钴基类赫斯勒合金半导体研究

  窄带隙半导体的热导率相较于金属更低，而电导率相较于绝缘体更高，这使得窄带隙半导体材料通常具备优异的热电效应品质因子。对这类材料的研究对发现具有卓越性能的热电材料具有重要意义。作为一类重要的窄带隙半导体材料，赫斯勒合金中包含了众多具备出色热电性能的材料体系。赫斯勒合金是由氯化钠子晶格和硫化锌子晶格嵌套形成的晶体结构的化合物。根据化学计量比的不同，1:1:1比例的赫斯勒合金被称为半赫斯勒合金，而2:1:1比例的赫斯勒合金被称为全赫斯勒合金。然而，赫斯勒合金的窄带隙数量有限，限制了人们对其热电性能的进一步挖掘。

  为解决这一问题，本工作推广了传统赫斯勒合金半导体的18电子数规则和24电子数规则，将其应用于化学计量比为1+x:1:1(0的钴基类赫斯勒合金中。这一新设计规则预测了20价电子数的Co_1.33YZ和22价电子数的Co_1.67YZ。这些不依赖于18电子数规则和24电子数规则半导体体系被称为非整数化学计量比的类赫斯勒合金半导体。本文着重讨论了Co_1.33YZ体系。通过对26个符合新设计规则的Co_1.33YZ体系进行第一性原理计算的高通量筛选，本工作发现了20余种可能形成窄带隙体的钴基类赫斯勒合金。由于赫斯勒合金的费米面附近主要由d 轨道电子占据，我们也对其磁性进行了深入研究。进一步的分析表明，这些可能的半导体中，11个形成了能带半导体，9个形成了具备新颖磁性反铁磁化合物。

  此次研究工作揭示了类赫斯勒合金体系中由结构和原子占据方式所导致的产生带隙与新颖磁性的核心机制——钴原子对之间的e-e_g轨道交叠机制。这一机制通过双原子双轨道模型得到了清晰的定量描述，同时说明了具有新颖磁性的体系可能导致关联效应。本文论的研究结果为实验研究强关联体系的热电性能提供了理论依据，并将有助于进一步探索具备优异性能的新型非证书化学计量比类赫斯勒合金半导体。

    Narrow-gap semiconductors have lower thermal conductivity than metals and higher electrical conductivity than insulators, which enhances their thermoelectric figure of merit. The research on narrow-gap semiconductors is essential to the discovery of high-performance thermoelectric materials. Heusler alloys are an important class of materials in the study of thermoelectric materials. narrow-gap Heusler semiconductors are found to perform well in thermoelectricity. The crystal structure of Heusler alloys is a nesting of Na-Cl sublattice and ZnS sublattice. According to the difference of stoichiometry, the Heusler alloys are classified as half Heusler with stoichiometry 1:1:1 and full Heusler with stoichiometry 2:1:1. However, the number of narrow-gap semiconductors in Heusler alloys is few, which barricades people’s further investigation.

    To solve this problem, we generalize the 18 and 24 electron rules to design the traditional Heusler semiconductors and apply the new rule into Co-based Heusler-like alloys with stoichiometry 1+x:1:1(0 . The systems consistent with the new rule are 20-VEC Co_1.33YZ and 22-VEC Co_1.67YZ. Since the formation of semiconductors in these systems is no longer constrained by 18 and 24 electron rules, they are named as off-stoichiometry Heusler-like alloys. In this work, we focus on Co_1.33YZ. The high-throughput screening of the designed 26 Co_1.33YZ systems, we found more than 20 possible narrow-gap semiconductors. Due to the dominating occupancy of d electrons near the Fermi surface in Heusler-like systems, we carefully investigate the magnetic properties. The study of magnetism shows that there are 11 band insulators and 9 antiferromagnetic compounds with novel magnetism.

    These results prevail the core mechanism, e-e_g orbital interplays, which is caused by the Heusler-like structure and the atom occupancy patterns. Through the 2-site 2-orbital model, this mechanism is described quantitatively and clearly. Meanwhile, the potential correlation effect is also found. This work will provide reliable theoretical basis and help the discovery of new high-performance thermoelectric materials.

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