Updated on 2024/05/21

写真a

 
TAKAYAMA, Akari
 
Affiliation
Faculty of Science and Engineering, School of Advanced Science and Engineering
Job title
Associate Professor
Degree
博士(理学) ( 東北大学 )

Research Experience

  • 2023.11
    -
    Now

    Tohoku University   Institute of Multidisciplinary Research for Advanced Materials

  • 2020.04
    -
    Now

    Waseda University   Department of Physics

  • 2018.04
    -
    2020.03

    Waseda University   Department of Physics   Assistant Professor

  • 2014.04
    -
    2018.03

    The University of Tokyo   Department of Physics   Assistant Professor

  • 2013.04
    -
    2014.03

    Tohoku University   WPI-AIMR   JSPS Research Fellowship for Young Scientists

  • 2010.04
    -
    2013.03

    Tohoku University   JSPS Research Fellowship for Young Scientists

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Education Background

  • 2008.04
    -
    2013.03

    Tohoku University   Department of Physics  

  • 2004.04
    -
    2008.03

    Fukushima University  

Committee Memberships

  • 2015.04
    -
    Now

    日本表面真空学会  関東支部幹事

  • 2023.09
    -
    2025.08

    放射光学会  行事委員

  • 2023.03
    -
    2025

    日本物理学会  第79~80期代議員

  • 2018.04
    -
    2019.03

    日本物理学会  領域運営委員(領域9)

Professional Memberships

  •  
     
     

    The Japanese Society for Synchrotron Radiation Research

  •  
     
     

    The Japan Society of Vacuum and Surface Science

  •  
     
     

    The Physical Society of Japan

Research Areas

  • Thin film/surface and interfacial physical properties

Research Interests

  • Surface Science

Awards

  • 一般財団法人エヌエフ基金 第7回研究開発奨励賞

    2018.11   一般財団法人エヌエフ基金  

  • 日本応用物理学会「薄膜・表面物理分科会 論文賞」

    2018.03  

    Winner: 一ノ倉 聖, 菅原 克明, 高山 あかり, 高橋 隆, 長谷川修司

  • 日本物理学会 日本物理学会若手奨励賞(領域9)

    2015.03  

  • 井上科学振興財団 第7回井上リサーチアウォード

    2014.12  

  • 日本表面科学会 講演奨励賞(新進研究者部門)

    2014.05  

  • 日本ロレアル ロレアル-ユネスコ女性科学者 日本奨励賞

    2013.09  

  • 東北大学青葉理学振興会 青葉理学振興会賞

    2013.03  

  • 東北大学 総長優秀学生賞

    2013.03  

  • 東北大学 総長賞

    2013.03  

  • 日本学術振興会 第3回育志賞

    2013.03  

  • 東北大学青葉理学振興会 黒田チカ賞

    2012.03  

  • 東北大学 物理学専攻賞

    2010.03  

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Papers

  • 角度分解光電子分光によるBi(111)上Sb超薄膜の電子状態

    H. ABE, M. IMAMURA, K. TAKAHASHI, A. TAKAYAMA

    Vacuum and Surface Science   66 ( 9 ) 548 - 552  2023.09

    DOI

  • Electronic structure of Sb ultrathin film on Bi(111) with large lattice mismatch

    Hiroko Abe, Daiki Iwasawa, Masaki Imamura, Kazutoshi Takahashi, Akari Takayama

    AIP Advances   13 ( 5 )  2023.05

     View Summary

    Topological phase transitions have been actively investigated to understand the fundamental properties and to develop applications for spintronics. In this study, we focused on Sb ultrathin films that undergo a topological phase transition by lattice strain. In order to fabricate samples with a lattice constant theoretically predicted to be topologically nontrivial, Sb ultrathin films are prepared on a Bi substrate. The surface structure and electronic states of the Sb/Bi heterostructure were confirmed by the low-energy electron diffraction measurement and angle-resolved photoemission spectroscopy. For 2 and 3 BL Sb films, we demonstrated the possibility of creating well-ordered immiscible ultrathin films of Sb on a Bi substrate and observed a “V”-shaped electronic band, which is significantly different from the electronic state of Bi thin films and freestanding Sb ultrathin films. This suggests that the electronic state of the Sb ultrathin film is affected by the Bi substrate. We propose that the “V”-shaped band originates from a topological phase transition due to expanded lattice constants and band hybridization between Bi and Sb. This heterostructure will provide a new platform for investigating the topological phase transitions and applications.

    DOI

    Scopus

  • Structure of χ3-Borophene Studied by Total-Reflection High-Energy Positron Diffraction (TRHEPD)

    Yuki Tsujikawa, Makoto Shoji, Masashi Hamada, Tomoya Takeda, Izumi Mochizuki, Toshio Hyodo, Iwao Matsuda, Akari Takayama

    Molecules   27 ( 13 ) 4219 - 4219  2022.06  [Refereed]  [Invited]

    Authorship:Last author, Corresponding author

     View Summary

    We have investigated the structure of χ3-borophene on Ag(111), a monolayer material of boron atoms, via total-reflection high-energy positron diffraction (TRHEPD). By comparing the experimental rocking-curves with ones for several structures calculated by using dynamical diffraction theory, we confirmed that the χ3-borophene layer has a flat structure. The distance from the topmost layer of the metal crystal is 2.4 Å, which is consistent with results reported by X-ray standing wave-excited X-ray photoelectron spectroscopy. We also demonstrated that the in-plane structure of χ3-borophene is compatible with the theoretical predictions. These structural properties indicate that χ3-borophene belongs to a group of epitaxial monolayer sheets, such as graphene, which have weak interactions with the substrates.

    DOI

    Scopus

    1
    Citation
    (Scopus)
  • Superconductivity near the saddle point in the two-dimensional Rashba system Si(111)−√3×√3−(Tl,Pb)

    T. Machida, Y. Yoshimura, T. Nakamura, Y. Kohsaka, T. Hanaguri, C.-R. Hsing, C.-M. Wei, Y. Hasegawa, S. Hasegawa, A. Takayama

    Phys. Rev. B   105 ( 6 ) 064507  2022.02  [Refereed]

    Authorship:Last author

     View Summary

    Two-dimensional Rashba superconductor Si(111)-3×3-(Tl,Pb) is a candidate platform of mixed spin-singlet and -triplet superconductivity. A recent scanning tunneling microscope (STM) experiment revealed a pseudogap at the vortex core, suggesting the finite triplet component [Nakamura, Phys. Rev. B 98, 134505 (2018)2469-995010.1103/PhysRevB.98.134505]. Detailed spectroscopic information of the superconducting gap and the low-energy band structure is necessary to establish the putative triplet superconductivity. Here, we performed high-energy-resolution spectroscopic imaging experiments on Si(111)-3×3-(Tl,Pb) using an ultralow temperature STM. We found that various spectroscopic features, including the vortex-core spectrum, are consistent with spin-singlet s-wave superconductivity, having no sign of the triplet component. The apparent contradiction with the previous STM result suggests that the nature of superconductivity changes within the same system. From the analysis of the quasiparticle interference patterns, we found that the Fermi energy is in the close vicinity of the saddle point near the M¯ point. We speculate that the nature of superconductivity varies depending on the saddle-point energy with respect to the Fermi energy, which is sample-dependent due to different band filling.

    DOI

    Scopus

    1
    Citation
    (Scopus)
  • Scattering of topological surface-state carriers at steps on surfaces

    Naoya Fukui, Rei Hobara, Akari Takayama, Ryota Akiyama, Toru Hirahara, Shuji Hasegawa

    Physical Review B   102 ( 11 ) 115418  2020.09  [Refereed]

     View Summary

    The resistance across a step on ultrathin films of three different topological insulators, Bi2Te3, Bi2Se3, and (Bi1-xPbx)(2)Te-3, was measured through anisotropy in two-dimensional resistivity by using the in situ square fourpoint probe method in ultrahigh vacuum. The step resistance was much larger for Bi2Te3 than for Bi2Se3 in the range of 1-10 quintuple-layer thickness, due to the smaller critical thickness for isolation of topological surface states in Bi2Te3. The transmission probability of carriers across a step is much higher for the bulk-insulating (Bi1-xPbx)(2)Te-3 than bulk-metallic Bi2Te3, due to prevention of scattering of surface-state carriers into the bulk states. We were able to deduce microscopic information concerning the transmission probability at individual steps from the resistance data obtained macroscopically.

    DOI

  • Controlling of the Dirac band states of Pb-deposited graphene by using work function difference

    Y. Tsujikawa, M. Sakamoto, Y. Yokoi, M. Imamura, K. Takahashi, R. Hobara, T. Uchihashi, A. Takayama

    AIP Advances   10 ( 8 ) 085314 - 085314  2020.08  [Refereed]

    Authorship:Last author, Corresponding author

    DOI

    Scopus

    2
    Citation
    (Scopus)
  • Superconducting proximity effect in a Rashba-type surface state of Pb/Ge(111)

    H Huang, H Toyama, L V Bondarenko, A Y Tupchaya, D V Gruznev, A Takayama, R Hobara, R Akiyama, A V Zotov, A A Saranin, S Hasegawa

    Superconductor Science and Technology   33 ( 7 ) 075007 - 075007  2020.07  [Refereed]

    DOI

    Scopus

    3
    Citation
    (Scopus)
  • Two-dimensional conducting layer on the SrTiO3 surface induced by hydrogenation

    Y. Takeuchi, R. Hobara, R. Akiyama, A. Takayama, S. Ichinokura, R. Yukawa, I. Matsuda, S. Hasegawa

    Physical Review B   101 ( 8 ) 085422  2020.02  [Refereed]

     View Summary

    We found that a surface state induced by hydrogenation on the surface of SrTiO3(001) (STO) did not obey the rigid-band model, which was confirmed by in situ electrical resistivity measurements in ultrahigh vacuum. With exposure of atomic hydrogen on the STO, a surface state (H-induced donor state, HDS) appears within the bulk band gap (an in-gap state), which donates electrons thermally activated to the conduction band, resulting in downward bending of the bulk bands beneath the surface. The doped electrons flow through the space-charge layer in two-dimensional manner parallel to the surface. The observed semiconductor behavior in the temperature dependence of electronic conductivity is explained by the thermal activation of carriers. The HDS and the conduction band are nonrigid in energy position; they come closer with increasing the hydrogen adsorption. Eventually the HDS saturates its position around 60 meV below the conduction-band minimum. The sheet conductivity, accordingly, also saturates at ∼1.0μS□ with increasing hydrogen adsorption, corresponding to completion of the hydrogenation of the surface.

