We theoretically propose a family of structurally stable monolayer halide perovskite A$_3$B$_2$C$_9$ (A=Rb, Cs; B=Pd, Pt; C=Cl, Br) with easy magnetization planes. These materials are all half-metals with large spin gaps over 1~eV accompanying with a
single spin Dirac point located at K point. When the spin-orbit coupling is switched on, we further show that Rb$_3$Pt$_2$Cl$_9$, Cs$_3$Pd$_2$Cl$_9$, and Cs$_3$Pt$_2$Cl$_9$ monolayers can open up large band gaps from 63 to 103 meV to harbor quantum anomalous Hall effect with Chern numbers of $mathcal{C}=pm1$, whenever the mirror symmetry is broken by the in-plane magnetization. The corresponding Berezinskii-Kosterlitz-Thouless transition temperatures are over 248~K. Our findings provide a potentially realizable platform to explore quantum anomalous Hall effect and spintronics at high temperatures.
We identify a valley-polarized Chern insulator in the van der Waals heterostructure, Pt$_{2}$HgSe$_{3}$/CrI$_3$, for potential applications with interplay between electric, magnetic, optical, and mechanical effects. The interlayer proximity magnetic
coupling nearly closes the band gap of Pt$_{2}$HgSe$_{3}$ and the strong intra-layer spin-orbit coupling further lifts the valley degeneracy by over 100 meV leading to positive and negative band gaps at opposite valleys. In the valley with negative gap, the interfacial Rashba spin-orbit coupling opens a topological band gap of 17.8 meV, which is enlarged to 30.8 meV by adding an $h$-BN layer. We find large orbital magnetization in Pt$_{2}$HgSe$_{3}$ layer that is much larger than spin, which can induce measurable optical Kerr effect. The valley polarization and Chern number are coupled to the magnetic order of the nearest neighboring CrI$_3$ layer, which is switchable by electric, magnetic, and mechanical means in experiments. The presence of $h$-BN protects the topological phase allowing the construction of superlattices with valley, spin, and layer degrees of freedoms.
We theoretically investigate the localization mechanism of quantum anomalous Hall Effect (QAHE) with large Chern numbers $mathcal{C}$ in bilayer graphene and magnetic topological insulator thin films, by applying either nonmagnetic or spin-flip (magn
etic) disorders. We show that, in the presence of nonmagnetic disorders, the QAHEs in both two systems become Anderson insulating as expected when the disorder strength is large enough. However, in the presence of spin-flip disorders, the localization mechanisms in these two host materials are completely distinct. For the ferromagnetic bilayer graphene with Rashba spin-orbit coupling, the QAHE with $mathcal{C}=4$ firstly enters a Berry-curvature mediated metallic phase, and then becomes localized to be Anderson insulator along with the increasing of disorder strength. While in magnetic topological insulator thin films, the QAHE with $mathcal{C=N}$ firstly enters a Berry-curvature mediated metallic phase, then transitions to another QAHE with ${mathcal{C}}={mathcal{N}}-1$ along with the increasing of disorder strength, and is finally localized to the Anderson insulator after ${mathcal{N}}-1$ cycling between the QAHE and metallic phases. For the unusual findings in the latter system, by analyzing the Berry curvature evolution, it is known that the phase transitions originate from the exchange of Berry curvature carried by conduction (valence) bands. At the end, we provide a phenomenological picture related to the topological charges to help understand the underlying physical origins of the two different phase transition mechanisms.
We numerically investigate the electronic transport properties between two mesoscopic graphene disks with a twist by employing the density functional theory coupled with non-equilibrium Greens function technique. By attaching two graphene leads to up
per and lower graphene layers separately, we explore systematically the dependence of electronic transport on the twist angle, Fermi energy, system size, layer stacking order and twist axis. When choose different twist axes for either AA- or AB-stacked bilayer graphene, we find that the dependence of conductance on twist angle displays qualitatively distinction, i.e., the systems with top axis exhibit finite conductance oscillating as a function of the twist angle, while the ones with hollow axis exhibit nearly vanishing conductance for different twist angles or Fermi energies near the charge neutrality point. These findings suggest that the choice of twist axis can effectively tune the interlayer conductance, making it a crucial factor in designing of nanodevices with the twisted van der Waals multilayers.
We theoretically demonstrate that the in-plane magnetization induced quantum anomalous Hall effect (QAHE) can be realized in atomic crystal layers of group-V elements with buckled honeycomb lattice. We first construct a general tight-binding Hamilton
ian with $sp^3$ orbitals via Slater-Koster two-center approximation, and then numerically show that for weak and strong spin-orbit couplings the systems harbor QAHEs with Chern numbers of $mathcal{C}=pm1$ and $pm2$ , respectively. For the $mathcal{C}=pm1$ phases, we find the critical phase-transition magnetization from a trivial insulator to QAHE can become extremely small by tuning the spin-orbit coupling strength. Although the resulting band gap is small, it can be remarkably enhanced by orders via tilting the magnetization slightly away from the in-plane orientation. For the $mathcal{C}=pm2$ phases, we find that the band gap is large enough for the room-temperature observation. Although the critical magnetization is relatively large, it can be effectively decreased by applying a strain. All these suggest that it is experimentally feasible to realize high-temperature QAHE from in-plane magnetization in atomic crystal layers of group-V elements.
Using first-principles calculation methods, we study the possibility of realizing quantum anomalous Hall effect in graphene from stable 3textit{d}-atomic adsorption via charge-compensated textit{n}-textit{p} codoping scheme. As concrete examples, we
show that long-range ferromagnetism can be established by codoping 3textit{d} transition metal and boron atoms, but only the Ni codopants can open up a global bulk gap to harbour the quantum anomalous Hall effect. Our estimated ferromagnetic Curie transition temperature can reach over 10 Kelvin for various codoping concentrations.
