Title: Obstructed atomic insulators: A rational strategy to reach two-dimensional quasi-particles

Abstract: Here, using the topological quantum chemistry theory, we present a strategy to reach two-dimensional (2D) quasi-particles, which is benefiting from featured floating surface states (FSSs) of a three-dimensional (3D) obstructed atomic insulator (OAI). Symmetry of obstructed Wannier charge centers guarantees that such FSSs degenerate at high symmetry points or invariant lines in the surface Brillouin zone. Based on symmetry analysis and first-principles calculations, we identify that a carbon allotrope with a 20-atom body-centered tetragonal (bct) unit cell (\textit{i.e.}, termed bct-C$_{20}$), where the OAI phase is verified, is an ideal candidate to achieve our strategy. There are four in-gap FSSs, which simultaneously form two kinds of surface Dirac cones, \textit{i.e.}, topological Dirac cones with linear dispersion and symmetry-enforced quadratic Dirac cones. The band topology of surface Dirac cone is depicted by the effective surface Hamiltonian and the emergence of a hinge state. Our work would absolutely inspire interest in research concerning revealing more 2D topological states and related practical applications.