Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 12064038), the Scientific Research Program of Guizhou Province, China (Grant Nos. KY[2019]179, KY[2017]315, TK[2021] General projects -034), the Outstanding Young Science and Technology Talents of Guizhou Province, China (Grants No. [2019]5673), and the Science and Technology Foundation of Tongren Science and Technology Bureau, China (Grant No. [2020]77)
Received Date:04 January 2021
Accepted Date:17 March 2021
Available Online:07 June 2021
Published Online:05 August 2021
Abstract:The quantum anomalous Hall effect is an intriguing quantum state that exhibits chiral edge states in the absence of a magnetic field. The chiral edge states are topologically protected and robust against electron scattering, which possesses great potential applications in designing low energy consumption and dissipation less spintronic devices. The experimental conditions are required to be very high, such as extremely low temperature (< 100 mK) due to the small band gap and the greatly accurate control of the extrinsic impurities. These greatly hinder their devices from being put into applications further. Hence, it would be meaningful to search for a new Chern insulator with a large band gap and high Curie temperature. According to the first-principles calculations, we predict the room temperature quantum anomalous Hall effect in the monolayer BaPb. The nontrivial topology of this new type of ferroelectric semi-metal material derives from fully spin-polarized quadratic non-Dirac bands. The quantum anomalous Hall effect can be realized in the monolayer BaPb with fully spin-polarized quadratic px,y non-Dirac bands with the nonzero Chern number (C = 1). Because of the trigonal symmetry of monolayer BaPb material, these bands composed of px,y orbitals are at the $ \varGamma $ point, which is different from the Dirac state formed by the pz orbital reported previously. In addition, it can still retain its original topological properties even if strongly hybridized with the substrate. The calculated phonon spectrum shows no imaginary frequency in the entire Brillouin zone, indicating that the monolayer BaPb system is dynamically stable. By using Monte Carlo simulation, we determine the Curie temperature of BaPb monolayer toreach up to 378 K. We also calculate the magnetic anisotropy energy of the BaPb cell, defined as $ \Delta E={E_{100}}-{E_{001}} $. Here, we consider two magnetization easy-axis directions, [100] and [001]. To our surprise, the MAE of monolayer BaPb is as high as 52.01 meV/cell by considering the spin-orbit coupling effect. Furthermore, the nontrivial band gap is opened with a magnitude of 177.39 meV when the spin-orbit coupling effect is included. The calculations of Berry curvature and edge states further prove that the monolayer BaPb system can realize the quantum anomalous Hall state. This discovery indicates that the monolayer BaPb materials can be used as a candidate for quantum anomalous Hall effect materials, thereby promoting the development of spintronics. Keywords:quantum anomalous Hall effect/ quadratic non-Dirac bands/ ferromagnetic half-metallicity/ first-principle
通过DFT计算发现, 无自旋极化的单层BaPb的总能量远高于自旋极化的单层BaPb. 这意味着尽管单分子层BaPb系统不包含磁性原子, 但它仍然具有磁性. 为了解系统的磁性来源, 如图2(a)和图2(b)所示, 计算了$2 \times 2$超胞的三种磁构型: 铁磁(ferromagnetic, FM)态、反铁磁(antiferromagnetic, AFM)态和非铁磁(non-magnetic, NM)态的能量, 以确定单层BaPb的磁基态. 图 2 单分子层BaPb的铁磁构型(a)和反铁磁构型(b); (c)利用蒙特卡罗模拟得到的单层BaPb的磁矩与温度的关系 Figure2. Two magnetic configurations for monolayer BaPb: (a) Ferromagnetic; (b) antiferromagnetic. (c) The average magnetic moment per unit cell with respect to temperature calculated for monolayer BaPb obtained by using Monte Carlo simulations.
单层BaPb最有趣的性质仍然反映在其电子结构上. 首先研究不考虑SOC作用的单层BaPb的能带结构. 如图3所示, 单分子层BaPb的能带结构是完全自旋极化的, 即, 自旋向上的能带展现出绝缘性, 而自旋向下的能带展现出金属性. 同时, 可以清楚地观测到自旋向下的能带在费米能级附近的Γ点上存在二次型的非狄拉克能带. 图 3 单分子层BaPb的自旋极化能带结构 Figure3. Spin-polarized band structure for monolayer BaPb.
