  There has been an upsurge in the discovery of topological quantum materials,
where various topological insulators and semimetals have been theoretically
predicted and experimentally observed. However, only very few of them contains
silicon, the most widely used element in electronic industry. Recently, ternary
compound CaAl2Si2 has been predicted to be a topological Dirac semimetal,
hosting Lorentz-symmetry-violating quasiparticles with a strongly tilted
conical band dispersion. In this work, by using high-resolution angle-resolved
photoemission spectroscopy (ARPES), we investigated the comprehensive
electronic structure of CaAl2Si2. A pair of topological Dirac crossings is
observed along the kz direction, in good agreement with the ab initio
calculations, confirming the topological Dirac semimetal nature of the
compound. Our study expands the topological material family on Si-containing
compounds, which have great application potential in realizing low-cost,
nontoxic electronic device with topological quantum states.
