Fons, Paul



Faculty of Science and Technology, Department of Electronics and Electrical Engineering (Yagami)




Books 【 Display / hide

  • Springer handbook of electronic and photonic materials

    Kasap, S. O. (Safa O.), Capper, Peter, Springer, 2017,  Page: xxxvi, 1536 p.

    Scope: Chapter 46: Phase-Change Memory Materials

  • Phase change materials : science and applications

    Raoux, Simone, Wutting, Matthias, Springer, 2009,  Page: xxii, 446 p.

Papers 【 Display / hide

  • Interfacial reaction behavior between ferromagnetic CoFeB and the topological insulator Sb<inf>2</inf>Te<inf>3</inf>

    Misako Morota, Shogo Hatayama, Yi Shuang, Shunsuke Mori, Yuji Sutou, Paul Fons, Yuta Saito

    Surfaces and Interfaces 51 2024.08

    ISSN  2468-0230

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    The successful prevention of interfacial reactions between ferromagnetic materials and topological insulators (TIs) is crucial for the realization of reliable spintronic devices using TIs. To this end, we investigated the magnetic properties and interfacial reaction behavior of a bilayer structure composed of ferromagnetic CoFeB and TI Sb2Te3. The effects of including a MgO interlayer was also investigated. Ferromagnetic resonance (FMR) studies showed a remarkably weak resonance peak for the sample annealed at 400 °C for the 2-nm-thick interlayer, and finally, no resonance peak for a film below 1 nm in thickness was observed. X-ray diffraction (XRD) results demonstrated that the Sb2Te3 peak intensities started to decrease upon annealing at 200 °C and completely disappeared for annealing temperatures >400 °C. Hard X-ray photoelectron spectroscopy (HAXPES) results also support that the core-level peaks of Sb and Te split upon annealing at temperatures >200 °C, suggesting the dissociation of Sb2Te3. These results indicate that Sb2Te3 and CoFeB react at the interface during annealing, resulting in a loss of the ferromagnetic properties of the CoFeB layer. Meanwhile, a sample containing a 3-nm-thick MgO layer retained its original Sb2Te3/MgO/CoFeB structure even after annealing at 400 °C, as evidenced by its unchanged XRD peak intensity and FMR spectrum. We expect that our findings will be highly valuable in developing TI-based spintronic devices.

  • Spin-dependent bandgap renormalization and state-filling effect in Bi2Se3 observed by ultrafast Kerr rotation

    Kazuhiro Kikuchi, Yu Mizukoshi, Takumi Fukuda, Paul Fons, Muneaki Hase

    Applied Physics Letters (AIP Publishing)  124 ( 23 )  2024.06

    ISSN  0003-6951

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    We investigate the ultrafast spin dynamics of the prototypical topological insulator Bi2Se3 using time-resolved Kerr rotation (polarization-change) measurements across near-infrared wavelengths. The Kerr rotation angle ΔθK of Bi2Se3 was found to significantly depend on the photon energy around a resonance transition (∼1.0 eV) of bulk states, as well as the ellipticity of the pump light, in the presence of spin excitation. The observed photon-energy dependence of ΔθK can be well simulated by assuming spin-dependent refractive-index changes in the presence of bandgap renormalization and state-filling effect upon photoexcitation. Our study delivers comprehensive insights into the opto-spintronic properties of bulk Bi2Se3 and the fundamental physical processes underlying polarization changes. These findings are expected to be crucial in developing ultrafast magneto-optical memory devices, which can perform read-and-write operations in the terahertz regime.

  • Coherent optical response driven by non-equilibrium electron–phonon dynamics in a layered transition-metal dichalcogenide

    Takumi Fukuda, Kotaro Makino, Yuta Saito, Paul Fons, Atsushi Ando, Takuya Mori, Ryo Ishikawa, Keiji Ueno, Jessica Afalla, Muneaki Hase

    APL Materials (AIP Publishing)  12 ( 2 )  2024.02

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    Layered transition-metal dichalcogenides (TMDs) are model systems to explore ultrafast many-body interactions and various nonlinear optical phenomena. For the application of TMD-based optoelectronic devices capable of ultrafast response, it is essential to understand how characteristic electron–hole and electron–phonon couplings modify ultrafast electronic and optical properties under photoexcitation. Here, we investigate the sub-picosecond optical responses of layered semiconductor 2H–MoTe2 in the presence of an electron–hole (e–h) plasma and a long-lived coherent phonon. Transient reflectivity measurements depending on photon energy reveal that the optical response for short-time delays (&amp;lt; 1ps) was significantly modified by band-gap renormalization and state filling due to the presence of the e–h plasma. Furthermore, octave, sum, and difference phonon frequencies transiently appeared for the early time delays (&amp;lt; 2ps). The emergent multiple phonon frequencies can be described as higher-order optical modulations due to deformation-potential electron–phonon coupling under resonant photoexcitation conditions. This work provides comprehensive insights into fundamental physics and the application of non-equilibrium quasiparticle generations on TMDs under time-periodic phonon driving forces.

