AbstractThe topological nodal-line semimetal (TNS) is a unique class of materials with a one dimensional line node accompanied by a nearly dispersionless two-dimensional surface state. However, a direct observation of the so called drumhead surface state within current nodal-line materials is still elusive. Here, using high-resolution angle-resolved photoemission spectroscopy (ARPES) along with first-principles calculations, we report the observation of a topological nodal-loop (TNL) in SrAs₃, whereas CaAs₃ exhibits a topologically trivial state. Our data reveal that surface projections of the bulk nodal-points are connected by clear drumhead surface states in SrAs₃. Furthermore, our magneto-transport and magnetization data clearly suggest the presence (absence) of surface states in SrAs₃ (CaAs₃). Notably, the observed topological states in SrAs₃ are well separated from other bands in the vicinity of the Fermi level. RAs₃ where R = Ca, Sr, thus, offers a unique opportunity to realize an archetype nodal-loop semimetal and establish a platform for obtaining a deeper understanding of the quantum phase transitions.