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論文

Dynamical spin-to-charge conversion on the edge of quantum spin Hall insulator

荒木 康史; 三澤 貴宏*; 野村 健太郎*

Physical Review Research (Internet), 2(2), p.023195_1 - 023195_11, 2020/05

We theoretically manifest that the edge of a quantum spin Hall insulator (QSHI), attached to an insulating ferromagnet (FM), can realize a highly efficient spin-to-charge conversion. Based on a one-dimensional QSHI-FM junction, the electron dynamics on the QSHI edge is analyzed, driven by a magnetization dynamics in the FM. Under a large gap opening on the edge from the magnetic exchange coupling, we find that the spin injection into the QSHI edge gets suppressed while the charge current driven on the edge gets maximized, demanded by the band topology of the one-dimensional helical edge states.

論文

水素分配制御によるアルミニウム合金の力学特性最適化

戸田 裕之*; 山口 正剛; 松田 健二*; 清水 一行*; 平山 恭介*; Su, H.*; 藤原 比呂*; 海老原 健一; 板倉 充洋; 都留 智仁; et al.

鉄と鋼, 105(2), p.240 - 253, 2019/02

 被引用回数:0 パーセンタイル:100(Metallurgy & Metallurgical Engineering)

本レビューでは、高強度アルミニウム合金の水素脆化に関する研究活動、特に様々なトラップサイトでの水素トラップとそれによる水素脆化への影響に焦点を当てて報告する。高亜鉛濃度Al-Zn-Mg合金において、高分解能TEM法による析出物のナノ構造及び界面構造の分析や、高分解能X線マイクロトモグラフィー技術による詳細な破壊マイクロメカニズムとマイクロ構造-破壊特性関係の調査がなされ、さらに、ごく最近実現された超高分解能X線顕微鏡により特徴的な局部的変形、亀裂の発生・成長が観察されている。また、第一原理シミュレーションによる数々の水素トラップサイトのトラップエネルギー導出を元に、変形・破壊中の水素再分配が解析された。水素の再分配と3つの異なるミクロ機構による水素脆化との間の関係を論じ、水素脆化が起こるための現実的な条件を説明する。

論文

Spin-electricity conversion induced by spin injection into topological insulators

塩見 雄毅*; 野村 健太郎*; 梶原 瑛祐*; 江藤 数馬*; Novak, M.*; 瀬川 耕司*; 安藤 陽一*; 齊藤 英治

Physical Review Letters, 113(19), p.196601_1 - 196601_5, 2014/11

 被引用回数:177 パーセンタイル:0.88(Physics, Multidisciplinary)

We report successful spin injection into the surface states of topological insulators by using a spin pumping technique. By measuring the voltage that shows up across the samples as a result of spin pumping, we demonstrate that a spin-electricity conversion effect takes place in the surface states of bulk-insulating topological insulators Bi$$_{1.5}$$Sb$$_{0.5}$$Te$$_{1.7}$$Se$$_{1.3}$$ and Sn-doped Bi$$_{2}$$Te$$_{2}$$Se. In this process, the injected spins are converted into a charge current along the Hall direction due to the spin-momentum locking on the surface state.

口頭

ITERにおける中性子計測を用いた燃料比測定法の開発

岡田 耕一; 近藤 恵太郎; 落合 謙太郎; 佐藤 聡; 今野 力; 西谷 健夫; 野村 健*; 岡本 敦*; 北島 純男*; 笹尾 真実子*

no journal, , 

ITERにおいて、燃焼領域中心の$$n_{mathrm{d}}$$/$$n_{mathrm{t}}$$燃料イオン比を測定することが求められている。燃料イオン比を測定するための一つの方法としてDT/DD反応比を利用したものがあり、われわれは中性子計測を用いて反応比を求めることを目標としている。本報告では、燃料比測定を目的とした中性子計測システムに対して要求される性能についての検討に対する報告を行う。計測器としては2結晶型のTOFスペクトロメータを採用した。ITERをモデル化し、プラズマでの中性子発生数から計測器への入射中性子数を求め、第1検出器の飽和が起こらない条件を導いた。また、燃焼制御のサポートを目的としていることから、100ms以内に統計精度10%を得ることを目標とし、それに合うような計数効率の条件、つまり、第1検出器に入射した中性子数に対するTOFスペクトロメータとしての計数の割合が要求に見合うような条件も導いた。モンテカルロコード(MCNP)を用いてTOFスペクトロメータで用いる検出器の配置の変化による計数効率への影響を調べ、上記2つの条件と合わせて検討した。