  • Nonvolatile Isomorphic Valence Transition in SmTe Films

    Shogo Hatayama, Shunsuke Mori, Yuta Saito, Paul J. Fons, Yi Shuang, Yuji Sutou

    ACS Nano (American Chemical Society (ACS))  18 ( 4 ) 2972 - 2981 2024.01

    ISSN  1936-0851

  • Electronic intraband scattering in a transition-metal dichalcogenide observed by double-excitation ultrafast electron diffraction

    Yui Iwasaki, Takumi Fukuda, Godai Noyama, Mizuki Akei, Hidemi Shigekawa, Paul J. Fons, Muneaki Hase, Yusuke Arashida, Masaki Hada

    Applied Physics Letters (AIP Publishing)  123 ( 18 )  2023.10

    ISSN  0003-6951

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    Electronic dynamics in the excited state of transition-metal dichalcogenides (TMDs) has attracted great interest. To understand the ultrafast intraband scattering process of excited electrons in the conduction band, we demonstrated ultrafast time-resolved electron diffraction measurements with double-optical-pulse excitation and ultrafast transient reflectivity measurements of a TMD material, 2H-MoTe2. Due to the saturable absorption (or Pauli blocking) effect present in 2H-MoTe2, the system does not absorb the second excitation pulse until the excited electrons generated by the first excitation pulse with a specific fluence are scattered in the conduction band. By exploiting the Pauli blocking effect in ultrafast time-resolved electron diffraction measurements with double-optical-pulse excitation, we found that the excited electrons were scattered within 100 fs comparable to the excitation optical pulse duration. Furthermore, the excited electrons were relaxed to the lowest energy level of the conduction band (K- or Σ-valley) within 1–2 ps.

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Reviews, Commentaries, etc. 【 Display / hide

  • Understanding local structure of Cr2Ge2Te6 phase change material for non-volatile memory application

    畑山祥吾, YI Shuang, FONS Paul, FONS Paul, 齊藤雄太, KOLOBOV Alexander V., KOLOBOV Alexander V., 小林啓介, 進藤怜史, 安藤大輔, 須藤祐司

    応用物理学会春季学術講演会講演予稿集(CD-ROM) 67th 2020

    Other, Joint Work

  • 不揮発性相変化メモリ用遷移金属カルコゲナイド相変化材料の開発

    齊藤雄太, 畑山祥吾, YI Shuang, 進藤怜史, FONS Paul, KOLOBOV Alexander V., 小林啓介, 小林啓介, 小林啓介, 須藤祐司

    応用物理学会春季学術講演会講演予稿集(CD-ROM) 66th 2019

    Other, Joint Work

  • スパッタ法による高配向Bi-Te薄膜の成膜と電子状態解析

    齊藤雄太, FONS Paul, 牧野孝太郎, MITROFANOV Kirill V., 上杉文彦, 竹口雅樹, KOLOBOV Alexander V., 富永淳二

    日本金属学会講演概要(CD-ROM) 164th 2019

    Other, Joint Work,  ISSN  2433-3093

  • 1T’-MoTe2バルク結晶におけるコヒーレントフォノンの観測

    福田拓未, 牧野孝太郎, 齊藤雄太, FONS Paul, KOLOBOV Alexander V., KOLOBOV Alexander V., 上野啓司, MONDAL Richarj, 長谷宗明

    応用物理学会秋季学術講演会講演予稿集(CD-ROM) 80th 2019

    Other, Joint Work

  • Erratum: Effects of RbF postdeposition treatment and heat-light soaking on the metastable acceptor activation of CuInSe<inf>2</inf> thin film photovoltaic devices (Applied Physics Letters (2018) 113 (063901) DOI: 10.1063/1.5031898)

    Ishizuka S., Shibata H., Nishinaga J., Kamikawa Y., Fons P.

    Applied Physics Letters (Applied Physics Letters)  113 ( 11 )  2018.09

    ISSN  00036951

     View Summary

    © 2018 Author(s). We have found an error in Fig. 3(d) in the original published version of our paper.1 The annotations -6V and +3.0V are incorrect and should be +0.6V and -30 V, respectively. Figure 1 is a corrected version for the original Fig. 3(d). (Figure Presented).

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

  • Understanding the crystallization mechanism of amorphous van der Waals layered materials


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


Courses Taught 【 Display / hide











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