Room-temperature quantum anomalous Hall effect in monolayer BaPb with large magnetocrystalline anisotropy energies
Authors: Zhang Wei-Xi , Li Yong , Tian Chang-Hai , She Yan-Chao ,   Department of Physics and Electronic Engineering, Tongren University, Tongren 554300, China
Published: June 2021

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 p<sub><i>x</i>,<i>y</i></sub> non-Dirac bands with the nonzero Chern number (<i>C</i> = 1). Because of the trigonal symmetry of monolayer BaPb material, these bands composed of p<sub><i>x</i>,<i>y</i></sub> orbitals are at the <inline-formula><tex-math id="M1">\begin{document}$ \varGamma $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20210014_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20210014_M1.png"/></alternatives></inline-formula> point, which is different from the Dirac state formed by the p<i><sub>z</sub></i> orbital reported previously. In addition, it can still retain its original topological properties even if strongly hybridized with the substrate. The calculated phonon spectrum shows no imaginary frequency in the entire Brillouin zone, indicating that the monolayer BaPb system is dynamically stable. By using Monte Carlo simulation, we determine the Curie temperature of BaPb monolayer toreach up to 378 K. We also calculate the magnetic anisotropy energy of the BaPb cell, defined as <inline-formula><tex-math id="M2">\begin{document}$ \Delta E={E_{100}}-{E_{001}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20210014_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20210014_M2.png"/></alternatives></inline-formula>. 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.