1.Department of Mathematics and Physics, Hebei Institute of Architecture Civil Engineering, Zhangjiakou 075000, China 2.College of Physics, Hebei Normal University, Shijiazhuang 050016, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 51971087), the Hebei Natural Science Foundation, China (Grant No. A2018205144), the Support Program of Scientific and Technological Research Project of Hebei, China (Grant No. 15211036), the Financial Support from the Science and Technology Plan Projects of Zhangjiakou City, China (Grant No.1611070A), and the Ph. D. Programs Foundation of Hebei Institute of Architecture Civil Engineering, China (Grant No. B-201807)
Received Date:11 June 2020
Accepted Date:06 September 2020
Available Online:06 January 2021
Published Online:20 January 2021
Abstract:Half-Heusler semiconductors exhibit similar properties: the differences among their properties lie only in the fact that in ternary compositions the zinc-blende binary substructure does not provide the required 18 electrons, but this is improved by adding an extra transition metal, which restores the electronic balance. Half-Heusler ternary compound with 18 valence electrons under an appropriate uniaxial strain is a topological insulating phase. Most importantly, it is proposed that in the half-Heusler family, the topological insulator should allow the incorporating of superconductivity and magnetism. Using the first-principle full-potential linearized augmented wave method we study the band structure of a series of Li(Na)AuS topological insulators. The electronic and magnetic properties of Heusler alloys are investigated by the WIEN2k package. The exchange-correlations are treated within the generalized gradient approximation of PerdeweBurke and Ernzerhof (GGA), the local spin density approximation (LSDA), by using the modified Becke-Johnson exchange potential and the correlation potential of the local-density approximation (MBJ). Spin-orbit coupling is treated by means of the second variational procedure with the scalar-relativistic calculation as basis. We first determine the equilibrium lattice constants by calculating the total energy. The theoretical lattice constant of LiAuS full-potential GGA is 6.02 ?, which is somewhat greater than the result of pseudopotential(5.99 ?). The calculated equilibrium lattice parameter is 5.86 ? for LSDA. Most of the half-Heusler compounds have band inversion, and open the nature band gaps, but the gap of MBJ is not very good. Smaller uniaxial stress damages the cubic structure and also such a natural band gap of topological insulators. By applying uniaxial tensile stress until the equilibrium position is reached in all directions of the structure, the system band gap value is about 0.2 eV, which is consistent with the result obtained from the band gap of cubic structure equilibrium position. When uniaxial tensile stress is 41%, the system turns into a tetragonal