    DOI

  • Structure of superconducting Ca-intercalated bilayer Graphene/SiC studied using total-reflection highenergy positron diffraction

    Y. Endo, Y. Fukaya, I. Mochizuki, A. Takayama, T. Hyodo, S. Hasegawa

    Carbon   157   857 - 862  2020.01  [Refereed]

    DOI

    Scopus

    24
    Citation
    (Scopus)
  • Atomic layer superconductors studied by in-situ surface electrical conductivity measurements in ultrahigh vacuum

    A. Takayama, S. Hasegawa

    Acc. Mater. Surf. Res.   ( 3 ) 87 - 96  2019.09

  • Superconducting single-atomic-layer Tl-Pb compounds on Ge(111) and Si(111) surfaces

    T. Nakamura, A. Takayama, R. Hobara, D.V. Gruznev, A.V. Zotov, A.A. Saranin, S. Hasegawa

    Applied Surface Science   479   679 - 684  2019  [Refereed]

    DOI

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    10
    Citation
    (Scopus)
  • Vortex-induced quantum metallicity in the mono-unit-layer superconductor NbSe2

    Satoru Ichinokura, Yuki Nakata, Katsuaki Sugawara, Yukihiro Endo, Akari Takayama, Takashi Takahashi, Shuji Hasegawa

    Phys. Rev. B   99   220501(R)  2019  [Refereed]

    DOI

    Scopus

    14
    Citation
    (Scopus)
  • Alkali-metal induced band structure deformation investigated by angle-resolved photoemission spectroscopy and first-principles calculations

    S. Ito, B. Feng, M. Arita, T. Someya, W. C. Chen, A. Takayama, T. Iimori, H. Namatame, M. Taniguchi, C. M. Cheng, S. J. Tang, F. Komori, I. Matsuda

    Physical Review B   97 ( 15 ) 155423  2018.04  [Refereed]

     View Summary

    Alkali-metal adsorption on the surface of materials is widely used for in situ surface electron doping, particularly for observing unoccupied band structures by angle-resolved photoemission spectroscopy (ARPES). However, the effects of alkali-metal atoms on the resulting band structures have yet to be fully investigated, owing to difficulties in both experiments and calculations. Here, we combine ARPES measurements on cesium-adsorbed ultrathin bismuth films with first-principles calculations of the electronic charge densities and demonstrate a simple method to evaluate alkali-metal induced band deformation. We reveal that deformation of bismuth surface bands is directly correlated with vertical charge-density profiles at each electronic state of bismuth. In contrast, a change in the quantized bulk bands is well described by a conventional rigid-band-shift picture. We discuss these two aspects of the band deformation holistically, considering spatial distributions of the electronic states and cesium-bismuth hybridization, and provide a prescription for applying alkali-metal adsorption to a wide range of materials.

    DOI

    Scopus

    5
    Citation
    (Scopus)
  • Weak localization in bilayer graphene with Li-intercalation/desorption

    Y. Endo, S. Ichinokura, R. Akiyama, A. Takayama, K. Sugawara, K. Nomura, T. Takahashi, S. Hasegawa

    Journal of Physics: Condensed Matter   30   305701  2018  [Refereed]

    DOI

    Scopus

    5
    Citation
    (Scopus)
  • Unconventional Superconductivity at Si(111)-√3×√3-(Tl, Pb) One-Atom-Layer Alloy

    T. Nakamura, H. Kim, S. Ichinokura, A. Takayama, A.V. Zotov, A.A. Saranin, Y. Hasegawa, S. Hasegawa

    Phys. Rev. B   98   134505  2018  [Refereed]

    DOI

    Scopus

    13
    Citation
    (Scopus)
  • Thickness dependence of surface structure and superconductivity in Pb atomic layers

    H. Toyama, H. Huang, T. Nakamura, L. V. Bondarenko, A. Y. Tupchaya, D. V. Gruznev, A. Takayama, A. V. Zotov, A. A. Saranin, S. Hasegawa

    J. Phys. Soc. Japan   87   113601  2018  [Refereed]

    DOI

    Scopus

    1
    Citation
    (Scopus)
  • Anisotropic electronic band structure of intrinsic Si(110) studied by angle-resolved photoemission spectroscopy and first-principles calculations

    Stephane Yu Matsushita, Akari Takayama, Erina Kawamoto, Chunping Hu, Satoshi Hagiwara, Kazuyuki Watanabe, Takashi Takahashi, Shozo Suto

    PHYSICAL REVIEW B   96 ( 12 ) 125302  2017.09  [Refereed]

     View Summary

    We have studied the electronic band structure of the hydrogen-terminated Si(110)-(1 x 1) [H:Si(110)-(1 x 1)] surface using angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations in the framework of density functional theory with local density approximation (LDA). The bulk-truncated H: Si(110)-(1 x 1) surface is a good template to investigate the electronic band structure of the intrinsic Si(110). In the ARPES spectra, seven bulk states and one surface state due to the H-H interaction are observed clearly. The four bulk states consisting of Si 3p(xy) orbitals exhibit anisotropic band dispersions along the high symmetric direction of (Gamma) over bar-(X) over bar and (Gamma) over bar-(X) over bar' directions, where one state shows one-dimensional character. The calculated band structures show a good agreement with the experimental results except the surface state. We discuss the exact nature of electronic band structures and the applicability of LDA. We have estimated the anisotropic effective masses of electrons and holes of Si(110) for device application.

    DOI

    Scopus

    4
    Citation
    (Scopus)
  • Emergence of charge density waves and a pseudogap in single-layer TiTe2

    P. Chen, Woei Wu Pai, Y. -H. Chan, A. Takayama, C. -Z. Xu, A. Karn, S. Hasegawa, M. Y. Chou, S. -K. Mo, A. -V. Fedorov, T. -C. Chiang

    NATURE COMMUNICATIONS   8   516  2017.09  [Refereed]

     View Summary

    Two-dimensional materials constitute a promising platform for developing nanoscale devices and systems. Their physical properties can be very different from those of the corresponding three-dimensional materials because of extreme quantum confinement and dimensional reduction. Here we report a study of TiTe2 from the single-layer to the bulk limit. Using angleresolved photoemission spectroscopy and scanning tunneling microscopy and spectroscopy, we observed the emergence of a (2 x 2) charge density wave order in single-layer TiTe2 with a transition temperature of 92 +/- 3 K. Also observed was a pseudogap of about 28 meV at the Fermi level at 4.2 K. Surprisingly, no charge density wave transitions were observed in two-layer and multi-layer TiTe2, despite the quasi-two-dimensional nature of the material in the bulk. The unique charge density wave phenomenon in the single layer raises intriguing questions that challenge the prevailing thinking about the mechanisms of charge density wave formation.

    DOI

    Scopus

    96
    Citation
    (Scopus)
  • Large-Gap Magnetic Topological Heterostructure Formed by Subsurface Incorporation of a Ferromagnetic Layer

    Toru Hirahara, Sergey V. Eremeev, Tetsuroh Shirasawa, Yuma Okuyama, Takayuki Kubo, Ryosuke Nakanishi, Ryota Akiyama, Akan Takayama, Tetsuya Hajiri, Shin-ichiro Ideta, Masaharu Matsunami, Kazuki Sumida, Koji Miyamoto, Yasumasa Takagi, Kiyohisa Tanaka, Taichi Okuda, Toshihiko Yokoyama, Shin-ichi Kimura, Shuji Hasegawa, Evgueni V. Chulkov

    NANO LETTERS   17 ( 6 ) 3493 - 3500  2017.06  [Refereed]

     View Summary

    Inducing magnetism into topological insulators is intriguing for utilizing exotic phenomena such as the quantum anomalous Hall effect (QAHE) for technological applications. While most studies have focused on doping magnetic impurities to open a gap at the surface-state Dirac point, many undesirable effects have been reported to appear in some cases that makes it difficult to determine whether the gap opening is due to the time-reversal symmetry breaking or not. Furthermore, the realization of the QAHE has been limited to low temperatures. Here we have succeeded in generating a massive Dirac cone in a MnBi2Se4/Bi2Se3 heterostructure, which was fabricated by self-assembling a MnBi2Se4 layer on top of the Bi2Se3 surface as a result of the codeposition of Mn and Se. Our experimental results, supported by relativistic ab initio calculations, demonstrate that the fabricated MnBi2Se4/Bi2Se3 heterostructure shows ferromagnetism up to room temperature and a clear Dirac cone gap opening of similar to 100 meV without any other significant changes in the rest of the band structure. It can be considered as a result of the direct interaction of the surface Dirac cone and the magnetic layer rather than a magnetic proximity effect. This spontaneously formed self-assembled heterostructure with a massive Dirac spectrum, characterized by a nontrivial Chern number C = -1, has a potential to realize the QAHE at significantly higher temperatures than reported up to now and can serve as a platform for developing future topotronics devices.