口頭

Dissipative spin pumping in two-dimensional quantum spin Hall insulator

荒木 康史; 三澤 貴宏*; 野村 健太郎*

no journal, , 

We present our theoretical work on spin pumping into a two-dimensional (2D) quantum spin Hall inslator (QSHI). Recent theories and experiments have demonstrated the QSHI phase in a monolayer of transition metal dichalcogenide 1T'-WTe2, which can be easily engineered in contrast to traditionally-known HgTe/CdTe and InAs/GaSb quantum wells. While the theory of spin pumping is well established in normal metals by focusing on the spin-dependent electron scattering at the interface, it is unreliable for topologically nontrivial interfaces in such systems. In the present work, we consider a junction of a ferromagnet and a 2D QSHI at its 1D edge, and demonstrate the pumping of angular momentum from the spin-precessing ferromagnet into the QSHI. Using the Floquet theory for the electrons on the helical edge states, we analytically show that the time-periodic precession of the magnetization drives a charge current on the edge, for the whole range of precession frequency. This edge current can be regarded as a consequence of the inverse spin Hall effect intrinsic to the QSHI, which converts the injected spin current into a transverse charge current. By varying the precession frequency of the magnetization and the coupling strength at the junction, we find a clear crossover between two regimes: the adiabatic regime, where the slow magnetization precession drives a quantized pumping, and the resonant regime, where the fast precession leads to a suppressed pumping. We also incorporate the effect of orbital dependence in the exchange coupling at the edge, and show numerically that it shifts the crossover point between the adiabatic and resonant regimes.

口頭

Dissipative spin pumping into two-dimensional quantum spin Hall insulator

荒木 康史; 三澤 貴宏*; 野村 健太郎*

no journal, , 

The two-dimensional quantum spin Hall insulator (2D QSHI) is the most primitive but quite important realization of topological insulator. It shows the helical edge states protected by time-reversal symmetry, whereas the quantized spin Hall conductivity in the bulk. In the present work, we theoretically investigate the spin pumping from a precessing ferromagnet into a 2D QSHI thoroughly from the adiabatic to nonadiabatic regimes, both analytically and numerically. We analytically treat the dynamics of the edge-state electrons coupled to the precessing ferromagnet by the Floquet theory, and derive the pumped current as a function of the exchange energy and the precession frequency. We find that a heat bath for the edge electrons governs the transition between the adiabatic and nonadiabatic regime: when the edge electrons are coupled with a heat bath, their spin and energy can dissipate into the bath by a certain rate, eventually reaching a periodic steady state. The pumped current on the becomes quantized when the exchange energy exceeds the dissipation rate. We also calculate the edge current numerically on the 2D lattice model, and find that the bulk states in the QSHI effectively serves as the heat bath for the edge electrons.

口頭

Adiabatic and resonant spin pumping in two-dimensional quantum spin Hall insulator

