Dec.2 2018

**Figure 1**

Atomic and band structures of free-standing Cu_{2}Si. **a** Top and side view. The orange and blue balls represent Cu and Si atoms, respectively. **b**, **c** Calculated band structures of Cu_{2}Si without and with spin-orbit coupling (SOC), respectively. The vertical axis *E*-*E*
_{F} corresponds to -*E*
_{B}, where *E*
_{B} is the binding energy. For simplicity, we label the three bands that cross the Fermi level *α*, *β*, and *γ*, respectively. The parity of mirror reflection symmetry for each band is labelled plus and minus signs in **b**. The zoom-in band structures in the blue and red ellipses are shown in **g**, **h**. **d** Band structure of Cu_{2}Si after artificially increasing the intrinsic SOC by 20 times. **e** Fermi surface of Cu_{2}Si without SOC. The blue, orange, and green lines correspond to bands *α*, *β*, and *γ*, respectively. **f** Momentum distribution of the nodal loops: NL1 (blue) and NL2 (orange). **g**, **h** Zoom-in band structures in the blue and red ellipses in **c**, which clearly show the SOC-induced gaps

**Figure 2**

ARPES results for monolayer Cu_{2}Si on Cu(111). **a** Schematic drawing of the Brillouin zones of Cu_{2}Si (blue hexagons) and Cu(111) (black hexagon). As the lattice of Cu_{2}Si is $\left(\sqrt{3}\times \sqrt{3}\right)$R30° with respect to the Cu(111)-1 × 1 lattice, the K point of Cu(111) is located at the Γ point of the second Brillouin zone of Cu_{2}Si. **b**–**e** Second derivative CECs measured using 30-eV *p*-polarized photons. Three closed contours have been observed: a hexagon, a hexagram, and a circle, as indicated by the dashed lines. **f**, **i**, **j** ARPES intensity plots along the Γ-K direction measured with different photon energies: 30, 35, and 25 eV, respectively. The black arrows mark the position of the crossing points. **g**, **h** ARPES intensity plots along the Γ-M direction measured with *p* and *s* polarized light, respectively. **k** ARPES intensity plots along the Γ-M direction measured with 60-eV circularly polarized light. The *γ* band is clearly observed while the *α* and *β* bands are suppressed

Topological nodal line semimetals, a novel quantum state of materials, possess topologically nontrivial valence and conduction bands that touch at a line near the Fermi level. The exotic band structure can lead to various novel properties, such as long-range Coulomb interaction and flat Landau levels. Recently, topological nodal lines have been observed in several bulk materials, such as PtSn_{4}, ZrSiS, TlTaSe_{2} and PbTaSe_{2}. However, in two-dimensional materials, experimental research on nodal line fermions is still lacking. Here, we report the discovery of two-dimensional Dirac nodal line fermions in monolayer Cu_{2}Si based on combined theoretical calculations and angle-resolved photoemission spectroscopy measurements. The Dirac nodal lines in Cu_{2}Si form two concentric loops centred around the Γ point and are protected by mirror reflection symmetry. Our results establish Cu_{2}Si as a platform to study the novel physical properties in two-dimensional Dirac materials and provide opportunities to realize high-speed low-dissipation devices.