    DOI

    Scopus

    127
    Citation
    (Scopus)
  • Two-Dimensional Superconductivity in Intercalated Bilayer Graphene

    TAKAHASHI Takashi, SUGAWARA Katsuaki, ICHINOKURA Satoru, TAKAYAMA Akari, HASEGAWA Shuji

    Hyomen Kagaku   38 ( 9 ) 460 - 465  2017

     View Summary

    <p>We have fabricated alkali metal (Li, Rb, Cs) and alkaline-earth metal (Ca) intercalated bilayer graphene on SiC substrate, and characterized them by low-energy electron diffraction, angle-resolved photoemission spectroscopy, and 4-point-probe measurements. We observed a free-electron-like state in the center of the Brillouin zone, called &ldquo;interlayer state&rdquo;, as well as the folded &pi;/&pi;* bands in Rb-, Cs-, and Ca-intercalated graphene, while it was absent in Li counterpart. Ca-intercalated bilayer graphene shows the zero-resistance below 4K, indicative of the two-dimensional superconductivity. These results suggest that the interlayer state plays an important role for the superconductivity in intercalated bilayer graphene.</p>

    DOI CiNii

  • Proving Nontrivial Topology of Pure Bismuth by Quantum Confinement

    S. Ito, B. Feng, M. Arita, A. Takayama, R. -Y. Liu, T. Someya, W. -C. Chen, T. Iimori, H. Namatame, M. Taniguchi, C. -M. Cheng, S. -J. Tang, F. Komori, K. Kobayashi, T. -C. Chiang, I. Matsuda

    PHYSICAL REVIEW LETTERS   117 ( 23 ) 236402  2016.12  [Refereed]

     View Summary

    The topology of pure Bi is controversial because of its very small (similar to 10 meV) band gap. Here we perform high-resolution angle-resolved photoelectron spectroscopy measurements systematically on 14-202 bilayer Bi films. Using high-quality films, we succeed in observing quantized bulk bands with energy separations down to similar to 10 meV. Detailed analyses on the phase shift of the confined wave functions precisely determine the surface and bulk electronic structures, which unambiguously show nontrivial topology. The present results not only prove the fundamental property of Bi but also introduce a capability of the quantum-confinement approach.

    DOI

    Scopus

    70
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  • Superconducting Calcium-Intercalated Bilayer Graphene

    Satoru Ichinokura, Katsuaki Sugawara, Akari Takayama, Takashi Takahashi, Shuji Hasegawa

    ACS NANO   10 ( 2 ) 2761 - 2765  2016.02  [Refereed]

     View Summary

    We report the direct evidence for superconductivity in Ca-intercalated bilayer graphene C6CaC6, which is regarded as the thinnest limit of Ca-intercalated graphite. We performed the electrical transport measurements with the in situ 4-point probe method in ultrahigh vacuum under zero- or nonzero-magnetic field for pristine bilayer graphene, Li-intercalated bilayer graphene (C6LiC6) and C6CaC6 fabricated on SiC substrate. We observed that the zero-resistance state occurs in C6CaC6 with the onset temperature (T-c(onset)) of 4 K, while the T-c(onset) is gradually decreased upon applying the magnetic field. This directly proves the superconductivity origin of the zero resistance in C6CaC6. On the other hand, both pristine bilayer graphene and C6LiC6 exhibit nonsuperconducting behavior, suggesting the importance of intercalated atoms and its species to drive the superconductivity.

    DOI

    Scopus

    205
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  • 19pPSA-31 Transport property of bilayer graphene studied by in situ four-point probe resistance measurements

    Endo Y., Ichinokura S., Suzuki K., Sugawara K., Akiyama R., Takayama A., Takahashi T., Hasegawa S.

    Meeting Abstracts of the Physical Society of Japan   71   2535 - 2535  2016

    DOI CiNii

  • 19aAJ-6 Edge state of Sb thin film studied by spin-resolved ARPES

    Takayama A., Sato T., Souma S., Oguchi T., Takahashi T.

    Meeting Abstracts of the Physical Society of Japan   71   2472 - 2472  2016

    DOI CiNii

  • 19aAJ-12 In situ Measurements of Transport Properties in Topological Insulators with Atomic Steps

    Fukui N., Hobara R., Takayama A., Akiyama R., Hasegawa S.

    Meeting Abstracts of the Physical Society of Japan   71   2478 - 2478  2016

    DOI CiNii

  • 22aAJ-8 Superconductivity in Ca-intercalated Bilayer Graphene

    Ichinokura S., Sugawara K., Takayama A., Takahashi T., Hasegawa S.

    Meeting Abstracts of the Physical Society of Japan   71   2629 - 2629  2016

    DOI CiNii

  • Superconductivity measurement on the superstructure (Tl,Pb)/Si(111) by scanning tunneling spectroscopy

    Nakamura Tomonori, Howon Kim, Ichinokura Satoru, Takayama Akari, Akiyama Ryota, A.V. Zotov, A.A. Saranin, Hasegawa Yukio, Hasegawa Shuji

    Abstract of annual meeting of the Surface Science of Japan   36   430 - 430  2016

    DOI CiNii

  • Quantum transport properties of bilayer graphene on SiC studied by in situ four-point probe resistance measurements

    Endo Y., Ichinokura S., Suzuki K., Sugawara K., Akiyama R., Takayama A., Takahashi T., Hasegawa S.

    Meeting Abstracts of the Physical Society of Japan   71   2450 - 2450  2016

    DOI CiNii

  • Observation of Superconductivity on the Rashba-Type Surface Reconstruction (Tl, Pb)/Si(111) by in situ Electrical Transport Measurements

    ICHINOKURA Satoru, HOBARA Rei, TAKAYAMA Akari, HASEGAWA Shuji, V. MATETSKIY Andrey, V. BONDARENKO Leonid, Y. TUPCHAYA Alexandra, V. GRUZNEV Dimitry, V. ZOTOV Andrey, A. SARANIN Alexander

    Hyomen Kagaku   37 ( 8 ) 363 - 368  2016

     View Summary

    <p>A surface reconstruction consisting of one monolayer of Tl and one-third monolayer of Pb on a Si(111) was found to exhibit a large Rashba-type spin splitting in the metallic surface-state bands together with two-dimensional superconductivity. Temperature dependent angle-resolved photoelectron spectroscopy revealed a strong electron-phonon interaction in one of the bands. In situ four-point-probe resistivity measurements with and without magnetic field demonstrated that the (Tl, Pb)/Si(111) system transited into the superconducting state at 2.3 K. The (Tl, Pb)/Si(111) is a prototypical system for prospective studies of intriguing properties of the space-inversion-symmetry-broken superconductor.</p>

    DOI CiNii

  • Two-Dimensional Superconductor with a Giant Rashba Effect: One-Atom-Layer Tl-Pb Compound on Si(111)

    A. V. Matetskiy, S. Ichinokura, L. V. Bondarenko, A. Y. Tupchaya, D. V. Gruznev, A. V. Zotov, A. A. Saranin, R. Hobara, A. Takayama, S. Hasegawa

    PHYSICAL REVIEW LETTERS   115 ( 14 ) 147003  2015.10  [Refereed]

     View Summary

    A one-atom-layer compound made of one monolayer of Tl and one-third monolayer of Pb on a Si(111) surface having root 3 x root 3 periodicity was found to exhibit a giant Rashba-type spin splitting of metallic surface-state bands together with two-dimensional superconducting transport properties. Temperature-dependent angle-resolved photoelectron spectroscopy revealed an enhanced electron-phonon coupling for one of the spin-split bands. In situ micro-four-point-probe conductivity measurements with and without magnetic field demonstrated that the (Tl, Pb)/Si(111) system transformed into the superconducting state at 2.25 K, followed by the Berezinskii-Kosterlitz-Thouless mechanism. The 2D Tl-Pb compound on Si(111) is believed to be the prototypical object for prospective studies of intriguing properties of the superconducting 2D system with lifted spin degeneracy, bearing in mind that its composition, atomic and electron band structures, and spin texture are already well established.

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    101
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  • Rashba effect of bismuth thin film on silicon studied by spin-resolved ARPES

    A. Takayama, T. Sato, S. Souma, T. Takahashi

    JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA   201   105 - 109  2015.05  [Refereed]

     View Summary

    We report high-resolution spin- and angle-resolved photoemission spectroscopy of bismuth thin film on Si(1 1 1) to discuss the spin structure of surface states. We found that, unlike conventional picture of the Rashba splitting, the magnitude of the in-plane spin polarization is asymmetric between two hole pockets across the Brillouin-zone center. In addition, these pockets exhibit a giant out-of-plane spin polarization as large as the in-plane counterpart, which changes the sign across the (Gamma) over bar (M) over bar line. We also revealed that the spin polarization of the Rashba surface states near the Brillouin-zone boundary (the (M) over bar point) is gradually reduced on decreasing the film thickness. Our result provides a new pathway to generate and control spin-polarized electrons in the Rashba system. (C) 2014 Elsevier B.V. All rights reserved.

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    5
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    (Scopus)
  • Topological proximity effect in a topological insulator hybrid

    T. Shoman, A. Takayama, T. Sato, S. Souma, T. Takahashi, T. Oguchi, Kouji Segawa, Yoichi Ando

    NATURE COMMUNICATIONS   6   6547  2015.03  [Refereed]

     View Summary

    It is well known that a topologically protected gapless state appears at an interface between a topological insulator and an ordinary insulator; however, the physics of the interface between a topological insulator and a metal has largely been left unexplored. Here we report a novel phenomenon termed topological proximity effect, which occurs between a metallic ultrathin film and a three-dimensional topological insulator. We study one bilayer of bismuth metal grown on the three-dimensional topological insulator material TlBiSe2, and by using spin-and angle-resolved photoemission spectroscopy, we found evidence that the topological Dirac-cone state migrates from the surface of TlBiSe2 to the attached one-bilayer Bi. We show that such a migration of the topological state occurs as a result of strong spin-dependent hybridization of the wave functions at the interface, which is also supported by our first-principles calculations. This discovery points to a new route to manipulating the topological properties of materials.

