Strain-tunable magnetism and topological states in layered VBi 2 Te 4 .

Similar to the magnetic topological insulator of MnBi 2 Te 4 , recent studies have demonstrated that VBi 2 Te 4 is also an ideal candidate to explore many intriguing quantum states. Different from the strong interlayer antiferromagnetic (AFM) coupling in layered MnBi 2 Te 4 , based on first-principles calculations, we find that the energy difference between AFM and ferromagnetic (FM) orders in layered VBi 2 Te 4 is much smaller than that of MnBi 2 Te 4 . Specifically, it is found that the interlayer FM coupling can be readily achieved by applying strain. Further electronic band structures reveal that the VBi 2 Te 4 bilayer is a time-reversal symmetry broken quantum spin Hall insulator with a spin Chern number of C S = 1, which is essentially different from the QAH state with a Chern number of C = 1 in the MnBi 2 Te 4 bilayer. Most strikingly, the topological states of the magnetic VBi 2 Te 4 bilayer can be well tuned by strain, whose topological phase diagram is mapped out as a function of strain by employing continuum model analyses. All of these results indicate that the layered VBi 2 Te 4 not only enriches the family of magnetic topological materials, but also provides a promising platform to explore more exotic quantum phenomena.