Symmetry Protected Topological Hopf Insulator and its Generalizations
  We study a class of $3d$ and $4d$ topological insulators whose topological nature is characterized by the Hopf map and its generalizations. We identify the symmetry $\mathcal{C}^\prime$, a generalized particle-hole symmetry that gives the Hopf insulator a $\mathbb{Z}_2$ classification. The $4d$ analogue of the Hopf insulator with symmetry $\mathcal{C}^\prime$ has the same $\mathbb{Z}_2$ classification. The minimal models for the $3d$ and $4d$ Hopf insulator can be heuristically viewed as "Chern-insulator$ \rtimes S^1$" and "Chern-insulator$\rtimes T^2$" respectively. We also discuss the relation between the Hopf insulator and the Weyl semimetals, which points the direction for its possible experimental realization. 
Chunxiao Liu, Farzan Vafa, Cenke Xu
cond-mat.str-el