    DOI

    Scopus

    55
    Citation
    (Scopus)
  • Role of Quantum and Surface-State Effects in the Bulk Fermi Level Position of Ultrathin Bi films

    T. Hirahara, T. Shirai, A. Takayama, T. Hajiri, M. Matsunami, K. Tanaka, S. Kimura, S. Hasegawa, K. Kobayashi

    Physical Review Letters   115   106803  2015.02  [Refereed]

    DOI

  • One-Dimensional Edge States with Giant Spin Splitting in a Bismuth Thin Film

    A. Takayama, T. Sato, S. Souma, T. Oguchi, T. Takahashi

    PHYSICAL REVIEW LETTERS   114 ( 6 ) 066402  2015.02  [Refereed]

     View Summary

    To realize a one-dimensional (1D) system with strong spin-orbit coupling is a big challenge in modern physics, since the electrons in such a system are predicted to exhibit exotic properties unexpected from the 2D or 3D counterparts, while it was difficult to realize genuine physical properties inherent to the 1D system. We demonstrate the first experimental result that directly determines the purely 1D band structure by performing spin-resolved angle-resolved photoemission spectroscopy of Bi islands on a silicon surface that contains a metallic 1D edge structure with unexpectedly large Rashba-type spin-orbit coupling suggestive of the nontopological nature. We have also found a sizable out-of-plane spin polarization of the 1D edge state, consistent with our first-principles band calculations. Our result provides a new platform to realize exotic quantum phenomena at the 1D edge of the strong spin-orbit-coupling systems.

    DOI

    Scopus

    75
    Citation
    (Scopus)
  • 16pCB-12 Circular dichroism in photovoltage at surface Rashba systems induced with visible-light laser : Bi and Bi absorbed Ag surfaces

    Ishihara H., Fukui N., Hobara R., Takayama A., Akiyama R., Hirahara T., Hasegawa S.

    Meeting Abstracts of the Physical Society of Japan   70   2260 - 2260  2015

    DOI CiNii

  • 16pCB-2 Two-dimensional superconductivity on the Rashba type surface reconstruction (Tl, Pb)/Si(111) studied by the in situ electrical transport measurement

    Ichinokura S., Hobara R., Akiyama R., Takayama A., Hasegawa S., Matetskiy A. V., Bondarenko L. V., Tupchaya A. Y., Gruznev D. V., Zotov A. V., Saranin A. A.

    Meeting Abstracts of the Physical Society of Japan   70   2250 - 2250  2015

    DOI CiNii

  • 19aCA-5 Transport property of MnX/Bi_2X_3 (X = Se, Te) thin film studied by four-point probe resistance measyrements

    Kubo T., Nakanishi R., Takayama A., Fukui N., Hobara R., Akiyama R., Hasegawa S.

    Meeting Abstracts of the Physical Society of Japan   70   2363 - 2363  2015

    DOI CiNii

  • Observation of superconductivity on the Rashba type surface reconstruction (Tl, Pb)/Si(111) by the in situ electrical transport measurements

    Ichinokura Satoru, Matetskiy A. V., Bondarenko L. V., Tupchaya A. Y., Gruznev D.V., Zotov A.V., Hobara Rei, Akiyama Ryota, Takayama Akari, Saranin A.A., Hasegawa Shuji

    Abstract of annual meeting of the Surface Science of Japan   35   100 - 100  2015

    DOI CiNii

  • 23pAS-6 Electronic structure of a topological insulator/magnetic insulator ultrathin film heterostructure

    Hirahara T., Kubo T., Mateckij Andrei, Takayama A., Matsunami M., Hajiri T., Tanaka K., Kimura S., Hasegawa S.

    Meeting Abstracts of the Physical Society of Japan   70   2630 - 2630  2015

    DOI CiNii

  • 21aCN-3 Topological proximity effect at the interface of Rashba metal and topological insulator studied by high-resolution ARPES

    Shoman T., Sato T., Takayama A., Souma S., Takahashi T., Oguchi T., Segawa K., Ando Y.

    Meeting Abstracts of the Physical Society of Japan   70   1419 - 1419  2015

    DOI CiNii

  • 21pPSB-6 Observation of circular dichroism in photovoltage by the Rashba effect of Bi and Bi/Ag thin film

    Ishihara H., Fukui N., Hanazuka M., Hobara R., Takayama A., Hirahara T., Hasegawa S.

    Meeting Abstracts of the Physical Society of Japan   70   2533 - 2533  2015

    DOI CiNii

  • 21pPSB-49 Transport property of magnetic insulator/topological insulator interface in MnSe/Bi_2Se_3 thin film

    Kubo T., Fukui N., Ichinokura S., Hobara R., Takayama A., Hasegawa S.

    Meeting Abstracts of the Physical Society of Japan   70   2575 - 2575  2015

    DOI CiNii

  • スピン分解ARPES によるBi エッジ構造の1 次元Rashba 効果の観測

    高山あかり, 佐藤宇史, 相馬清吾, 小口多美夫, 高橋隆

    表面科学   36 ( 8 ) 412 - 417  2015

  • Rashba effect in antimony and bismuth studied by spin-resolved ARPES

    A. Takayama, T. Sato, S. Souma, T. Takahashi

    NEW JOURNAL OF PHYSICS   16   055004  2014.05  [Refereed]

     View Summary

    We have performed spin- and angle-resolved photoemission spectroscopy of antimony (Sb) and bismuth (Bi) thin films grown on Si(111) to elucidate the nature of the Rashba effect in the spin-split surface bands. In Sb, we revealed spin polarization with the in-plane vortical texture on an elongated hole-like Fermi surface. The spin polarization is strongly momentum-dependent, almost vanishing at the region away from the Brillouin zone center. Such unusual suppression of the spin polarization is not observed in Bi, pointing to a strong influence from the quantized bulk bands on the surface spin polarization in Sb. The present result strongly suggests that bulk-surface interband scattering should be properly taken into account to understand the Rashba surface states in group-V semimetals.

    DOI

    Scopus

    28
    Citation
    (Scopus)
  • Relationship between Fermi surface warping and out-of-plane spin polarization in topological insulators: A view from spin- and angle-resolved photoemission

    M. Nomura, S. Souma, A. Takayama, T. Sato, T. Takahashi, K. Eto, Kouji Segawa, Yoichi Ando

    Physical Review B - Condensed Matter and Materials Physics   89 ( 4 )  2014.01  [Refereed]

     View Summary

    We have performed spin- and angle-resolved photoemission spectroscopy of the topological insulator Pb(Bi,Sb)2Te4 and observed significant out-of-plane spin polarization on the hexagonally warped Dirac-cone surface state. To put this into context, we carried out quantitative analysis of the warping strengths for various topological insulators [Pb(Bi,Sb)2Te4, Bi2Te3, Bi2Se3, and TlBiSe2] and elucidated that the out-of-plane spin polarization Pz is systematically correlated with the warping strength. However, the magnitude of Pz is found to be only half that expected from the k·p theory when the warping is strong. Besides confirming a universal relationship between the spin polarization and the surface state structure, our data provide an empirical guiding principle for tuning the spin polarization in topological insulators. © 2014 American Physical Society.

    DOI

    Scopus

    57
    Citation
    (Scopus)
  • Relationship between Fermi surface warping and out-of-plane spin polarization in topological insulators: A view from spin- and angle-resolved photoemission

    M. Nomura, S. Souma, A. Takayama, T. Sato, T. Takahashi, K. Eto, Kouji Segawa, Yoichi Ando

    PHYSICAL REVIEW B   89 ( 4 ) 045134  2014.01  [Refereed]

     View Summary

    We have performed spin- and angle-resolved photoemission spectroscopy of the topological insulator Pb(Bi,Sb)(2)Te-4 and observed significant out-of-plane spin polarization on the hexagonally warped Dirac-cone surface state. To put this into context, we carried out quantitative analysis of the warping strengths for various topological insulators [Pb(Bi,Sb)(2)Te-4, Bi2Te3, Bi2Se3, and TlBiSe2] and elucidated that the out-of-plane spin polarization P-z is systematically correlated with the warping strength. However, the magnitude of P-z is found to be only half that expected from the k . p theory when the warping is strong. Besides confirming a universal relationship between the spin polarization and the surface state structure, our data provide an empirical guiding principle for tuning the spin polarization in topological insulators.

    DOI

    Scopus

    57
    Citation
    (Scopus)
  • Spin Polarization of Gapped Dirac Surface States Near the Topological Phase Transition in TlBi(S1-xSex)(2)

    S. Souma, M. Komatsu, M. Nomura, T. Sato, A. Takayama, T. Takahashi, K. Eto, Kouji Segawa, Yoichi Ando

    PHYSICAL REVIEW LETTERS   109 ( 18 ) 186804  2012.11  [Refereed]

     View Summary

    We performed systematic spin- and angle-resolved photoemission spectroscopy of TlBi(S1-xSex)(2) which undergoes a topological phase transition at x similar to 0.5. In TlBiSe2 (x = 1.0), we revealed a helical spin texture of Dirac-cone surface states with an intrinsic in-plane spin polarization of similar to 0.8. The spin polarization still survives in the gapped surface states at x &gt; 0: 5, although it gradually weakens upon approaching x = 0.5 and vanishes in the nontopological phase. No evidence for the out-of-plane spin polarization was found, irrespective of x and momentum. The present results unambiguously indicate the topological origin of the gapped Dirac surface states, and also impose a constraint on models to explain the origin of mass acquisition of Dirac fermions.

