Magnetotransport and Ab Initio Calculation Studies on the Layered Semimetal CaAl2Si2 Hosting Multiple Nontrivial Topological States

We report herein the results of magnetotransport measurements and ab initio calculations on single crystalline CaAl2Si2 semimetal. The transport properties could be understood in connection with the two-band model, agreeing well with the theoretical calculations indicating four main sheets of Fermi surface consisting of three hole pockets centered at the Gamma point and one electron pocket centered at the M point in the Brillouin zone. Magnetotransport measurements showed striking Shubnikov-de Haas oscillations associated with a nontrivial Berry phase, which originate from a hole Fermi pocket indicated by the ab initio calculations. The calculations also unveiled that the hole Fermi surface encloses a nodal line setting around the Gamma point close to the Fermi level without considering the spin-orbit coupling (SOC). Once the SOC is included, the fragile nodal-line will be gapped and a pair of Dirac points emerge along the high symmetric Gamma-A direction located at the Brillouin zone coordinates (0, 0, k(z)(D) approximate to +/- 0.278 x 2 pi/c), about 1.22 eV below the Fermi level. In addition, the SOC can also induce a topological insulator state along the Gamma-A direction with a gap of about 3 meV. The results demonstrate CaAl2Si2 as an excellent platform for the study of novel topological physics with multiple nontrivial topological states.