structure, the equilibrium lattice constant is a = 5.2477 ? and c/a = 1.41. We use the method of substitution of homologous elements to ensure the properties of topological insulator of materials without changing the cubic structure, and open the bandgap of materials under the equilibrium lattice constant of the system, thereby improving the feasibility of experimental synthesis of topological insulator materials. Our results for the doping suggest that epitaxial strain encountered during experiment can result in electronic topological transition. We hope that the results presented here conduce to further experimental investigation of the electronic topological transition in half-Heusler compounds. Keywords:topological insulators/ Heusler alloys/ first principle/ band structure
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3.1.不同关联泛函LiAuS能带结构
通过计算发现LiAuS和NaAuS具有反带结构, 这个结论和文献[15]中采用GW近似方法得到的结果一致; 另外, 对于反带结构和反转强度的讨论, 文献[16]中进行了细致的讨论, 不再作为本文讨论的重点. 利用GGA, LSDA和MBJ三种关联泛函计算了LiAuS. 各个化合物的平衡晶格常数都是通过能量优化得到的. 通过比较能量优化得到的晶格常数, 发现采用全势GGA泛函得到的平衡晶格常数为0.602 nm, 稍大于VASP赝势软件GGA计算值的0.599 nm, 而采用LSDA泛函优化得到的平衡晶格常数为0.586 nm, 可见局域密度泛函得到的平衡晶格常数为最小. 通过各自平衡晶格常数计算得到的能带图结构均为具有反带结构的拓扑绝缘体结构, 且均打开了自然带隙, 结果和文献一致, 但是在带隙数值上MBJ并没有显现出优势. 对于LaPtBi材料一般在平衡状态为反带结构, 但是并没有带隙, 普遍认为具有此类反带结构的立方半Heusler化合物要想打开体能隙, 常用的方法就是施加等体积单轴应力, 施加拉应力可以增大带隙, 从而得到真正意义上的拓扑绝缘体材料, 这种方法的实质是通过破坏立方对称性的保护而打开带隙[17,18]. 图1表示对LiAuS在保证体积不变的基础上对c轴施加单轴应力得到的能量优化曲线, 由于施加拉应力可以增大带隙, 所以应力施加是以拉应力为主, 图中横坐标的–10表示施加10%的单轴压应力, 而横坐标的正值均表示拉应力. 由图中可知, 当单轴拉力施加到约41%处能量最低, 此时破坏了立方对称性之后成为四方结构后空间群为$119 (I\bar4m2)$, 且平衡晶格常数为a = 0.52477 nm, c/a = 1.41时系统又趋于平衡状态. 图 1 对LiAuS在保证体积不变的基础上对c轴施加单轴应力得到的能量优化曲线, c/a = 1.41时得到平衡晶格常数为a = 0.52477 nm Figure1. Calculated total energies as functions of the uniaxial strain along [001] direction with constant volume for LiAuS, the equilibrium lattice constant is a = 0.52477 nm and c/a = 1.41.
研究认为有必要对碱金属系列半Heusler化合物的拓扑绝缘体进行计算和分析, 另外由于全势的WIEN2k软件包的MBJ是原生的, 因此采用LSDA和MBJ两种关联泛函计算此系列的能带进行比较. 图2给出了利用WIEN2k软件分别采用LSDA和MBJ两种关联泛函计算得到的对LiAuS施加1%的单轴拉力得到的能带结构图, 可以发现较小的单轴应力破坏立方结构后也破坏了此类拓扑绝缘体的自然带隙, 通过计算得到施加较小的拉力和压力均会使系统由拓扑绝缘体转变为拓扑金属. 图 2 施加1%拉应力后破坏立方结构的能带结构, 左图为LSDA 计算能带图, 右图为MBJ计算得到的能带图 Figure2. Band structure of the LiAuS compound with 1% uniaxial tensile stress, on the left with LSDA, and on the right with MBJ.
图3给出了单轴应力下平衡状态下采用LSDA和MBJ两种关联泛函计算得到的LiAuS的能带结构图. 从图3可以看出, 通过施加单轴拉应力直到四方结构的平衡位置时, 系统带隙的值为0.2 eV左右, 这与立方结构平衡位置得到的带隙结果一致. 一般情况下, 计算半导体和绝缘体材料, 利用GGA方法计算得到的能带带隙要比MBJ的要小, 也就是通常所说的GGA会低估带隙, 发现在此系列的拓扑绝缘体带隙计算中, 每种关联泛函得到的带隙是一致的, 所以证实这是材料本身的属性. 但是, 在结果中通过MBJ泛函计算得到的能带图中, 费米能级明显穿过了价带. 通过计算发现, 在此体系的半Heusler化合物中, MBJ泛函在拓扑绝缘体的计算中并没有明显优势. 而在LSDA计算得到的能带图中, 在拉力作用下, Au的部分s轨道明显能量下降, 在局域密度近似当中尤其明显, 拉力为84%严重破坏立方对称性时, 带隙值仍然约为0.2 eV. 图 3 四方结构平衡晶格常数(a = 0.52477 nm, c/a = 1.41)下的能带结构 (a) LSDA 计算能带图; (b) MBJ计算得到的能带图 Figure3. Band structure of the tetragonal structure LiAuS compound with the equilibrium lattice constant (a = 0.52477 nm and c/a = 1.41): (a) LSDA; (b) MBJ.