    DOI

    Scopus

    42
    Citation
    (Scopus)
  • Anomalous Rashba effect of bismuth(111) thin films studied by high-resolution spin- and angle-resolved photoemission spectroscopy

    Akari Takayama, Takafumi Sato, Seigo Souma, Takashi Takahashi

    JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B   30 ( 4 ) 04E107  2012.07  [Refereed]

     View Summary

    The authors have developed an ultrahigh-resolution spin-resolved photoemission spectrometer equipped with a highly efficient mini Mott detector and a high-intensity xenon plasma discharge lamp. An electron deflector situated between the hemispherical electron-energy analyzer and the Mott detector enables the determination of the electron's spin-polarization in three independent directions and the spectrometer achieves an energy resolution of 0.9 and 8 meV for nonspin-resolved and spin-resolved modes, respectively. By using this spectrometer, we have performed spin-and angle-resolved photoemission spectroscopy of bismuth thin films on Si(111) to investigate the spin structure of surface states. Unlike conventional Rashba splitting, the magnitude of the in-plane spin polarization is asymmetric across the zone center between the two elongated surface hole pockets and there is a giant out-of-plane spin polarization. The authors discuss these unusual spin textures in terms of a possible time-reversal symmetry breaking. (C) 2012 American Vacuum Society. [http://dx.doi.org/10.1116/1.4731467]

    DOI

    Scopus

    4
    Citation
    (Scopus)
  • Tunable Spin Polarization in Bismuth Ultrathin Film on Si(111)

    Akari Takayama, Takafumi Sato, Seigo Souma, Tamio Oguchi, Takashi Takahashi

    NANO LETTERS   12 ( 4 ) 1776 - 1779  2012.04  [Refereed]

     View Summary

    We performed a spin- and angle-resolved photoemission spectroscopy of bismuth ultrathin film on Si(111) with various film thickness d. We found that the spin polarization of spin-split Rashba surface states near the Brillouin-zone boundary, which is high (0.7) at d = 40 BL (bilayers), is gradually reduced on decreasing d and almost vanishes at d = 8 BL. This finding provides a novel method to generate spin-polarized electrons with tunable spin-polarization.

    DOI

    Scopus

    64
    Citation
    (Scopus)
  • Anomalous Rashba effect of bismuth(111) thin films studied by high-resolution spin- and angle-resolved photoemission spectroscopy

    Akari Takayama, Takafumi Sato, Seigo Souma, Takashi Takahashi

    Journal of Vacuum Science and Technology B:Nanotechnology and Microelectronics   30 ( 4 )  2012  [Refereed]

     View Summary

    The authors have developed an ultrahigh-resolution spin-resolved photoemission spectrometer equipped with a highly efficient mini Mott detector and a high-intensity xenon plasma discharge lamp. An electron deflector situated between the hemispherical electron-energy analyzer and the Mott detector enables the determination of the electron's spin-polarization in three independent directions and the spectrometer achieves an energy resolution of 0.9 and 8 meV for nonspin-resolved and spin-resolved modes, respectively. By using this spectrometer, we have performed spin- and angle-resolved photoemission spectroscopy of bismuth thin films on Si(111) to investigate the spin structure of surface states. Unlike conventional Rashba splitting, the magnitude of the in-plane spin polarization is asymmetric across the zone center between the two elongated surface hole pockets and there is a giant out-of-plane spin polarization. The authors discuss these unusual spin textures in terms of a possible time-reversal symmetry breaking. © 2012 American Vacuum Society.

    DOI

    Scopus

    4
    Citation
    (Scopus)
  • 高分解能スピン分解光電子分光装置の開発と表面ラシュバ効果の研究

    高山あかり, 相馬清吾, 高橋隆

    表面科学   33 ( 3 ) 165 - 171  2012

  • Evolution of surface states in Bi1-xSbx alloys across the topological phase transition

    H. Guo, K. Sugawara, A. Takayama, S. Souma, T. Sato, N. Satoh, A. Ohnishi, M. Kitaura, M. Sasaki, Q. -K. Xue, T. Takahashi

    PHYSICAL REVIEW B   83 ( 20 ) 201104(R)  2011.05  [Refereed]

     View Summary

    We performed a systematic, high-resolution, angle-resolved photoemission spectroscopy of Bi1-x Sb-x alloys to study the evolution of electronic states as a function of Sb concentration x. We revealed that the number of Fermi-level crossings is different between the topologically trivial (x = 0.023) and nontrivial (x = 0.17) phases: four and five times for the former and latter cases, respectively. We also found a systematic expansion of the bandwidth of spin-split surface bands at the (Gamma) over bar point upon substituting Bi with Sb, which is indicative of the monotonous reduction of the spin-orbit coupling strength.

    DOI

    Scopus

    38
    Citation
    (Scopus)
  • Direct Measurement of the Out-of-Plane Spin Texture in the Dirac-Cone Surface State of a Topological Insulator

    S. Souma, K. Kosaka, T. Sato, M. Komatsu, A. Takayama, T. Takahashi, M. Kriener, Kouji Segawa, Yoichi Ando

    PHYSICAL REVIEW LETTERS   106 ( 21 ) 216803  2011.05  [Refereed]

     View Summary

    We have performed spin-and angle-resolved photoemission spectroscopy of Bi2Te3 and present the first direct evidence for the existence of the out-of-plane spin component on the surface state of a topological insulator. We found that the magnitude of the out-of-plane spin polarization on a hexagonally deformed Fermi surface of Bi2Te3 reaches maximally 25% of the in-plane counterpart, while such a sizable out-of-plane spin component does not exist in the more circular Fermi surface of TlBiSe2, indicating that the hexagonal deformation of the Fermi surface is responsible for the deviation from the ideal helical spin texture. The observed out-of-plane polarization is much smaller than that expected from the existing theory, suggesting that an additional ingredient is necessary for correctly understanding the surface spin polarization in Bi2Te3.

    DOI

    Scopus

    169
    Citation
    (Scopus)
  • Giant Out-of-Plane Spin Component and the Asymmetry of Spin Polarization in Surface Rashba States of Bismuth Thin Film

    A. Takayama, T. Sato, S. Souma, T. Takahashi

    PHYSICAL REVIEW LETTERS   106 ( 16 ) 166401  2011.04  [Refereed]

     View Summary

    We have performed high-resolution spin-and angle-resolved photoemission spectroscopy of bismuth thin film on Si(111) to investigate the spin structure of surface states. Unlike the conventional Rashba splitting, the magnitude of the in-plane spin polarization is asymmetric between the two elongated surface hole pockets across the zone center. Moreover, we uncovered a giant out-of-plane spin polarization as large as the in-plane counterpart which switches the sign across the (Gamma) over bar-(M) over bar line. We discuss the present finding in terms of the symmetry breaking and the many-body effects.

    DOI

    Scopus

    85
    Citation
    (Scopus)
  • Erratum: Ultrahigh-resolution spin-resolved photoemission spectrometer with a mini Mott detector [Review of Scientific Instruments (2010) 81 (095101))

    S. Souma, A. Takayama, K. Sugawara, T. Sato, T. Takahashi

    Review of Scientific Instruments   81 ( 10 )  2010.10  [Refereed]

    DOI

    Scopus

  • Ultrahigh-resolution spin-resolved photoemission spectrometer with a mini Mott detector

    S. Souma, A. Takayama, K. Sugawara, T. Sato, T. Takahashi

    REVIEW OF SCIENTIFIC INSTRUMENTS   81 ( 9 ) 095101  2010.09  [Refereed]

     View Summary

    We have developed an ultrahigh-resolution spin-resolved photoemission spectrometer with a highly efficient mini Mott detector and an intense xenon plasma discharge lamp. The spectrometer achieves the energy resolutions of 0.9 and 8 meV for non-spin-resolved and spin-resolved modes, respectively. Three-dimensional spin-polarization is determined by using a 90 degrees electron deflector situated before the Mott detector. The performance of spectrometer is demonstrated by observation of a clear Rashba splitting of the Bi(111) surface states. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3480542]

    DOI

    Scopus

    36
    Citation
    (Scopus)
  • Magnetic Phase Transition of CeSb Studied by Low-Energy Angle-Resolved Photoemission Spectroscopy

    Akari Takayama, Seigo Souma, Takafumi Sato, Toshiyuki Arakane, Takashi Takahashi

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   78 ( 7 ) 073702  2009.07  [Refereed]

     View Summary

    We have performed low-energy ultrahigh-resolution angle-resolved photoemission spectroscopy on CeSb to elucidate the bulk electronic structure responsible for the magnetic phase transition. By using a newly developed xenon-plasma discharge lamp, we clearly observed the splitting of Sb 5p(3/2) and Cc 5d bands near the Fermi level as well as their strong hybridization below the Neel temperature (T-N = 16 K). We also revealed distinct differences in the band structure between the antiferropara- and antiferromagnetic phases. These experimental results are discussed in terms of the p-f/p-d mixing effect, the exchange splitting, and the magnetic band-folding. The present result also explains the emergence of fine structures in the optical conductivity in the magnetic phase.

    DOI

    Scopus

    18
    Citation
    (Scopus)
  • Observation of Fermi-surface-dependent nodeless superconducting gaps in Ba(0.6)K(0.4)Fe(2)As(2)

    H. Ding, P. Richard, K. Nakayama, K. Sugawara, T. Arakane, Y. Sekiba, A. Takayama, S. Souma, T. Sato, T. Takahashi, Z. Wang, X. Dai, Z. Fang, G. F. Chen, J. L. Luo, N. L. Wang

    EPL   83 ( 4 ) 47001  2008  [Refereed]

     View Summary

    We have performed a high-resolution angle- resolved photoelectron spectroscopy study on the newly discovered superconductor Ba(0.6)K(0.4)Fe(2)As(2) (T(c) = 37 K). We have observed two superconducting gaps with different values: a large gap (Delta similar to 12 meV) on the two small hole-like and electron-like Fermi surface (FS) sheets, and a small gap (similar to 6meV) on the large hole-like FS. Both gaps, closing simultaneously at the bulk transition temperature (T(c)), are nodeless and nearly isotropic around their respective FS sheets. The isotropic pairing interactions are strongly orbital dependent, as the ratio 2 Delta/k(B)T(c) switches from weak to strong coupling on different bands. The same and surprisingly large superconducting gap due to strong pairing on the two small FSs, which are connected by the (pi, 0) spin-density-wave vector in the parent compound, strongly suggests that the pairing mechanism originates from the inter-band interactions between these two nested FS sheets. Copyright (c) EPLA, 2008.