荒木 康史; 三澤 貴宏*; 野村 健太郎*

no journal, , 

We present our theoretical work on spin pumping into a two-dimensional (2D) quantum spin Hall inslator (QSHI). QSHI is a topological insulator in 2D exhibiting gapless helical edge stats, which are responsible for the quantized spin Hall conductivity. Recent theories and experiments have demonstrated the QSHI phase in a monolayer of transition metal dichalcogenide 1T'-WTe2, which can be easily engineered in contrast to traditionally-known HgTe/CdTe and InAs/GaSb quantum wells. While the theory of spin pumping is well established in normal metals by focusing on the spin-dependent electron scattering at the interface, it is unreliable for topologically nontrivial interfaces in such systems. In the present work, we consider a junction of a ferromagnet and a 2D QSHI at its 1D edge, and demonstrate the pumping of angular momentum from the spin-precessing ferromagnet into the QSHI. Using the Floquet theory for the electrons on the helical edge states, we analytically show that the time-periodic precession of the magnetization drives a charge current on the edge, for the whole range of precession frequency. This edge current can be regarded as a consequence of the inverse spin Hall effect intrinsic to the QSHI, which converts the injected spin current into a transverse charge current. By varying the precession frequency of the magnetization and the coupling strength at the junction, we find a clear crossover between two regimes: the adiabatic regime, where the slow magnetization precession drives a quantized pumping, and the resonant regime, where the fast precession leads to a suppressed pumping. We also incorporate the effect of orbital dependence in the exchange coupling at the edge, and show numerically that it shifts the crossover point between the adiabatic and resonant regimes.

口頭

Spin-to-charge conversion on the edge of quantum spin Hall insulator

荒木 康史; 三澤 貴宏*; 野村 健太郎*

no journal, , 

We present our theoretical work on dynamical spin-to-charge conversion at the edge of a quantum spin Hall insulator (QSHI), namely a two-dimensional topological insulator with helical edge states. Interconversion between spin- and charge-related quantities has been a key idea in making use of magnetic materials, especially in the context of spintronics. QSHI is a typical system showing a universal charge-to-spin conversion behavior, namely the quantum spin Hall effect, whereas the spin-to-charge conversion therein is still not clearly understood. At a lateral heterojunction of a ferromagnet (FM) and a QSHI, it has been theoretically demonstrated that magnetization dynamics induces a charge current along the edge of QSHI; however, its mechanism from the viewpoint of spin-to-charge conversion still remains to be clarified. In order to understand the spin transfer and the spin-to-charge conversion mechanism in QSHI, we investigate the many-body dynamics of the electrons under the magnetization dynamics at the QSHI-FM junction. We analytically treat the electron dynamics in terms of the Floquet-Keldysh formalism, and compare two physical quantities present on the edge: the spin injection rate from the FM into the QSHI, and the charge current induced along the edge. Whereas the edge current seen in the previous works is reproduced, we find that it is not proportional to the spin injection rate, especially when the exchange interaction at the junction is strong enough. This relation implies that the spin-to-charge conversion in this system cannot be considered as the inverse spin Hall effect, while it can be rather seen as the inverse Edelstein effect, in which an electron spin accumulation at the junction is converted to a charge current. We also focus on the energy transfer at the junction, and interpret this phenomenon in terms of magnon exchange.

口頭

Spin-to-charge conversion on the edge of quantum spin Hall insulator

荒木 康史; 三澤 貴宏*; 野村 健太郎*

no journal, , 

We present our theoretical work on dynamical spin-to-charge conversion at the edge of a quantum spin Hall insulator (QSHI), namely a two-dimensional topological insulator with helical edge states. Interconversion between spin- and charge-related quantities has been a key idea in making use of magnetic materials, especially in the context of spintronics. QSHI is a typical system showing a universal charge-to-spin conversion behavior, namely the quantum spin Hall effect, whereas the spin-to-charge conversion therein is still not clearly understood. We here investigate the many-body dynamics of the electrons under the magnetization dynamics at the QSHI-ferromagnet (FM) junction. We analytically treat the electron dynamics in terms of the Floquet-Keldysh formalism, and compare the spin injection rate from the FM into the QSHI and the charge current induced along the edge. Whereas the edge current seen in the previous works is reproduced, we find that it is not proportional to the spin injection rate, especially when the exchange interaction at the junction is strong enough. This relation implies that the spin-to-charge conversion in this system cannot be considered as the inverse spin Hall effect, while it can be rather seen as the inverse Edelstein effect.

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