Tailoring Robust Quantum Anomalous Hall Effect via Entropy-Engineering.

The development of quantum materials and the tailoring of their functional properties is of fundamental interest in materials science. Here, a new design concept is proposed for the robust quantum anomalous Hall (QAH) effect via entropy engineering in 2D magnets. As a prototypical example, the configurational entropy of monolayer transition metal trihalide VCl is manipulated by incorporating four different transition-metal cations [Ti,Cr,Fe,Co] into the honeycomb structure made of vanadium, such that all in-plane mirror symmetries, inversion and/or roto-inversion are broken. Monolayer VCl is a ferromagnetic Dirac half-metal in which spin-polarized Dirac dispersion at valley momenta is accompanied by bulk states at the Γ-point and thus the spin-orbit interaction-driven QAH phase does not exhibit fully gapped bulk band dispersion. Entropy-driven bandstructure renormalization, especially band flattening in combination with red- and blue-shifts at different momenta of the Brillouin zone and crystal-field effects, transforms Dirac half-metal to a Dirac spin-gapless semiconductor and leads to a robust QAH phase with fully gapped bulk band dispersion and, thus, a purely topological edge state transport without mixing with dissipative bulk channels. These findings provide a paradigm for designing entropy-engineered 2D materials for the realization of robust QAH effect and quantum device applications.