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| 1.
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Yamaguchi, Naoya ; Ishii, Fumiyuki ; 山口, 直也 ; 石井, 史之
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金沢大学ナノマテリアル研究所<br />We investigate the effect of tensile strain on spin splitting at the n-type interface of LaAlO3/SrTi
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O3 in terms of spin–orbit coupling coefficient α and spin texture in the momentum space using first-principles density-functional calculations. Our results show that α could be controlled by the tensile strain, and can be enhanced by up to 5 times for a tensile strain of 7%, and the effect of the tensile strain leads to a persistent spin helix, which has a long spin lifetime. The strain effect on the LaAlO3/SrTiO3 interface is important for various applications such as spin field-effect transistors and spin-to-charge conversion.
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Minami, Susumu ; Ishii, Fumiyuki ; Mizuta, Yo Pierre ; Saito, Mineo ; 見波, 将 ; 石井, 史之 ; 斎藤, 峯雄
概要:
金沢大学自然科学研究科数物科学専攻<br />We have performed systematic density functional calculations and evaluated thermoelectric propert
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ies (Seebeck coefficient and anomalous Nernst coefficient) of half-Heusler compounds CoMSb (M = Sc, Ti, V, Cr, and Mn). The carrier concentration dependence of the Seebeck coefficient in nonmagnetic compounds is in good agreement with experiments. We found that the half-metallic ferromagnetic CoMnSb shows a large anomalous Nernst effect originating from Berry curvature near the Brillouin zone boundary. These results help us understand the mechanism of a large anomalous Nernst coefficient and give us a clue to design high performance magnetic thermoelectric materials. © 2018 Author(s).<br />Embargo Period 12 months
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Tanaka, Yuto ; Saito, Mineo ; Ishii, Fumiyuki ; 斎藤, 峯雄 ; 石井, 史之
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金沢大学理工研究域数物科学系<br />By using the nonequilibrium Green's function (NEGF) method, we perform first-principles calculations
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for phosphorene and bismuthene and evaluate their thermoelectric properties on the basis of the Landauer formalism. We find that the figures of merit strongly depend on the transport direction: the figures for the zigzag (ZZ) direction are substantially different from those for the armchair direction. These anisotropies are found to originate from the difference between the transmission functions in the AM and ZZ directions. We find that p-type bismuthene has a large figure of merit when the transport is in the ZZ direction, then we conclude that bismuthene has some advantages over phosphorene as a thermoelectric material. We also clarify that the spin-orbit coupling has a substantial effect on the thermoelectric properties of bismuthene. © 2018 The Japan Society of Applied Physics.<br />Embargo Period 12 months
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Manaf, Muhamad Nasruddin ; Minami, Susumu ; Ishii, Fumiyuki ; Saito, Mineo ; 見波, 将 ; 石井, 史之 ; 斎藤, 峯雄
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金沢大学理工研究域数物科学系<br />We present the calculation of anomalous muonium in silicon which has been extensively studied but th
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ere still remains unresolved problems in the first-principles calculations. We perform calculations using spin-polarized density functional theory within the general gradient approximation or the local density approximation. We check the size effect of supercells and find that we need to use large sizes of supercells to obtain reliable results. Some quantitative disagreement between the experimental and theoretical values in previous studies may be due to the use of insufficient sizes of supercells. We clarify that the negative Fermi contact interaction constant (FCIC) is induced by the electron correlation effect. By using the Hubbard model, we find that the FCIC is zero when we neglect the correlation effect and the negative value of the FCIC is induced by the correlation effect. © 2019 The Japan Society of Applied Physics.<br />Embargo Period 12 months
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Kotaka, Hiroki ; Ishii, Fumiyuki ; Saito, Mineo ; Sawada, Keisuke ; Uramoto, Yuki ; Nagao, Tadaaki ; Yaginuma, Shin ; 石井, 史之 ; 斎藤, 峯雄
概要:
金沢大学理工研究域数物科学系<br />We perform fully-relativistic first-principles calculations on Bi{012}films which are stable when th
