[论文解读] Electrical spin injection into graphene from a topological insulator in a van der Waals heterostructure

Jifa Tian, Ting‐Fung Chung+2 more|arXiv (Cornell University)|Jul 9, 2016

Topological Materials and Phenomena参考文献 31被引用 4

一句话总结

本研究通过范德华异质结，实现了从铋基拓扑绝缘体（Bi2Te2Se）到石墨烯的全电控自旋注入，利用了拓扑表面态中的自旋-动量锁定效应。两种材料中均观测到阶梯状电压信号，证实了电流诱导的螺旋自旋极化：信号极性随电流方向反转，且与偏置电流呈线性关系，证明了无需磁性材料的电控自旋注入。

ABSTRACT

All-electrical (magnetic-material-free) spin injection is one of the outstanding goals in spintronics. Topological insulators (TIs) have been recognized as a promising electrically controlled spin source thanks to the strong spin-orbit coupling and in particular, the spin-momentum locked topological surface states (TSS) supporting helically spin polarized currents. Many TI materials such as Bi-based chalcogenides are also layered 2D materials and can be incorporated into van der Waals (vdW) coupled heterostructures, opening the possibility of the utilization of TIs for electrical spin injection into other 2D materials. Here, we demonstrate electrical injection of helically spin-polarized current into graphene through a 3D TI in a mechanically stacked heterostructure between Bi2Te2Se (a TI) and chemical vapor deposition (CVD)-grown graphene, using the spin potentiometric measurement. When a dc current is flowing from the TI to graphene, we detect a striking step-like voltage change (spin signal) in both the TI and graphene using ferromagnetic (FM) probes. The sign of the spin signal can be reversed by reversing the direction of the dc bias current, and the corresponding amplitude of the spin signal increases linearly with the bias current, indicative of a current-induced helical spin polarization in both TI and graphene. In contrast, the graphene itself exhibits usual nonlocal spin valve signal when the spins are injected using an FM electrode. We discuss possible origins of the helical spin polarization injected into the graphene in our TI/graphene heterostructure that may include TSS as well as the spin-orbit coupled Rashba states. Our findings show electrical injection of a spin helical current into graphene through a TI and demonstrate TIs as potential spin sources for future spintronic devices wherein spin manipulation is achieved electrically.

研究动机与目标

• 在不使用铁磁电极的条件下，实现对石墨烯的全电控自旋注入。
• 探索拓扑绝缘体因其自旋-动量锁定表面态而作为自旋源的潜力。
• 通过Bi2Te2Se与化学气相沉积生长的石墨烯的范德华异质结，实现向石墨烯的自旋注入。
• 通过自旋压阻测量验证拓扑绝缘体和石墨烯中螺旋自旋极化电流的存在。
• 研究异质结中自旋极化的来源，包括拓扑表面态和Rashba态的贡献。

提出的方法

• 通过范德华堆叠方式制备了机械剥离的Bi2Te2Se（三维拓扑绝缘体）与化学气相沉积生长的石墨烯异质结。
• 从拓扑绝缘体向石墨烯注入直流电流以诱导自旋注入。
• 使用铁磁探针进行自旋压阻测量，以检测Bi2Te2Se和石墨烯中的自旋信号。
• 测量电压变化与所加偏置电流的关系，以评估自旋信号的幅度和极性。
• 反转直流偏置电流方向，以测试自旋信号的可逆性。
• 将结果与使用铁磁电极在石墨烯上进行的传统非局域自旋阀测量进行对比。

实验结果

研究问题

• RQ1 拓扑绝缘体能否作为范德华异质结中石墨烯的电驱动自旋源？
• RQ2 当由拓扑绝缘体施加直流电流时，是否向石墨烯注入了螺旋自旋极化电流？
• RQ3 异质结中的自旋信号是否表现出与电流相关的幅度变化和极性反转，表明实现了电控自旋注入？
• RQ4 石墨烯层中自旋极化的主导贡献来自拓扑表面态还是Rashba型态？
• RQ5 能否在二维异质结中实现无需铁磁材料的全电控自旋注入？

主要发现

• 在施加直流电流后，拓扑绝缘体和石墨烯层中均检测到指示自旋信号的阶梯状电压变化。
• 当直流偏置电流方向反转时，自旋信号极性也随之反转，证实了自旋极化的电控性。
• 自旋信号幅度随所加偏置电流线性增加，证明了电流诱导的螺旋自旋极化。
• 当使用铁磁电极探测时，石墨烯层表现出典型的非局域自旋阀信号，证实了基线自旋输运性能。
• 观测到的自旋注入可归因于拓扑表面态中的自旋-动量锁定，以及异质结中可能存在的Rashba型自旋-轨道耦合态。
• 结果确立了拓扑绝缘体作为未来自旋电子器件中电驱动、无磁性材料自旋源的可行候选。