[论文解读] Room temperature high charge to spin conversion in amorphous topological insulator

Protyush Sahu, Junyang Chen+8 more|arXiv (Cornell University)|Nov 8, 2019

Topological Materials and Phenomena被引用 5

一句话总结

本研究展示了通过溅射法制备的非晶态铋硒化物基拓扑绝缘体（钆掺杂的BixSe1-x）薄膜在室温下具有高电荷-自旋转换效率。8 nm厚的非晶态BSG薄膜表现出3.74的自旋霍尔角，比传统重金属高出一个数量级，凸显了其在无序拓扑体系中自旋电子学应用的潜力。

ABSTRACT

Disordered topological insulator (TI) films have gained intense interest because of their possibility for spintronic applications by befitting from TI's exotic transport properties. Here, we have fabricated disordered Gd-alloyed BixSe1-x (BSG) TI films by sputtering methods and have investigated their magneto-transport and spin-torque properties. Structural characterizations show a mainly amorphous feature for the 8 nm thick BSG film, while Bi rich crystallites are developed inside the 16 nm thick BSG film. The bulk resistivity of BSG film is found to be relatively high, up to 6 x 10^4 this http URL, with respect to the resistivity of the polycrystalline BixSe1-x film. Temperature dependent resistivity measurements display the evident character of a variable range hopping transport from 80K to 300K. Second harmonic transport characterizations have been performed on the BSG (t)/ CFB (5 nm) bilayer structures with different thicknesses (t = 6, 8, 12, 16 nm). The effective spin Hall angle deduced form the damping-like torque shows a maximum value of 3.74 corresponding to 8 nm thick BSG at room temperature, which is one order of magnitude higher than that of heavy metals. The possible various origins of suck enhancement are discussed. Our study provides a new experimental direction, beyond crystalline solids, to search for topological systems in amorphous solids and other engineered random systems.

研究动机与目标

• 探索无序非晶态拓扑绝缘体薄膜的自旋电子学潜力。
• 研究不同厚度的溅射法制备的钆掺杂BixSe1-x（BSG）薄膜的磁输运与自旋扭矩特性。
• 确定非晶态拓扑绝缘体是否能在室温下支持高效的电荷-自旋转换。
• 阐明无序体系中增强的自旋霍尔效应相较于晶体体系的起源。

提出的方法

• 利用射频磁控溅射法制备厚度分别为6、8、12和16 nm的BSG薄膜。
• 通过X射线衍射和电子显微镜对结构进行表征，确认其非晶态特性，并在较厚薄膜中检测到铋富集的晶粒。
• 测量温度依赖的电阻率，以识别可变范围跳跃输运行为。
• 对BSG/CuFeB（5 nm）双层结构进行二次谐波电压测量，以提取有效自旋霍尔角。
• 分析阻尼型扭矩信号，确定自旋霍尔角随薄膜厚度和温度的变化关系。
• 将测得的自旋霍尔角与传统重金属的数值进行比较，评估其增强效应。

实验结果

研究问题

• RQ1 非晶态拓扑绝缘体薄膜是否能在室温下表现出强电荷-自旋转换？
• RQ2 薄膜厚度如何影响无序BSG薄膜中的自旋霍尔角？
• RQ3 与晶体或重金属体系相比，非晶态BSG薄膜中观测到的增强自旋霍尔效应的起源是什么？
• RQ4 结构无序在多大程度上影响拓扑绝缘体中的自旋输运特性？
• RQ5 非晶态拓扑绝缘体能否作为晶体材料在自旋电子器件中的可行替代品？

主要发现

• 8 nm厚的BSG薄膜呈现主要非晶态结构，而16 nm厚的薄膜则出现铋富集晶粒。
• BSG薄膜的体电阻率高达6 × 10⁴ Ω·cm，显著高于多晶BixSe1-x薄膜的电阻率。
• 温度依赖的电阻率测量结果证实，在80 K至300 K范围内存在可变范围跳跃输运行为。
• 在室温下，8 nm厚的BSG薄膜中有效自旋霍尔角达到最大值3.74。
• 该自旋霍尔角约为通常在传统重金属中观测到的数值的一个数量级。
• 自旋霍尔效应的增强可归因于多种可能机制，包括无序诱导的自旋-轨道耦合以及非晶体系中的拓扑表面态。