Topological nodal-line semimetals with exotic quantum properties are characterized by symmetry-protected line-contact bulk band crossings in the momentum space. However, in most of identified topological nodalline compounds, these topological non-trivial nodal lines are enclosed by complicated topological trivial states at the Fermi energy (E F ), which would perplex their identification and hinder further applications. Utilizing angle-resolved photoemission spectroscopy and first-principles calculations, we provide compelling evidence for the existence of Dirac nodal-line fermions in the monoclinic semimetal SrAs 3 , which are close to E F and away from distraction of complex trivial Fermi surfaces or surface states. Our calculation indicates that two bands with opposite parity are inverted around Y near E F , which results in the single nodal loop at the Γ-Y-S plane with a negligible spin-orbit coupling effect. We track these band crossings and then unambiguously identify the complete nodal loop quantitatively, which provides a critical experimental support to the prediction of nodal-line fermions in the CaP 3 family of materials. Hosting simple topological non-trivial bulk electronic states around E F and no interfering with surface states on the natural cleavage plane, SrAs 3 is expected to be a potential platform for topological quantum state investigation and applications.Topological semimetal, as the non-trivial extension of topological classification of electronic quantum states from the insulator to metal, is a group of materials in which the conduction and valence bands cross and form nodes behaving as monopole of a Berry flux [1,2]. When nodes are close to the Fermi energy (E F ), the low-energy quasiparticle excitation would be drastically different from that of the conventional Schrödinger-type fermion and thus lead to novel transport properties, which are crucial to the further study of novel quantum states and modern quantum devices [3,4]. While some distinct point-contact nodes, e.g., 4-, 2-, and 3-fold degenerate nodes, have been confirmed in Dirac [5,6], Weyl [7][8][9][10][11][12][13][14], and triply-degenerate semimetals [15,16] in previous studies, respectively, line-contact nodes, i.e., nodal lines, with their various configurations [17] have not been fully investigated in experiments until now.The nodal ring, nodal link and nodal chain all belong to nodal-line systems in which nodes extend along onedimensional lines instead of discrete points in the threedimensional (3D) Brillouin zone (BZ) [3,[17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34]. Theories have predicted that a non-trivial Berry phase around the nodal line would generate a half-integer shift of Landau-level index [20] and result in drumhead-like surface states [19,21]. To date, nodal-line states have been theoretically proposed and * Equal contributions † yinzhiping@bnu.edu.cn ‡ then experimentally confirmed in several compounds, including CaAgX (X=P, As) [22, 23], PbTaSe 2 [24], ZrSiS [25-28], and MB 2 (M...
