Topological nature of large bulk band gap materials Sr3Bi2 and Ca3Bi2

Topological materials are an emerging class of materials attracting the attention of the scientific community due to their potential applications in the fields of spintronics and quantum computing. Using first-principles calculations, the structural, electronic, and topological properties of Sr3Bi2 and Ca3Bi2 compounds without and with spin-orbit coupling are investigated. In the absence of spin-orbit coupling, the projected bulk band structure revealed that the Sr3Bi2 compound host a type-I Dirac point along the F-Gamma direction. Since the compound possesses time-reversal and space-inversion symmetries, this Dirac point is associated with the nodal line. The existence of a type-I nodal ring around the Gamma-point in the k(z) = 0 planes, as well as a drumhead-like surface state within the nodal ring, suggested that Sr3Bi2 is a type-I nodal-line semimetal with no spin-orbit coupling. The inclusion of spin-orbit coupling introduced an energy gap of 0.36 eV between the valence band and conduction band at Dirac point. The topological surface states forming a Dirac cone between the bulk bandgap for (001) surface of Sr3Bi2 compound is calculated with spin-orbit coupling. The Z(2) topological invariants (1;000), as calculated by using parity product criteria, suggested that Sr3Bi2 is a strong topological insulator. Ca3Bi2, another compound with a similar crystal structure, is also predicted to behave similarly to Sr3Bi2 compound without and with spin-orbit coupling. This research broadens the application of topological insulators and existing platforms for developing novel spintronic devices.