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石川 卓門*; 松尾 衛; 加藤 岳生*
Physical Review B, 107(5), p.054426_1 - 054426_9, 2023/02
被引用回数：0 パーセンタイル：0(Materials Science, Multidisciplinary)We study the temperature dependence of spin-Hall magnetoresistance (SMR) in antiferromagnetic-insulator/metal bilayer systems. We calculate the amplitude of the SMR signal by using a quantum Monte Carlo simulation and examine how the SMR depends on the amplitude of the spin, thickness of the antiferromagnetic-insulator layer, and randomness of the exchange interactions. Our results for simple quantum spin models provide a useful starting point for understanding SMR measurements on atomic layers of magnetic compounds.
荒木 康史; 三澤 貴宏*; 野村 健太郎*
Physical Review Research (Internet), 3(2), p.023219_1 - 023219_15, 2021/06
本論文では、トポロジカルディラック半金属(TDSM)表面のギャップレス状態を介した、長距離スピン伝送を理論面から提案する。次世代のスピントロニクス素子の構築のためには、散逸の少ないスピン流を実現することが必要である。主要なスピン流のキャリアは金属中の伝導電子や磁性絶縁体中のスピン波であるが、これらはジュール熱やギルバート緩和により伝播距離が制限される問題がある。本研究ではTDSM(Cd
As
, NaBi等)のスピン・ヘリカルな表面状態が、乱れに対して頑強である性質を用いて、低散逸で長距離のスピン輸送を提案する。2つの磁性絶縁体とTDSMの接合系を考え、一方の磁性体の磁化ダイナミクスにより、TDSM表面を介して他方の磁性体に注入されるスピン流に注目する。表面における輸送理論と、格子模型による実時間発展シミュレーションを併用することにより、TDSM表面を流れるスピン流は準量子化された値をとり、その値は界面の微視的な結合の構造によらないことを示す。さらに、このスピン流は長距離においても乱れに対して強いことを示し、TDSMがスピントロニクス素子へ応用可能な表面状態をもつことを提案する。
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.
荒木 康史
no journal, ,
The main focus of this presentation is the theory of spin current generation in topological Dirac semimetals (TDSMs), the newly-found three-dimensional topological materials. TDSMs are characterized by pair(s) of doubly-degenerate nodal points (Dirac points) in their momentum-space band structures, which are observed by angle-resolved photoemission spectroscopy (ARPES) in NaBi and CdAs. I focus on a lattice-strained TDSM, to obtain an additional contribution to the spin current generation. I propose that an electric field applied to the strained TDSM gives rise to a nonlinear spin Hall current, namely the spin current perpendicular to and quadratic in the electric field. The spin current response is obtained by the Boltzmann transport theory, regarding the strain as a pseudomagnetic field for the Dirac electrons. This nonlinear effect implies that one can obtain a rectified (dc) pure spin current out of an alternating (ac) electric field, which renders the TDSM an efficient spin-current injector.
荒木 康史
no journal, ,
Topological Dirac seimetals (TDSMs) form a new class of three-dimensional topological semimetals, characterized by pair(s) of doubly-degenerate nodal points (Dirac points) in their momentum(k)-space band structures. They show the intrinsic spin Hall effect (SHE), which comes from the k-space topology around the Dirac points. This spin Hall conductivity is topologically protected, while it cannot be easily tuned or enhanced at linear response. In order to overcome this problem, I theoretically propose that an electric field applied to a lattice-strained TDSM gives rise to an additional "nonlinear spin Hall current", namely the spin current perpendicular to and quadratic in the electric field. The spin current response is obtained by the Boltzmann transport theory, regarding the strain as a pseudomagnetic field for the Dirac electrons. The nonlinear SHE arises as the hybrid of the regular Hall effect driven by the pseudomagnetic field (strain) and the anomalous Hall effect from the k-space topology. This behavior implies that one can obtain a rectified (dc) pure spin current out of an alternating (ac) electric field, which renders the TDSM an efficient spin-current injector.
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.
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.
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.
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.
荒木 康史
no journal, ,
Spin current, namely the flow of spin angular momentum carried by electrons or spin waves in materials, plays the key role in designing next-generation spintronics devices. For efficient use of spin current, we need to lower the energy dissipation in the conversion process between charge and spin currents, and the transmission process of spin current. In this talk, I introduce my recent theoretical works for achieving low-dissipation spin-charge conversion and spin current transmission by making use of topological edge states. In particular, we have considered the roles of helical edge states on two-dimensional (2D) quantum spin Hall insulators (QSHIs) and 3D topological Dirac semimetals (TDSMs), which are (quasi-)1D channels protected by the bulk topology and hence robust against disorder. Helical edge states are suitable for spin-charge conversion and spin current transmission, due to their spin-helical nature, where the electrons propagate in directions depending on their spins (up/down). For spin-charge conversion, we consider spin pumping from a ferromagnet (FM) into a QSHI or a TDSM. The injected spin current is converted into a charge current flowing at the interface, and we evaluate the conversion rate from the spin current to the charge current. As a result, we find that the conversion rate is enhanced under a strong exchange coupling at the interface, around 100 times larger than the conversion rate at 2D interfaces of complex oxides. For spin current transmission, we consider the transmission between two FMs (FM1/FM2) mediated by the helical edge states of a QSHI or a TDSM. We evaluate the spin current injected from FM1 into FM2 analytically and numerically, which exerts a torque on FM2 switching its magnetization. As a result, we find that the spin current is semi-quantized depending on the number of edge channels, and is robustly transmitted against disorder over a long range.