    DOI

    Scopus

    901
    Citation
    (Scopus)

▼display all

Books and Other Publications

  • High-Resolution Spin-Resolved Photoemission Spectrometer and the Rashba Effect in Bismuth Thin Films (Springer Theses)

    Akari Takayama( Part: Sole author)

    Springer  2015

    ASIN

Presentations

  • 角度分解光電子分光とその場伝導測定で探る 表面原子層の超伝導

    高山あかり  [Invited]

    日本物理学会2019年秋季学会 

    Presentation date: 2019.09

  • Structural analysis of bilayer-graphene intercalation compounds on SiC substrate by total-reflection high-energy positron diffraction

    Akari Takayama  [Invited]

    Presentation date: 2019.03

  • Exotic phenomena caused by spin-orbit coupling

    A. Takayama  [Invited]

    60 years of Physical Review Letters, APS/JPS Joint Symposium, JPS autumn meeting 2018  JPS

    Presentation date: 2018.09

  • 高速化データ駆動科学への期待:陽電子回折を中心に

    高山あかり  [Invited]

    ポスト「京」重点課題7研究会「データ駆動科学と高速計算科学」 

    Presentation date: 2018.07

  • Rashba 系表面超構造の超伝導

    高山あかり  [Invited]

    日本表面科学会 2017年真空・表面科学合同講演会  (横浜) 

    Presentation date: 2017.08

  • スピン分解光電子分光によるV族半金属薄膜におけるラシュバ効果の研究

    高山あかり  [Invited]

    日本物理学会第70回年次大会(領域9若手奨励賞受賞講演) 

    Presentation date: 2015.03

  • 五感で感じる電子の世界

    高山あかり  [Invited]

    第3回世界トップレベル研究拠点合同シンポジウムサイエンストークライブ 

    Presentation date: 2013.12

▼display all

Research Projects

  • Electronic states of artificially-stacked atomic-layer thinfilms with Rashba effect and topological electronic states

    Japan Society for the Promotion of Science  Grants-in-Aid for Scientific Research

    Project Year :

    2023.04
    -
    2026.03
     

  • Study of borophene by structural analysis using positrons and electronic structure observation using photons

    Japan Society for the Promotion of Science  Grants-in-Aid for Scientific Research

    Project Year :

    2019.04
    -
    2022.03
     

    Takayama Akari

     View Summary

    In this study, we have performed structure analysis of one-atomic-layer borophene, which exhibits a two-dimensional Dirac electron system, using total-reflection high-energy positron diffraction (TRHEPD). Our experimental results show that the structure of χ3-borophene on Ag(111) is a flat sheet structure without buckling and a triangular lattice, which is characteristic of borophene. We also investigated the structure of honeycomb borophene on Al(111) substrates. Two structural models have been proposed in previous studies, and we discussed the validity of these structures. We also studied the electronic structure of the borophene layer in borides and their hydrogenated borophenes.

  • 原子層半導体薄膜における歪みダイナミクス測定の新手法開発

    東芝メモリ  東芝メモリFS研究

    Project Year :

    2018.09
    -
    2020.03
     

  • Spin-texture mapping in k-space by spin-resolved AREPS

    Japan Society for the Promotion of Science  Grants-in-Aid for Scientific Research

    Project Year :

    2014.04
    -
    2018.03
     

    Souma Seigo, TAKAYAMA Akari

     View Summary

    Spin-orbit coupling (SOC) in matter strongly affects an electron’s spin which points to various directions depending on the type of SOC interaction and crystal symmetry. In this study, we have developed our spin-resolved photoemission spectrometer and, by using this, studied the electronic structure of high functional materials in which a relation between spin and momentum plays an important role in this functionality. Consequently, we have observed various intriguing electronic states; Surface Fermi arc in Weyl semimetals, As 4p hole-band in dilute magnetic semiconductor GaMnAs, one dimensional edge state in heavy metal film, unexpected surface state in line node semimetals, and so on. Based on the above results, we conclude that this project has succeeded in establishing of new topological materials and unraveling of mechanism of ferromagnetism in GaMnAs

  • 独立駆動4探針・磁性探針STMによるスピン偏極した1次元電子系の研究

    日本学術振興会  科学研究費助成事業

    Project Year :

    2015.04
    -
    2018.03
     

  • Study of Electron/Spin Transport at Molecules by Functionalized Four-Tip STM

    Project Year :

    2013.06
    -
    2018.03
     

     View Summary

    By using our four-tip scanning tunneling microscope prober with functionalized tips, we have measured charge and spin transport properties of various molecular objects and surfaces of electrodes which are components necessary for "molecular architechtonics" . (1) The conductivity of Nickel dithiolene nanosheets is very high and changes with redox control. (2) Bilayer graphene becomes superconducting around 2 K by Calcium intercalation. (3) The Berry phase of bilayer graphene changes from 0 to π by Li-intercalation and thermal desorption treatment. (4) A probe for pure spin injection has been developed

  • 多探針STM を用いた1次元Rashba 効果およびトポロジカルエッジ状態の研究

    井上科学振興財団  井上リサーチアウォード

    Project Year :

    2015.04
    -
    2017.03
     

  • Studies on Electronic and Spin Transport at Topological Surfaces and Edge States

    Project Year :

    2013.04
    -
    2016.03
     

     View Summary

    The present research on transport phenomena at surfaces of strong spin-orbit coupling materials has provided the following results. (1) Direct detection of suppression of back scattering of surface-state carriers on topological insulators. (2) Detection of photogalvanic effect (spin-polarized current induced by irradiation of circularly polarized light) at Rashba-type spin-split surface states. (3) Detection of energy-gap opening in a topological Dirac-cone type state by proximity effect of magnetic material. (4) Detection of spin Hall effect at ultrathin films of topological insulators. (5) Discovery of superconductivity at Rashba-type spin-split surface states. (6) Discovery of superconductivity at Ca-intercalated double-layer graphene

  • 高分解能スピン分解光電子分光による金属超薄膜の電子状態の研究

    日本学術振興会  科学研究費補助金

    Project Year :

    2013.04
    -
    2014.03
     

  • スピン分解超高分解能光電子分光装置の建設と磁性表面薄膜の研究

    日本学術振興会  科学研究費補助金

    Project Year :

    2010.04
    -
    2013.03
     

    高山 あかり

     View Summary

    これまでに建設を進めてきた超高分解能スピン分解光電子分光装置の最終調整を行った。検出器の感度比やバックグラウンドの補正方法を確立し、データの信頼性を向上させた。バックグラウンドノイズをより減少させるため、長時間のベーク・高電圧印加を行った。各電極における印加電圧の調整、残留ガスの除去や流入ガスの遮蔽による到達真空度の向上など、装置全体の性能も高めた。
    建設した装置を用いて、Si上に作成したV族半金属Bi薄膜のRashba効果についてスピン偏極率の膜厚依存性を測定した。その結果、膜厚が薄くなるにつれて表面バンドのスピン偏極率が小さくなることを明らかにし、SiとBiの界面においても、表面と同様なRashba効果を示す可能性を初めて明らかにした。以上の結果は米国化学誌Nano Lettersに掲載され、国内外の学会にて発表を行った他、多数の新聞で報道された。また、Rashba効果による表面バンドのスピン偏極率がバルクバンドから受ける影響について明らかにするため、Sbのスピン分解ARPES測定を行った。その結果、バルクバンド射影内において表面バンドのスピン偏極率が減少する様子を観測し、バルクバンドと表面バンドの混成がスピン偏極率に与える影響を実験的に明らかにした。この結果は、日本物理学会(2012年秋季大会)にて発表した。さらに、トポロジカル相転移による表面ディラックバンドのスピン構造の変化を明らかにするため、TlBi(S_<1-x>Se_x)_2のスピン偏極率の波数および組成依存性をスピン分解ARPESにて決定した。xを変化させた各組成でスピン偏極率を測定した結果、Se濃度(x)の減少とともにスピン偏極率が弱くなり、TlBiS_2(x=0.0)ではスピン偏極率がゼロであることを実験的に初めて観測した。以上の結果は、米国物理学誌Phys. Rev. Lett.に掲載された。

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Misc

  • Superconductivity on surface superstructure with Rashba-split states observed by ultra-lowe temperature scanning tunneling spectroscopy

    町田理, 町田理, 吉村優輝, 中村友謙, 花栗哲郎, 長谷川幸雄, 長谷川修司, 高山あかり

    日本物理学会講演概要集(CD-ROM)   76 ( 1 )  2021

    J-GLOBAL

  • Anisotropic electronic band structure of the intrinsic Si(110)-(1×1) surface: Experiment and theory

    Matsushita S. Y., Takayama A., Kawamoto E., Hu S., Watanbe K., Takahashi T., Suto Shozo

    Meeting Abstracts of the Physical Society of Japan   72 ( 0 ) 2236 - 2236  2017

    DOI CiNii

  • 10pAS-1 Topological proximity effect in Bi/TlBiSe_2 studied by high-resolution ARPES

    Shoman T., Takayama A., Sato T., Souma S., Takahashi T., Oguchi T., Segawa K., Ando Y.

    Meeting abstracts of the Physical Society of Japan   69 ( 2 ) 677 - 677  2014.08

    CiNii

  • 10pAS-1 Topological proximity effect in Bi/TlBiSe_2 studied by high-resolution ARPES

    Shoman T., Takayama A., Sato T., Souma S., Takahashi T., Oguchi T., Segawa K., Ando Y.

    Meeting abstracts of the Physical Society of Japan   69 ( 2 ) 461 - 461  2014.08

    CiNii

  • 28aBF-9 Electronic states of Bi-deposited TlBiSe2 studied by High-resolution ARPES

    Shoman T., Takayama A., Tanaka Y., Sugawara K., Souma S., Sato T., Takahashi T., Eto K., Segawa K., Ando Y.

    Meeting abstracts of the Physical Society of Japan   69 ( 1 ) 706 - 706  2014.03

    CiNii

  • 27pAQ-3 Edge state of Bi thin film studied by spin-resolved ARPES

    Takayama A., Sato T., Souma S., Oguchi T., Takahashi T.

