Title: Hybrid Topological Superconductivity and Hinge Majorana Flat Band in Type-II Dirac Semimetals

Abstract: Type-II Dirac semimetals (DSMs) have a distinct Fermi surface topology, which allows them to host novel topological superconductivity (TSC) different from type-I DSMs. By performing a low-energy analysis on a generic DSM model, we introduce an inherited pairing through band hybridization, which describes the second-order TSC. In a type-II DSM model, it is a two-dimensional~(2D) first-order time-reversal invariant topological superconductor for the $k_z = 0/\pi$ slice, whereas for the $k_z$ slices other than 0 or $\pi$, they belong to 2D second-order topological superconductors. Our analysis reveals a Majorana flat band on the $z$-directed hinge of the type-II DSM, which penetrates through the whole hinge Brillouin zone. These higher-order hinge modes are protected by $C_{4v}$ symmetry and can even host strong stability against finite $C_{4z}$ rotation symmetry-breaking order. We suggest that experimental realization of these findings can be achieved in transition metal dichalcogenides.