随后, 研究考虑SOC作用的磁矩分别沿x方向平面内和沿z方向平面外的单层BaPb的能带结构, 如图4所示. 结果表明, 磁矩沿z方向, 二次型的非狄拉克能带打开了全局域带隙177.39 meV, 体系呈现出拓扑绝缘态, 而在平面内沿x方向有磁矩的带隙结构则显示出金属态. 这意味着通过施加电场或构造异质结改变磁化方向, 可以实现体系从金属态到拓扑绝缘态的相变. 图 4 考虑SOC时单层BaPb的能带结构 (a) 磁矩平面内沿x方向; (b) 磁矩平面外沿z方向 Figure4. Band structures (including SOC) for monolayer BaPb: (a) With magnetic moments oriented in-plane along x; (b) with magnetic moments oriented out of the plane (along z directions).
23.4.量子反常霍尔效应 -->
3.4.量子反常霍尔效应
在这一部分, 通过计算贝利曲率和Chern数来研究单层BaPb的拓扑性质. 首先, 利用公式${\sigma _{xy}} = C{e^2}/h$计算出反常霍尔电导, 如图5(a)所示. Chern数C [36]可以通过对第一布里渊区的倒空间的贝利曲率积分$C \!= \!\dfrac{1}{{2{\text{π}}}}\!\displaystyle\int_{BZ} \!{{{\rm{d}}^2}k\varOmega (k)}$得到, 其中 图 5 (a) 反常霍尔电导率随费米能级移动的变化; (b) 动量空间中带有SOC的Berry曲率; (c) 计算得到的单层BaPb半无限片的边缘状态 Figure5. (a) Anomalous Hall conductivity when the Fermi level is shifted around the original Fermi level; (b) the Berry curvature with SOC in the momentum space; (c) calculated edge state of a semi-infinite sheet for monolayer BaPb.
考虑到二维材料的电子能带结构和磁性质的稳定性对于量子器件的实验制备和发展非常重要[37,38]. 本部分以六方氮化硼(hexagonal boron nitride, h-BN)为衬底研究了单层BaPb的能带结构. 在h-BN衬底上生长的单层BaPb可能的晶格结构的俯视图和侧视图如图6(a)和图6(b)所示. 定义晶格失配率为$\left| {{a_{{\rm{h}} \text- {\rm{BN}}}} - {a_{{\rm{BaPb}}}}} \right|/{a_{\text{h-}{\rm{BN}}}}$, 其中${a_{{\rm{h}} \text- {\rm{BN}}}}$和${a_{{\rm{BaPb}}}}$分别为h-BN衬底和单层BaPb的晶格常数. 在本文的模型中, 使用了1 × 1的BaPb和$\sqrt 7 \times \sqrt 7 $的h-BN衬底, 计算得到两者的晶格失配率非常小, 为5.71%. 图 6 (a), (b) h-BN/BaPb异质结(h-BN/BaPb)体系中单分子层BaPb的结构晶体结构的俯视图和侧视图; (c) h-BN/BaPb异质结自旋极化能带结构; (d) 考虑SOC效应时h-BN/BaPb异质结体系中单分子层BaPb的带结构(考虑SOC效应) Figure6. Crystal structure of monolayer BaPb in the h-BN/BaPb heterostructure system: (a) Top and (b) side views. Here, the black solid lines denote the unit cell. (c) Spin-polarized band structure of h-BN/BaPb heterostructure; (d) the band structures (including SOC) of monolayer BaPb in the h-BN/BaPb heterostructure system.
如图6(c)所示, 在h-BN/BaPb异质结构中, 费米能级附近的二次型非狄拉克带色散仍然存在. 考虑SOC效应时, 二次型非狄拉克能带打开了全局带隙82.11 meV, 如图6(d)所示. 这些结果表明, h-BN衬底对单分子层BaPb的能带结构几乎没有影响. 因此, 在狄拉克锥上观察到的SOC诱导的非平庸体带隙可能表明在该异质结构中可以实现量子反常霍尔绝缘态, 用于潜在的器件应用. 最后, 还用蒙特卡罗模拟了h-BN/BaPb异质结中磁矩与温度的关系, 如图7所示. 结果表明, 由于基底导致的近邻效应可能影响异质结中电子跃迁和磁交换机制, 所以交换作用参数J值变为2.39 meV, 相应的Tc变为54 K. 但由于非磁性的基底h-BN与BaPb是通过范德瓦耳斯作用结合的, 因此体系的磁各向异性能几乎不变, 为51.32 meV. 图 7 利用蒙特卡罗模拟得到的h-BN/BaPb异质结的磁矩与温度的关系 Figure7. Average magnetic moment per unit cell with respect to temperature calculated for the h-BN/BaPb heterojunction obtained by using Monte Carlo simulation.