$\mathbb{Z}_2$-Topological metals and semimetals
We report two theoretical discoveries for $\mathbb{Z}_2$-topological metals and semimetals. It is shown first that any dimensional $\mathbb{Z}_2$ Fermi surface is topologically equivalent to a Fermi point. Then the famous conventional no-go theorem, which was merely proven before for $\mathbb{Z}$ Fermi points in a periodic system without any discrete symmetry, is generalised to that the total topological charge is zero for all cases. Most remarkably, we find and prove a new and unconventional strong no-go theorem: all $\mathbb{Z}_2$ Fermi points have the same topological charge $\nu_{\mathbb{Z}_2} =1$ or $0$ for periodic systems. Moreover, we also establish realistic models of $\mathbb{Z}_2$ Dirac points for physical dimensions.