピン分極の三重安定状態の実験的調査を行った． ＮＭＲ測定を用いて核四極子相互作用について直接的に調べるため，ｒｆ磁場を試料に効率よく照射するための電気回路のインピーダンス整合が必要であった．そこで簡易な遺伝的アルゴリズムを用いて回路の最適化を試みたが，必要な広帯域での整合は叶わなかったため，インピーダンス整合を行わずに高周波アンプを用いてｒｆ磁場を増幅する手法を選択した．現在は，複数の任意波形発生器を用いた測定シーケンスの実装を行っている． 数値シミュレーションでは，異常ハンル効果が発現する条件を探るために核四極子相互作用の主軸分布に着目した計算を実装した．計算結果から，結晶成長方向から大きくずれた主軸や四極子相互作用の二軸性が異常ハンル効果の発現，特に光注入した電子スピンと直交する核スピン分極の形成に必要であるという画期的な結果を得た． 核スピン分極の三重安定性は本研究で提案したモデルにおいて理論的に予言されていたが，これまで観測されていなかった．そこで実験的な観測を目指して定常発光測定を行った．提案モデルでは電子ー核スピン間の相関時間が非常に重要なパラメータであったため，実験では試料温度の変化からこれの制御を試みた．実験結果からは相関時間が実際に制御できたことが示唆されたが，三重安定性の発見には至らなかった．一方で，相関時間の変化に伴って核スピン分極の双安定曲線が一重から二重になる様子を初めて観測した．