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

https://doi.org/10.1103/PhysRevResearch.2.023195

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.

Negative Te spin polarization responsible for ferromagnetic order in the doped topological insulator V(SbBi)Te

Ye, M.*; Xu, T.*; Li, G.*; Qiao, S.*; 竹田 幸治; 斎藤 祐児; Zhu, S.-Y.*; Nurmamat, M.*; 角田 一樹*; 石田 行章*; et al.

Physical Review B, 99(14), p.144413_1 - 144413_7, 2019/04

https://doi.org/10.1103/PhysRevB.99.144413

We investigate the microscopic origin of ferromagnetism coupled with topological insulators in V-doped (Sb,Bi)Te employing X-ray magnetic circular dichroism and angle-resolved two-photon photoemission spectroscopies, combined with first-principles calculations. We found an magnetic moment at the Te site anti-parallel to that of the V and Sb sites, which plays a key role in the ferromagnetic order. We ascribe it to the hybridization between Te 5 and V 3 majority spin states at the Fermi energy, consistent with the Zener-type - exchange interaction scenario. The substitution of Bi for Sb suppresses the bulk ferromagnetism by introducing extra electron carriers in the majority spin channel of the Te states that compensates the antiparallel magnetic moment on the Te site. Our findings reveal important clues to designing magnetic topological insulators with higher Curie temperature that work under ambient conditions.

Carrier-mediated ferromagnetism in the magnetic topological insulator Cr-doped (Sb,Bi)Te

Ye, M.*; Li, W.*; Zhu, S.-Y.*; 竹田 幸治; 斎藤 祐児; Wang, J.*; Pan, H.*; Nurmamat, M.*; 角田 一樹*; Ji, F.*; et al.

Nature Communications (Internet), 6, p.8913_1 - 8913_7, 2015/11

AA2015-0647.pdf:0.64MB

https://doi.org/10.1038/ncomms9913

磁性元素を添加したトポロジカル絶縁体は、量子異常ホール効果や無散逸伝導などの魅力的な現象の発現が予言され、低消費電力スピンデバイスの開発につながっていくものと期待されている。既に、いくつかの磁性添加トポロジカル絶縁体で長距離磁気秩序が確認されている。しかし、量子異常ホール効果の発現は、極低温におけるCrを添加した(Sb,Bi)Te系に限られており、強磁性の微視的な起源はほとんど分かっていない。そこで、今回、X線磁気円二色性実験による元素選択的研究を行うことにより、本物質系の強磁性は、母体の正孔キャリアーを媒介としたものであり、Crの3d電子とSbやTeのp電子の相互作用が極めて重要であることを明らかにした。この結果は、異常量子ホール素子の実現に向けても重要である。

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.

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.

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.