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Rashba效应是一种因自旋轨道耦合效应以及对称性破缺而引起的能带自旋劈裂现象。Rashba参量aR是评价材料Rashba自旋劈裂强度的重要指标。材料的电输运性质极易受到Rashba参量的影响，因此通过Rashba参量来综合调控体系的电输运性质，找到具有最佳功率因子的Rashba参量范围，对于设计新型Rashba热电材料具有重大的意义。同时，对于Rashba参量的调控，不仅有利于热电方面的应用，对于其他 Rashba效应相关的功能性质，也具有重要意义。
上海大学材料基因组工程研究院的杨炯教授、南方科技大学物理系的张文清教授等，通过自行开发的“五点法”，从62个待选掺杂BiTeI中高通量地筛选了32个具有Rashba效应的体系。该五点法是根据材料本身在Rashba自旋劈裂中产生的自旋极化分布特性所提出的，即在自旋劈裂的中心点（BiTeI中为A点）两边，垂直于电场的平面内，对称点的自旋大小相等，方向相反，而平面外自旋为0。因此只需要5个点的计算便可判断体系是否具有Rashba效应，而不用复杂的能带结构计算。这一方法具有普适性，未来可以用于其它Rashba体系的筛选。32个具有Rashba效应的掺杂BiTeI体系构成了一张aR-掺杂元素图表。不同元素由于各自的自旋轨道耦合强度、原子尺寸以及对带隙的影响，可将aR在0～4.05 eV·Å的宽泛范围内调节，适用于不同相关功能性质的需求。在热电性能方面，通过电输运性质的计算，得到了最优功率因子的aR范围在2.75～3.55 eV·Å之间，可在aR-掺杂元素图表中很容易找出6种掺杂元素满足这一条件。这一发现成为一种并列于能带工程、散射工程的电输运性能调控方法——Rashba工程，即通过调节材料的Rashba自旋劈裂属性来改变能带结构，进而提升热电性能。该文近期发表于npj Computational Materials 6: 107 (2020)，英文标题与摘要如下，点击左下角“阅读原文”可以自由获取论文PDF。
Defect-Mediated Rashba Engineering for Optimizing Electrical Transport in Thermoelectric BiTeI
Xin Li, Ye Sheng, Lihua Wu, Shunbo Hu, Jiong Yang, David J. Singh, Jihui Yang, and Wenqing Zhang
The Rashba effect plays a vital role in electronic structures and related functional properties. The strength of the Rashba effect can be measured by the Rashba parameter aR; it is desirable to manipulate aR to control the functional properties. The current work illustrates how aR can be systematically tuned by doping, taking BiTeI as an example. A five-point-spin-texture method is proposed to efficiently screen doped BiTeI systems with the Rashba effect. Our results show that aR in doped BiTeI can be manipulated within the range of 0~4.05 eV·Å by doping different elements. The dopants change aR by affecting both the spin-orbit coupling strength and band gap. Some dopants with low atomic masses give rise to unexpected and sizable aR, mainly due to the local strains. The calculated electrical transport properties reveal an optimal aR range of 2.75~3.55 eV·Å for maximizing the thermoelectric power factors. aR thus serves as an effective indicator for high-throughput screening of proper dopants and subsequently reveals a few promising Rashba thermoelectrics. This work demonstrates the feasibility of defect-mediated Rashba engineering for optimizing the thermoelectric properties, as well as for manipulating other spin-related functional properties.
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