第119回先端計測オープンセミナー

2017年12月12日 (火) 11:00~12:00

国立研究開発法人 物質・材料研究機構
千現地区 研究本館8階 中セミナー室
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Dr. Aires Ferreira (Department of Physics, University of York, York YO10 5DD, U.K.)

Giant charge-to-spin conversion in atomically-thin van der Waals heterostructures

In the past decade, graphene has emerged as a strong contender for next-generation spintronic devices due to its long spin diffusion lengths and gate tunable spin transport at room temperature [1]. However, the lack of a band gap and its weak spin-orbit coupling (SOC) pose major limitations for injection and control of spin currents. The recent capability to assemble 2D crystals into layered heterostructures offers a realistic prospect of overcoming the weaknesses of graphene [2]. When graphene is paired with semiconducting dichalcogenide monolayers [MX2 (M = Mo, W; X = S, Se)] its band structure develops rich spin-orbital textures with helical (in-plane) and out-of-plane components. The proximity-induced SOC enhancement experienced by the Dirac electrons at the graphene layer was confirmed by several groups [3-4], providing an exciting new direction towards realising novel types of spin devices from ultra-thin, high-mobility and gate-tuneable van der Waals (VDW) heterostructures.

In this talk, I will present our latest results on graphene with interface-induced SOC [5-6]. We find that graphene/MX2 generally supports current-driven spin polarization; a relativistic transport phenomenon known as inverse spin galvanic effect (or Edelstein effect). Owing to the characteristic spin winding of interfacial states in graphene heterostructures, the Edelstein effect shows striking similarities to charge-to-spin conversion (CSC) generated by ideal topologically protected surfaces. A detailed study shows that the CSC efficiency is little sensitive to the scattering strength of random impurities and can be as great as 30% at room temperature (for typical SOC energy scales smaller than kBT).

The robust inverse spin galvanic effect in a VDW heterostructure promises unique advantages for lowpower spintronic applications, including the tuning of spin polarization by a gate voltage. Implications of our quantum transport theory [4-6] beyond standard VDW materials will be discussed. For example, ferromagnetic 2D layered (VDW) materials have been recently discovered [7], which could be used to generate strong interfacial exchange interactions and corresponding non-equilibrium spin phenomena.

[1] N. Tombros et al. Nature 448, 571 (2007); M. Kamalakar et al. Nat. Comm. 6, 6766 (2015);
[2] A.K. Geim, and I.V. Grigorieva. Nature 499, 419 (2013);A. Soumyanarayanan, N. Reyren, A. Fert, and C. Panagopoulos. Nature 539, 509 (2016).
[3] A. Avsar et al. Nat. Comm. 5, 4875 (2014); Z. Wang, et al. PRX 6, 041020 (2016);B. Wang, et al. PRB 96, 041409(R) (2017); T. Volkl, et al. ibidem 125405
[4] A. Ferreira et al. PRL 112, 066601 (2014). A. Pachoud et al, PRB 90 035444 (2014);M. Milletari, and A. Ferreira, PRB 94, 134202 (2016); ibidem 201402(R)
[5] M. Offidani et al. Physical Review Letters 119, 196801 (2017)
[6] M. Milletari et al. Physical Review Letters, in press.
[7] C. Gong et al. Nature 546, 265 (2017). B. Huang et al, ibidem 270