High-Temperature Quantum Anomalous Hall Insulators in Lithium-Decorated Iron-Based Superconductor Materials

Quantum anomalous Hall (QAH) insulator is the key material to study emergent topological quantum effects, but its ultralow working temperature limits experiments. Here, by first-principles calculations, we find a family of stable two-dimensional (2D) structures generated by lithium decoration of layered iron-based superconductor materials $\mathrm{Fe}X(X=\mathrm{S},\mathrm{Se},\mathrm{Te})$, and predict room-temperature ferromagnetic semiconductors together with large-gap high-Chern-number QAH insulators in the 2D materials. The extremely robust ferromagnetic order is induced by the electron injection from Li to Fe and stabilized by strong ferromagnetic kinetic exchange in the 2D Fe layer. While in the absence of spin-orbit coupling (SOC), the ferromagnetism polarizes the system into a half Dirac semimetal state protected by mirror symmetry, the SOC effect results in a spontaneous breaking of mirror symmetry and introduces a Dirac mass term, which creates QAH states with sizable gaps (several tens of meV) and multiple chiral edge modes. We also find a 3D QAH insulator phase featured by a macroscopic number of chiral conduction channels in bulk $\mathrm{LiOH}\text{−}\mathrm{Li}\mathrm{Fe}X$. The findings open new opportunities to realize novel QAH physics and applications at high temperatures.

Figure 1

Atomic structure of monolayer LiFeSe. (a) Potential energy surface of Li atoms on monolayer FeSe. (b),(c) Side and top views of monolayer LiFeSe. The symmetry operations are displayed. The 2D material has a ferromagneitc ground state with spin magnetic moment of $3.0{\mu }_B$ per Fe atom (labeled by the orange arrows).

Figure 2

Electronic and magnetic properties of monolayer LiFeSe. (a) Band structure of monolayer LiFeSe without the SOC for spin up (left) and spin down (right) states. The contribution of $d_{z^2}$, $d_{xy}$, $d_{x^2-y^2}$, and $d_{xz,yz}$ orbitals to the Bloch states are denoted by blue, red, green, and orange dots, respectively. (b) Schematic diagram displaying the electron injection from Li to Fe. (c) Schematic diagram of $3d$ orbital occupations for the Fe atoms in monolayer FeSe and LiFeSe. (d) Schematic diagram of the ferromagnetic kinetic exchange coupling between Fe atoms in monolayer LiFeSe.

Figure 3

Topological properties of monolayer LiFeSe. (a) Band structure with the SOC. (b) Anomalous hall conductance ${\sigma }_{xy}$ as a function of Fermi energy, which displays a quantized plateau at the value of $2e^2/h$ within the bulk gap. (c) 3D band structures of Dirac fermions near the Fermi level excluding (left) and including (right) the SOC. (d) Distribution of Berry curvature contributed by occupied valence bands in the momentum space. (e) Topological edge states calculated along the (100) direction. (f) Dependence of band gap and Chern number on the spin orientation quantified by a polar angle $\theta$, where $\theta =0$, $\pi /2$, and $\pi$ denote the $+z$, $+x$, and $-z$ directions.

Figure 4

Atomic and electronic structures of 3D LiOH-LiFeSe and potential applications. (a) Atomic structure and the first Brillouin zone of 3D LiOH-LiFeSe. (b) Band structure of 3D LiOH-LiFeSe with the SOC. (c),(d) A natural heterostructure between superconductor and QAH insulator (LiFeSe) built by gate-controlled lithium injection, which could be used to detect the chiral topological superconductivity and Majorana fermions. An isolation layer (colored green) is inserted at the interface for blocking the lithium diffusion into the superconductor, which may use thin flakes of graphene or boron nitride.