相对论效应在Mg2Si、Mg2Ge和Mg2Sn电子结构和热电势计算中的重要性
Importance of relativistic effects in electronic structure and thermopower calculations for Mg2Si, Mg2Ge and Mg2Sn
摘要：
本文从理论上研究了相对论效应对Mg_2X（X= Si，Ge，Sn）半导体电子能带结构和热电势的影响。采用全势KKR方法，详细比较了全相对论和半相对论电子结构特征。结果表明，自旋-轨道（S-O）相互作用使γ点处的价带结构发生分裂，这种分裂与实验结果吻合较好，且这种效应强烈依赖于X原子.利用玻尔兹曼方法研究了以γ为中心的类空穴费米表面口袋的拓扑结构的S-O修饰导致电子输运性质的变化。此外，本文还提出了一种简单有效的计算态密度有效质量m* 的方法，并用来考察相对论效应对m* 的影响。结果发现，S-O耦合的价带减少有效质量，因此显着降低热电势，主要是在Mg 2Sn，但也在Mg 2Ge。分析了温度（10-900 K）和载流子浓度（10^18-10^22 cm-3）对p型Mg 2X热电性能的不利影响。有趣的是，在n型Mg 2X中的类似计算显示，S-O相互作用对最低导带的影响可以忽略不计，因此也对塞贝克系数的影响可以忽略不计。
英文摘要：
We present a theoretical study of the influence of the relativistic effects on electronic band structure and thermopower of Mg2X(X= Si, Ge, Sn) semiconductors. The full potential Korringa-Kohn-Rostoker (KKR) method is used, and the detailed comparison between the fully relativistic and semi-relativistic electronic structure features is done. We show that the spin-orbit (S-O) interaction splits the valence band structure at Gamma point in good agreement with the experimental data, and this effect strongly depends on X atom. The S-O modifications of the topology of the Gamma-centered hole-like Fermi surface pockets lead to a change in electron transport properties, which are investigated using the Boltzmann approach. In addition, the simple and efficient method is presented for the calculation of density of states effective mass m*, and then used to examine the impact of relativistic effects on m*. It is found that S-O coupling of the valence bands reduces effective mass and therefore significantly lowers the thermopower, primarily in Mg2Sn, but also in Mg2Ge. A detrimental influence of the S-O interaction on thermoelectric performance of p-type Mg2X is analyzed in function of temperature (10-900 K) and carrier concentration (10^18-10^22 cm-3). Interestingly, similar calculations in n-type Mg2X, show negligible effect of the S-O interaction on lowest conduction bands and consequently also on the Seebeck coefficient.
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