Relativistic Effects in Complex Compounds Containing Heavy Elements: Ternary Cu-Tl-X (X = S/Se/Te)
Relativistic Effects in Complex Compounds Containing Heavy Elements: Ternary Cu-Tl-X (X = S/Se/Te)
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含重元素的复杂化合物中的相对论效应：三元 Cu-Tl-X (X = S/Se/Te)
DOI：
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发表时间：
2023
影响因子：
3.3
通讯作者：
Yubo Zhang
中科院分区：
文献类型：
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作者：
Da Ke;Leiqiang Li;Yubo Zhang
铜硫族化合物是一类多功能化合物，广泛应用于光化学和光电子学领域。它们的带隙尺寸通常随着元件质量而减小，例如，对于CuAlSe 2、CuGaSe 2和CuInSe 2分别为2.68、1.68和1.04 eV。具有更重的铊（Tl）的Cu-Tl-X（X = S/Se/Te）作为拓扑绝缘体和高性能热电转换器最近受到关注。虽然新的应用可能与Tl相对论效应有关，但这些复杂化合物的第一原理研究很少。在这里，我们揭示了Cu-Tl-X中的相对论效应，使用定制的密度泛函理论方法。质量-速度、达尔文和自旋-轨道耦合这三个相对论项起着不同的作用。在类金刚石CuTlX 2中，质量速度校正降低了导带位置，并有助于使带隙最小化。在不考虑相对论效应的情况下，CuTlS 2的相对论带隙为0.11eV，远小于1.7eV。在CuTlTe 2中，自旋轨道耦合分裂价带，导致奇异的带反转。CuTlSe 2位于正常和倒置带拓扑的边界上。有趣的是，相对论核心收缩是如此之强，它可能有利于非中心对称的缺陷结构与立体活性孤对电子。缺陷结构的带隙要大得多，使得系统几乎没有机会形成反向带拓扑。我们的工作为理解复杂Cu-Tl-X化合物的相对论能带拓扑结构提供了深刻的见解。
Cu-chalcogenides are a large group of multifunctional compounds traditionally used in photovoltaics and optoelectronics. Their bandgap sizes usually decrease with the element masses, e.g., 2.68, 1.68, and 1.04 eV for CuAlSe2, CuGaSe2, and CuInSe2, respectively. Cu-Tl-X (X = S/Se/Te) with even heavier thallium (Tl) has received recent attention as topological insulators and high-performance thermoelectric converters. While the novel applications may be related to Tl relativistic effects, first-principle investigations are scarce for these complex compounds. Here, we reveal the relativistic effects in Cu-Tl-X using a tailored density-functional-theory approach. Three relativistic terms of mass-velocity, Darwin, and spin-orbit-coupling play distinct roles. In diamond-like CuTlX2, the mass-velocity correction reduces the conduction band position and contributes to minimizing the bandgaps. CuTlS2's relativistic bandgap of 0.11 eV is substantially smaller than 1.7 eV without considering the relativistic effects. In CuTlTe2, the spin-orbit-coupling splits the valence bands, resulting in an exotic band inversion. CuTlSe2 lies on the boundary of normal and inverted band topologies. Interestingly, relativistic core contraction is so strong that it may favor non-centrosymmetric defective structures with stereoactive lone-pair electrons. The bandgap of the defective structure is much larger, leaving the system little chance to develop an inverted band topology. Our work provides deep insights into understanding the relativistic band topologies of the complex Cu-Tl-X compounds.