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Topological invariants for three dimensional Dirac semimetals and four dimensional topological rotational insulators

Dirac semimetal is a class of semi-metallic phase protected by certain types of crystalline symmetries, and its low-energy effective Hamiltonian is described by Dirac equations in three dimensions (3D). Despite of various theoretical studies, theories that describe the topological nature of Dirac semimetals have not been well established. In this work, we define a topological invariant for 3D Dirac semimetals by establishing a mapping between a 3D Dirac semimetal and a topological crystalline insulator in four dimension (4D). We demonstrate this scheme by constructing a tight-binding model for 4D topological crystalline insulators that are protected by rotational symmetry. A new type of topological invariant, "rotational Chern number", is shown to characterize the topology of this system. As a consequence of the rotational Chern number, gapless Dirac points are found on the 3D surface of this 4D system. For a slab with two surfaces, we find that the corresponding low-energy effective theory of two surface states can be directly mapped to that of a 3D Dirac semimetal, suggesting that topological nature of 3D Dirac semimetals can be characterized by rotational Chern number which is defined in 4D. Our scheme provides a new systematic approach to extract topological nature for topological semimetal phases.