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...

We calculate the density of states (DOS) and the Mulliken population of the diamond and the co-doped diamonds with different concentrations of lithium (Li) and phosphorus (P) by the method of the density functional theory, and analyze the bonding situations of the Li-P co-doped diamond thin films and the impacts of the Li-P co-doping on the diamond conductivities. The results show that the Li-P atoms can promote the split of the diamond energy band near the Fermi level, and improve the electron conductivities of the Li-P co-doped diamond thin films, or even make the Li-P co-doped diamond from semiconductor to conductor. The effect of Li-P co-doping concentration on the orbital charge distributions, bond lengths and bond populations is analyzed. The Li atom may promote the split of the energy band near the Fermi level as well as may favorably regulate the diamond lattice distortion and expansion caused by the P atom.