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e film thickness is very small. We find that the 2ML free-standing film has bistability, i,e, the two top atoms are buckled in one geometry and they are nearly flat in the other geometry. The calculated amplitude of the buckling in the buckled structures is 0.17 Å whereas the amplitude is 0.41 Å when the spin-orbit interaction (SOI) is not included in the calculation. Furthermore, the bond lengths between the top and the second top atoms in the buckled structure are alternant when the SOI is included in the calculation. Therefore, the SOI has a significant effect on the geometry. The total energy difference between the two structures is less than 1 meV. Therefore, we expect that one of the two structures appears depending on circumstance such as the interaction between the substrate and the film. [DOI: 10.1380/ejssnt.2009.13]
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Sawada, Keisuke ; Ishii, Fumiyuki ; Saito, Mineo ; 石井, 史之 ; 斎藤, 峯雄
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金沢大学理工研究域数物科学系<br />We study magnetism of zigzag graphene nanoribbons (ZGNRs) whose ribbon widths are 1.8–2.2 nm by perf
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orming first-principles density functional theory calculations. In contrast with freestanding ZGNRs, ZGNRs directly adsorbed on Ni(111) do not show flat-band magnetism due to strong orbital hybridization between edge-localized C p orbitals and Ni d orbitals. The flat-band magnetism of the ZGNR is recovered by introduction of a graphene sheet between the ZGNR and Ni(111) as a buffer layer which weakened the orbital hybridization. In this case, a parallel configuration of spin moments at the two edges has lower energy than the antiparallel spin configuration whereas the magnetic ground state of the freestanding ZGNR has an antiparallel spin configuration. We explore the effects of orbital hybridization and charge transfer on the magnetic stability of ZGNRs on graphene/Ni(111).
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Sawada, Keisuke ; Ishii, Fumiyuki ; Saito, Mineo ; 石井, 史之 ; 斎藤, 峯雄
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金沢大学理工研究域数物科学系<br />We studied magnetism in bilayer and multilayer zigzag graphene nanoribbons (ZGNRs) through first-pri
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nciples density functional theory calculations. We found that the magnetic ground state of bilayer ZGNRs is the C-type antiferromagnetic (AFM) state, which is the AFM order between intraplane-edge carbon atoms and ferromagnetic (FM) order between interplane edge carbon atoms. In the cases of infinitely stacked multilayer ZGNRs, i.e., zigzag graphite nanoribbons, the C-type AFM state is also the most stable. By carrier doping, we found that the magnetic ground state changed from the C-AFM state to the FM state and, thus, realized two-dimensional FM surface (edge) states of graphite with a metallic conductivity.Part of this research has been funded by the MEXT HPCI Strategic Program. This work was partly supported by Grants-in-Aid for Scientific Research (Nos. 21560030, 22016003, 25104714, 25790007, and 25390008) from the JSPS. The computations in this research have been performed using the supercomputers at the ISSP, University of Tokyo, the RCCS, Okazaki National Institute, and the IMR, Tohoku University.<br />Embargo Period 12 months
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| 8.
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石井, 史之 ; Ishii, Fumiyuki
概要:
金沢大学ナノマテリアル研究所<br />今年度は貴金属fcc(111)表面, fcc金属(111)ビスマス表面合金, ビスマス酸化物(Bi2O3)バルク, Bi2O3/貴金属fcc(111)界面のスピン軌道結合係数を定量的に求め、これまで報
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告されている実験結果との比較、実験グループで実施中の測定結果と比較をおこなった。また、運動量空間のスピン構造を計算し、詳細にスピン軌道結合係数を明らかにした。fcc金属(111)Bi表面合金については、Rashba効果が発現する最小モデルであるBiM2(M=Cu, Ag, Au, Ni, Co, Fe)超薄膜についてfcc遷移金属, fcc貴金属について、系統的に調べた。これらの結果から、大きなスピン流ー電流変換が報告されているBi/Ag系のみが特殊であり、Rashba係数の貴金属の層数依存性を調べた結果、Bi/Agのみ層数にほとんど依存せず、ラシュバ状態が強く局在していることが明らかになった。この成果は、第18回結晶成長国際会議（ICCGE-18）にて発表をおこない、論文としてまとめられた（N. Yamaguchi, H. Kotaka, and F. Ishii, J. Cryst. Growth, in press, arXiv:1609.09782）。スピン変換現象の一つである異常ネルンスト効果について、前年に引き続き、Skyrmion結晶系について詳しく調べた。特に、スキルミオン結晶のサイズが大きくなればなるほど、異常ホール効果、異常ネルンスト効果が大きくなることが明らかになり、スキルミオン結晶が高効率な熱電変換材料の候補となりうる可能性を示した。この成果は論文としてまとめられた（Y. P. Mizuta and F. Ishii, Scientific Reports, 6, 28076(2016)）。<br />研究課題/領域番号:15H01015, 研究期間(年度):2015-04-01 – 2017-03-31
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| 9.