    Meeting abstracts of the Physical Society of Japan   69 ( 1 ) 873 - 873  2014.03

    CiNii

  • Anomalous electronic structure of Bi/Si(111) surface studied by high-resolution ARPES

    Takayama A., Sato T., Souma S., Oguchi T., Takahashi T.

    Meeting abstracts of the Physical Society of Japan   68 ( 2 ) 786 - 786  2013.08

    CiNii

  • 18pFE-6 High-resolution spin-resolved ARPES study of Sb thin film

    Takayama A., Sato T., Souma S., Takahashi T.

    Meeting abstracts of the Physical Society of Japan   67 ( 2 ) 815 - 815  2012.08

    CiNii

  • 20aFJ-12 Spin-dependent electronic states near the quantum phase transition of a Tl-based topological insulator

    Sato T., Souma S., Komatsu M., Nomura M., Takayama A., Takahashi T., Eto K., Segawa K., Ando Y.

    Meeting abstracts of the Physical Society of Japan   67 ( 2 ) 698 - 698  2012.08

    CiNii

  • 20aEA-9 Spin-resolved ARPES study of TlBi(S, Se)_2

    Souma S., Komatsu M., Nomura M., Sato T., Takayama A., Takahashi T., Eto K., Segawa K., Ando Y.

    Meeting abstracts of the Physical Society of Japan   67 ( 2 ) 632 - 632  2012.08

    CiNii

  • 24pCK-3 Film-thickness dependence of spin polarization of surface electronic states in Bi/Si(111) thin film

    Takayama A., Sato T., Souma S., Oguchi T., Takahashi T.

    Meeting abstracts of the Physical Society of Japan   67 ( 1 ) 939 - 939  2012.03

    CiNii

  • 23pTR-11 Direct observation of z-spin component in Dirac-cone surface state of Bi_2Te_3 by spin-resolved ARPES

    Souma S., Kosaka K., Sato T., Komatsu M., Takayama A., Takahashi T., Kriener M., Segawa K., Ando Y.

    Meeting abstracts of the Physical Society of Japan   66 ( 2 ) 701 - 701  2011.08

    CiNii

  • 23aHA-8 Anisotropic Rashba effect of Bi(111) thin film studied by high-resolusion spin-resolved ARPES

    Takayama A., Sato T., Souma S., Takahashi T.

    Meeting abstracts of the Physical Society of Japan   66 ( 2 ) 929 - 929  2011.08

    CiNii

  • 26aTH-8 Anomalous Rashba effect of Bi(111) surface studied by high-resolusion spin-resolved ARPES

    Takayama A., Souma S., Sato T., Takahashi T.

    Meeting abstracts of the Physical Society of Japan   66 ( 1 ) 915 - 915  2011.03

    CiNii

  • 25aWX-3 Spin-resolved electronic structure of Bi(111) surface

    Takayama A., Kosaka K., Sugawara K., Souma S., Sato T., Takahashi T.

    Meeting abstracts of the Physical Society of Japan   65 ( 2 ) 873 - 873  2010.08

    CiNii

  • 26aWJ-11 Anomalous electronic structure of CeSb : xenon-plasma-light ARPES

    Kosaka K., Takayama A., Souma S., Arakane T., Sato T., Takahashi T.

    Meeting abstracts of the Physical Society of Japan   65 ( 2 ) 620 - 620  2010.08

    CiNii

  • 24pRE-9 High-resolution ARPES study of Bi_<1-x>Sb_x(x=0.023)

    Guo H., Sugawara K., Takayama A., Souma S., Sato T., Sato N., Sasaki M., Takahashi T.

    Meeting abstracts of the Physical Society of Japan   65 ( 2 ) 703 - 703  2010.08

    CiNii

  • 20pGQ-1 Ultrahigh-resolution spin-resolved ARPES study of Bi thin film

    Takayama A., Sugawara K., Souma S., Sato T., Takahashi T.

    Meeting abstracts of the Physical Society of Japan   65 ( 1 ) 931 - 931  2010.03

    CiNii

  • 23pHL-9 Development of spin-resolved photoemission spectrometer based on mini-Mott detector: III

    Souma S., Takayama A., Sugawara K., Sato T., Takahashi T.

    Meeting abstracts of the Physical Society of Japan   65 ( 1 ) 811 - 811  2010.03

    CiNii

  • 27aYG-4 Development of a mini-Mott spin detector and Rashba effect in surface electronic states of group-V semimetals

    Takayama A., Souma S., Sugawara K., Sato T., Takahashi T.

    Meeting abstracts of the Physical Society of Japan   64 ( 2 ) 834 - 834  2009.08

    CiNii

  • 27pYE-6 High-resolution ARPES study of high quality growth graphene

    Sugawara K., Takayama A., Souma S., Sato T., Takahashi T.

    Meeting abstracts of the Physical Society of Japan   64 ( 2 ) 783 - 783  2009.08

    CiNii

  • 26pXA-9 Development of spin-resolved photoemission spectrometer based on a mini-Mott detector: II

    Souma S., Takayama A., Sugawara K., Sato T., Takahashi T.

    Meeting abstracts of the Physical Society of Japan   64 ( 2 ) 636 - 636  2009.08

    CiNii

  • 30aVA-10 Development of spin-resolved photoemission spectrometer based on mni-Mott detector

    Souma S., Takayama A., Mori T., Sugawara K., Sato T., Takahashi T.

    Meeting abstracts of the Physical Society of Japan   64 ( 1 ) 763 - 763  2009.03

    CiNii

  • 30aVA-2 Direct observation of superconducting gap in iron-based superconductor Ba_<1-x>K_xFe_2As_2 by ARPES

    Sato T., Ding H., Richard P., Nakayama K., Sugawara K., Arakane T., Sekiba Y., Takayama A., Souma S., Wang Z., Dai X., Fang Z., Chen G. F., Luo J. L., Wang N. L., Takahashi T.

    Meeting abstracts of the Physical Society of Japan   64 ( 1 ) 761 - 761  2009.03

    CiNii

  • 28aYH-6 Sample growth and high-resolution ARPES study of single- and multi-layer graphene

    Ise K., Sugawara K., Takayama A., Souma S., Sato T., Takahashi T.

    Meeting abstracts of the Physical Society of Japan   64 ( 1 ) 828 - 828  2009.03

    CiNii

  • 21aYG-8 Xenon-plasma-light high-resolution ARPES study of CeSb

    Takayama A., Souma S., Nakayama K., Sekiba Y., Sato T., Takahashi T.

    Meeting abstracts of the Physical Society of Japan   63 ( 2 ) 639 - 639  2008.08

    CiNii

  • 20aQF-10 Xenon-plasma-light high-resolution PES study of Yb_4As_3

    Souma S., Arakane T., Nakayama K., Takayama A., Sekiba Y., Sato T., Takahashi T., Ochiai A.

