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Theory of spin pumping and transport on topological edge states

トポロジカルエッジ状態におけるスピンポンピング及びスピン輸送理論

荒木 康史  

Araki, Yasufumi

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.

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