Shubnikov-de Haas and de Haas-van Alphen oscillations in the topological semimetal CaAl4

We report the magnetotransport properties of CaAl4 single crystals with C2/m structure at low temperature. CaAl4 exhibits large unsaturated magnetoresistance similar to 3000% at 2.5 K and 14 T. The nonlinear Hall resistivity is observed, which indicates the multi-band feature. The first-principles calculations show the electron-hole compensation and the complex Fermi surface in CaAl4, to which the two-band model with over-simplified carrier mobility can't completely apply. Evident quantum oscillations have been observed with B perpendicular to ab and B//ab configurations, from which the nontrivial Berry phase is extracted by the multi-band Lifshitz-Kosevich formula fitting and Landau level index fan diagram. The calculations also elucidate that CaAl4 possesses a Dirac nodal line type band structure around the Gamma point in the Z-Gamma-L plane, which is protected by the mirror symmetry as well as the space inversion and time reversal symmetries. Once the spin-orbit coupling is included, the crossed nodal line opens a negligible gap (less than 3 meV). The Z(2) invariant of CaAl4 equals 1, indicating its nontrivial topological properties. The open-orbit topology is also found in the electron-type Fermi surfaces by the first-principles calculations, which is believed to help enhance the magnetoresistance observed.