    Meeting abstracts of the Physical Society of Japan   63 ( 2 ) 444 - 444  2008.08

    CiNii

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Syllabus

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Sub-affiliation

  • Affiliated organization   Global Education Center

Research Institute

  • 2022
    -
    2024

    Waseda Research Institute for Science and Engineering   Concurrent Researcher

Internal Special Research Projects

  • 非従来型原子層超伝導における基板依存性の解明

    2023   Lars Konermann, 孝橋麟太郎, 今村真幸, 高橋和敏, 角田一樹, 宮本幸治, 奥田太一, 望月出海

     View Summary

    Rashba効果によるスピン分裂した電子状態を持つ系での超伝導は、スピン三重項超伝導の可能性が予測され、非BCS的な超伝導物質の候補として現在精力的に研究が行われている。本研究グループでは、このRashba型超伝導体の解明に向けて、表面原子層合金による超構造を対象としてこれまで研究を行っており、非BCS的超伝導ギャップの観測などを報告してきた。Rashba型超伝導においては、超伝導転移温度TcとRashba効果の大きさの関係性は、超構造の平坦性、すなわち基板との相互作用に起因する構造にも関連することが予測されているが、Rashba型超伝導となる候補物質自体が少ないことが物性解明の課題となっていた。最近、新たな候補物質となる(Pb, Bi)超構造の存在が報告され、本研究では、SiおよびGe基板上に作製した試料を用いて、角度分解光電子分光(ARPES)による電子状態の観測を行った。実験の結果、いくつかの構造で金属的な電子状態を観測したが、我々の観測した電子状態は理論予測とは異なっており、提案された構造モデルとは異なる構造をもつ可能性が示された。(Pb, Bi)合金系においては、SiとGe基板で作製できる超構造の種類やBiとPbの組成比に違いがあることが実験により再現された。また、スピン分解ARPES実験により、これらの電子状態がスピン偏極を持つことを初めて観測した。さらに、これらの物質について全反射高速陽電子回折(TRHEPD)実験による構造解析を試みた。本研究では合金に加え、Pbのみ、Biのみの表面超構造も作製して測定を行い、これらの構造との比較も行うことで、表面合金の形成過程についても議論した。角度分解光電子分光実験の結果については、国内・国際学会での発表を行い、現在論文執筆中である。@font-face {font-family:"Cambria Math"; panose-1:2 4 5 3 5 4 6 3 2 4; mso-font-charset:0; mso-generic-font-family:roman; mso-font-pitch:variable; mso-font-signature:-536870145 1107305727 0 0 415 0;}@font-face {font-family:游明朝; panose-1:2 2 4 0 0 0 0 0 0 0; mso-font-charset:128; mso-generic-font-family:roman; mso-font-pitch:variable; mso-font-signature:-2147482905 717749503 18 0 131231 0;}@font-face {font-family:"Apple Color Emoji"; panose-1:0 0 0 0 0 0 0 0 0 0; mso-font-charset:0; mso-generic-font-family:auto; mso-font-pitch:variable; mso-font-signature:3 402653184 335544320 0 1 0;}@font-face {font-family:Cambria; panose-1:2 4 5 3 5 4 6 3 2 4; mso-font-charset:0; mso-generic-font-family:roman; mso-font-pitch:variable; mso-font-signature:-536870145 1073743103 0 0 415 0;}@font-face {font-family:"\@游明朝"; mso-font-charset:128; mso-generic-font-family:roman; mso-font-pitch:variable; mso-font-signature:-2147482905 717749503 18 0 131231 0;}p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-unhide:no; mso-style-qformat:yes; mso-style-parent:""; margin:0mm; text-align:justify; text-justify:inter-ideograph; mso-pagination:none; font-size:12.0pt; font-family:"游明朝",serif; mso-ascii-font-family:游明朝; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:游明朝; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:游明朝; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi; mso-font-kerning:1.0pt;}.MsoChpDefault {mso-style-type:export-only; mso-default-props:yes; font-size:12.0pt; mso-ansi-font-size:12.0pt; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi; mso-ligatures:none;}div.WordSection1 {page:WordSection1;}Rashba効果によるスピン分裂した電子状態を持つ系での超伝導は、スピン三重項超伝導の可能性が予測され、非BCS的な超伝導物質の候補として現在精力的に研究が行われている。本研究グループでは、このRashba型超伝導体の解明に向けて、表面原子層合金による超構造を対象としてこれまで研究を行っており、非BCS的超伝導ギャップの観測などを報告してきた。Rashba型超伝導においては、超伝導転移温度TcとRashba効果の大きさの関係性は、超構造の平坦性、すなわち基板との相互作用に起因する構造にも関連することが予測されているが、Rashba型超伝導となる候補物質自体が少ないことが物性解明の課題となっていた。最近、新たな候補物質となる(Pb, Bi)超構造の存在が報告され、本研究では、SiおよびGe基板上に作製した試料を用いて、角度分解光電子分光(ARPES)による電子状態の観測を行った。実験の結果、いくつかの構造で金属的な電子状態を観測したが、我々の観測した電子状態は理論予測とは異なっており、提案された構造モデルとは異なる構造をもつ可能性が示された。(Pb, Bi)合金系においては、SiとGe基板で作製できる超構造の種類やBiとPbの組成比に違いがあることが実験により再現された。また、スピン分解ARPES実験により、これらの電子状態がスピン偏極を持つことを初めて観測した。さらに、これらの物質について全反射高速陽電子回折(TRHEPD)実験による構造解析を試みた。本研究では合金に加え、Pbのみ、Biのみの表面超構造も作製して測定を行い、これらの構造との比較も行うことで、表面合金の形成過程についても議論した。角度分解光電子分光実験の結果については、国内・国際学会での発表を行い、現在論文執筆中である。

  • 2次元ディラックノーダルライン物質の構造と電子状態および輸送特性の解明

    2022   秋山了太

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    本研究ではSi基板上のCu2Si単原子層を作製し,放射光のエネルギーおよび偏光を適切に設定したARPES実験を行った。その結果,Cu2Siにおいてノーダルラインを形成するのに重要となる電子バンドを実験で初めて観測した。また,Si基板上Cu2Siではノーダルラインが非占有状態側に位置することを明らかにした。さらに,極低温・強磁場下での電気伝導測定を行った結果,本試料は極低温で弱半局在的な振る舞いを示すことを観測した。この特異な輸送特性はCu2Siのノーダルライン型電子状態が関係していることを示唆している。また,構造解析の結果,Si基板上のCu2Siはバックリングをもつ構造であることがわかった。

  • 原子層Sb超薄膜におけるトポロジカル相転移の実証

    2022   阿部浩子

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    本研究では,Sb超薄膜のトポロジー制御・スピン物性の増幅実現を目標として,Bi基板上の原子層薄膜Sbのスピン特性を明らかにすることを目的にスピン分解ARPES測定を行った。実験は広島大学放射光科学研究センターBL9A/Bで行った。3BL-Sb薄膜/Bi(111)において新たに観測されたV字型のバンドについて面内方向のスピン分解ARPES測定を行った結果,有限のスピン偏曲率を観測した。Free-standingな3BLのSb薄膜は理論的にはスピン偏極成分を持たないため,このスピン偏極成分はBiのRashba分裂表面状態からの染み出し効果やSb超薄膜の格子引き伸ばしによるトポロジカル相転移によって生じた可能性が高いことが示唆される。

  • 対称性の破れた原子層構造における非従来型超伝導現象の直接観測

    2021  

     View Summary

    本研究では、対称性の破れた原子層構造における非従来型超伝導現象、すなわちRashba型表面超伝導の電子状態および超伝導特性について知見を得ることを目的として理化学研究所創発物性計測研究チームと共同で研究を行った。 (Tl, Pb)/Si(111)表面超構造について極低温・強磁場下での走査トンネル電子顕微鏡/分光(STM/STS)測定を行った結果、STM観測からは以前の報告と全く違いがないにもかかわらず、STS測定では従来型の超伝導特性を示すことが明らかとなった。これらの結果は、フェルミ準位がわずかに異なることでフェルミ面の形状が変化し物性が変化するリフシッツ転位によるものだと結論付けた。

  • グラフェンのディラック電子系における超伝導ギャップの直接観測

    2020  

     View Summary

    本研究において超伝導ギャップ測定を行う予定であった実験施設への立ち入りが制限されたため、本実験予定を変更し、ディラック電子系の一種であるディラックノーダルライン型電子状態を持ち、かつ超伝導発現が理論的に示唆されている単原子層Cu2Siの電子状態および超伝導発現の有無について研究を行った。極低温・磁場下での電気伝導測定および解析の結果、弱半局在効果を観測した。一方、理論の予測とは異なり超伝導は発現しなかったが、このことはSi基板の効果による歪みが超伝導発現を阻害した可能性を示唆している。さらに、電子状態測定実験と比較した結果、この弱半局在効果はディラック電子系由来のものであると考えられる。

  • 合金系表面超構造における2次元超伝導体の探索

    2019   辻川夕貴

     View Summary

     本研究は、表面超構造における低次元超伝導の物性解明を目指す研究の基盤を形成するため、新たな2次元超構造を探索することを目的して研究を行った。ディラック電子をもつ炭素の二次元シートであるグラフェンにおいて、超伝導体であるPbを数原子層蒸着するとグラフェン上にアイランドが成長するが、STMおよび電気伝導測定の結果、Pbアイランドどうしが直接接触していなくても試料全体が超伝導を示すことを見出した。このことは、Pbアイランドからの近接効果によってグラフェン上を超伝導電流が流れていることを意味しており、この系が二次元超伝導の新たな研究舞台になることを提案した。

  • ランダム欠陥を取り入れた超巨大構造としての二次元物質解析

    2019   星健夫, 辻川夕貴

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    本研究では⽋陥を持つグラフェンの正確な物性理解と評価のため,以下の研究を行った。(i)⽋陥を含むグラフェンの作成:脱離法によりSiC基板上に作成したグラフェンを初期構造とし、Arスパッタリングによる物理的破壊によりグラフェンへ欠損を導入した。(ii)物性測定:Arスパッタリングの印加電圧依存性(0, 0.2, 1.0 kV)をSTMおよび電気伝導測定を用いて調べた。STM測定の結果、グラフェンのテラスの概形は1.0 kVでのスパッタリング後も観測された。また、電気伝導測定の結果、0.2 kVでのスパッタリングではグラフェンの電気抵抗値の変化は少なく、1.0 kVでグラフェンからの伝導度がなくなることがわかった。今後、この試料について構造解析を行う予定である。

  • 原子層薄膜における歪みダイナミクス測定の新手法開発

    2018   保原麗

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     本研究では「柔軟性の高いシート上」の「柔軟性の高いグラフェン」を基板として薄膜を作成し、東京大学において連携研究者とともに歪み物性測定を行った。SiC基板上に作成した2層グラフェンからカプトンテープを用いてグラフェンの剥離・転写し、走査型電子顕微鏡(SEM)を用いて観察した。この試料について電気伝導測定を行い、SiC基板上の2層グラフェンの抵抗値とおおよそ同じオーダーの抵抗値を観測した。グラファイトを用いて同様の実験を行った結果、SiC基板上のグラフェンと比べて転写率は高いものの様々な層数の薄片が混在し、層数制御は難しいことが確認できた。以上の結果は学位論文(学士)として提出・受理された。

  • 機能性原子層物質における物性・構造の完全解明

    2018  

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    本研究は、角度分解高電子分光を用いて機能性原子層構造の電子構造の決定と物性の発現メカニズムの解明を目指した実験を行った。超伝導とディラック電子系の関係性に着目し、Pbを蒸着した2層グラフェンを用いて測定した結果、Pbアイランドによる金属的なバンド分散、さらにPb蒸着によってグラフェン特有のディラック型バンド分散がフェルミ準位側にシフトする様子を観測した。またCu/Si(111)を用いた実験では、第2ブリルアンゾーン内において新たに観測したバンドがノーダルライン型バンド分散である可能性が高いと見出した。以上の結果は、以上の成果は学位論文(学士)として提出・受理された。

  • 全反射高速陽電子回折を用いたノーダルライン半金属Cu2Siの構造解析

    2018  

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    本研究は、ノーダルライン半金属の可能性が示唆されているCu/Si(111)の構造を詳細に決定するため、全反射⾼速陽電⼦回折(TRHEPD)法による鏡⾯反射スポット強度の視射⾓依存性について実験およびシミュレーションを用いて構造解析を行った。先行研究において提案されている3種類の銅シリサイド構造について、Si基板との距離やバックリング度合いを変化させた様々な構造モデルを構築し、その理論曲線を実際の測定結果と⽐較した結果、バックリングのない構造において、Cu2SiおよびCuSi2のロッキング曲線が実験結果と⽐較的近い曲線を⽰すことを⾒出した。このことは以上の成果は学位論文(学士)として提出・受理され、4月に学会発表を予定している。

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