Escape from black hole analogs in materials: Type-II Weyl semimetals and generic edge states

Type-II Weyl semimetals are dictated by bulk excitations with tilted light cones, resembling the inside of black holes. We obtain generic boundary conditions for surface boundaries of the type-II Weyl semimetals near Weyl nodes, and show that for a certain boundary condition edge states can escape out of the ``black hole'' event horizon. This means that for realization of the material ``black hole'' by the type-II Weyl semimetals, a careful choice of the boundary condition is necessary.