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We report the magneto-transport properties of CaAl$_4$ single crystals with$C2/m$ structure at low temperature. CaAl$_4$ exhibits large unsaturatedmagnetoresistance $\sim$3000$\%$ at 2.5 K and 14 T. The nonlinear Hallresistivity is observed, which indicates the multi-band feature. Thefirst-principles calculations show the electron-hole compensation and thecomplex Fermi surface in CaAl$_4$, to which the two-band model withover-simplified carrier mobility can't completely apply. Evident quantumoscillations have been observed with B//c and B//ab configurations, from whichthe nontrivial Berry phase is extracted by the multi-band Lifshitz-Kosevichformula fitting. An electron-type quasi-2D Fermi surface is found by theangle-dependent Shubnikov-de Haas oscillations, de Haas-van Alphen oscillationsand the first-principles calculations. The calculations also elucidate thatCaAl$_4$ owns a Dirac nodal line type band structure around the $\Gamma$ pointin the $Z$-$\Gamma$-$L$ plane, which is protected by the mirror symmetry aswell as the space inversion and time reversal symmetries. Once the spin-orbitcoupling is included, the crossed nodal line opens a negligible gap (less than3 meV). The open-orbit topology is also found in the electron-type Fermisurfaces, which is believed to help enhance the magnetoresistance observed.

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