Tunable double-Weyl Fermion semimetal state in the SrSi$_2$ materials class.

We discuss first-principles topological electronic structure of noncentrosymmetric SrSi$_2$ materials class based on the hybrid exchange-correlation functional. Topological phase diagram of SrSi$_2$ is mapped out as a function of the lattice constant with focus on the semimetal order. A tunable double-Weyl Fermion state in Sr$_{1-x}$Ca$_{x}$Si$_2$ and Sr$_{1-x}$Ba$_{x}$Si$_2$ alloys is identified. Ca doping in SrSi$_2$ is shown to yield a double-Weyl semimetal with a large Fermi arc length, while Ba doping leads to a transition from the topological semimetal to a gapped insulator state. Our study indicates that SrSi$_2$ materials family could provide an interesting platform for accessing the unique topological properties of Weyl semimetals.