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石井, 史之 ; Ishii, Fumiyuki
概要:
金沢大学理工研究域数物科学系<br />ペロブスカイト型遷移金属酸化物LaAlO3/SrTiO3のヘテロ界面の電子状態を詳しく調べた。ヘテロ界面をモデル化するため、人工超格子(LaAlO3)n/(SrTiO3)nによる計算と有効遮蔽体法によ
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る計算の二つの方法で密度汎関数法に基づき、スピン軌道相互作用を考慮した第一原理電子状態計算をおこなった。界面状態のスピン軌道分裂から見積もった、ラシュバ係数は両手法で大きく変わらず、実験結果とも同じオーダーであることが明らかになった。また、人工超格子系については、内部電場を見積り、ラシュバ係数の層数依存性について調べ、内部電場とラシュバ係数の相関を明らかにした。人工超格子の研究成果については、Molecular Simulation, published online DOI:10.1080/08927022.2014.987986として、論文発表をおこなった。界面近傍の歪みについて詳しく解析した結果については、投稿準備中である。遷移金属酸化物人工超格子によるトポロジカル界面状態の理論設計のため、トポロジカル不変量で分類される、トポロジカル絶縁体（バルク）の探索をおこなった。他方、典型的なトポロジカル絶縁体Bi2Te3, Bi2Se3の表面状態のスピン構造の解析、また、Z2不変量の計算によるBi2Te3, Bi2Se3内部パラメータの変化によるトポロジカル相転移を明らかにした。これらの研究成果は、Molecular Simulation, published online DOI:10.1080/08927022.2014.964476, ならびにJPS Conf. Proc. 5, 011022 (2015)に論文発表をおこなった。<br />研究課題/領域番号:25104714, 研究期間(年度):2013-04-01 – 2015-03-31
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| 10.
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石井, 史之 ; Ishii, Fumiyuki
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金沢大学ナノマテリアル研究所<br />物質の端もしくは異なる物質の接合面（界面）では、通常の物質と異なる性質を示す。特に、電子のスピンを活用した、エレクトロニクスであるスピントロニクスの応用に繋がる、特徴的な電子の状態が出現する。本研究で
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は、地球上に豊富に存在する酸素の化合物である、酸化物の表面や界面、新しい炭素材料であるグラフェンとの接合面などについて、その電子状態について詳しく調べた。その結果、酸化物誘電体とその界面においては電気分極を指標とする内部電場で生ずる強い有効磁場が生ずることが明らかになった。この性質はスピンエレクトロニクスのみならず、熱エネルギーを電気エネルギーに変換する熱電効果への応用が期待される。<br />At the surface of the materials and at the interface of different materials, they show properties different from ordinary bulk materials, the electronic state which can be used for spintronics applications. In this study, we have investigated the electronic state at the surface and interface of oxides material which abundantly present on Earth and the interface of graphene and oxide. As a result, we found that an effective magnetic field is generated by the internal electric field characterized by electric polarization at the interface between oxide and other substances. This property is expected to be applied not only for spin electronics but also for thermoelectric effect that converts thermal energy into electric energy.<br />研究課題/領域番号:25790007, 研究期間(年度):2001-2006
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