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 PtSn4, ZrSiS, TlTaSe2 and PbTaSe2. 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 Cu2Si based on combined theoretical calculations and angle-resolved photoemission spectroscopy measurements. The Dirac nodal lines in Cu2Si form two concentric loops centred around the Γ point and are protected by mirror reflection symmetry. Our results establish Cu2Si as a platform to study the novel physical properties in two-dimensional Dirac materials and provide opportunities to realize high-speed low-dissipation devices.

We determine the band structure and spin texture of WTe2 by spin-and angle-resolved photoemission spectroscopy (SARPES). With the support of first-principles calculations, we reveal the existence of spin polarization of both the Fermi arc surface states and bulk Fermi pockets. Our results support WTe2 to be a type-II Weyl semimetal candidate and provide important information to understand its extremely large and non-saturating magnetoresistance. 71.15.Mb,72.15.Gd Weyl semimetals, a novel state of topological quantum matter, have attracted significant attention in the recent years 1-5 . The low-energy electronic excitations (quasiparticles) in Weyl semimetal behave as Weyl fermions, a long-sought fundamental particle that has not been discovered until now. In a Weyl semimetal, the Weyl points always appear in pairs with opposite chirality, and can be described as magnetic monopoles in the momentum space. Near each Weyl point, the bands disperse linearly along all three momentum directions, thus forming three-dimensional Weyl cones. Besides the intriguing bulk bands, there exist topological non-trivial Fermi arcs that connect the projections of bulk Weyl points on the surface. The Weyl semimetals can be further classified into two types. Type-I has a point-like Fermi surface with symmetric Weyl cones, which has been realized in the TaAs family 3-9 ; in the type-II, the Lorentz invariance is strongly violated and the Weyl cones, tilted over one side, appear at the contact points between electron and hole pockets 10 . Recently, evidence of type-II Weyl semimetal has been reported in LaAlGe 11 , MoTe 2 12-16and WTe 2 17-19 . However, the Weyl semimetal character of WTe 2 has not yet been validated because neither the bulk Weyl points nor the non-trivial Fermi arcs has been experimental confirmed.Weyl semimetals are expected to host novel properties such as quantum anomalous Hall effect, extremely large magnetoresistance (XMR), and so forth 20-24 . Indeed, before being found as a type-II Weyl semimetal, WTe 2 has already been reported to host XMR which can reach 13 million percent in a magnetic field of 60 T without signature of saturation 25 . The origin of XMR in WTe 2 is still unclear until now. One explanation is that electrons and holes perfectly compensate in WTe 2 25-30 , but later experiments by high-resolution angle resolved photoemission spectroscopy (ARPES) and magneto-transport measurements revealed that the electron and hole densities are slightly imbalanced [31][32][33] . Recently, new mechanisms for XMR have been proposed based on the dynamics of spinpolarized electrons under the external magnetic field. For example, D. Rhodes et al. 32 suggest that the electronic structure of WTe 2 is composed of spin-split bands due to the spin-orbit interaction. The spin-polarized Fermi surface evolves sensitively with the magnetic field due to the Zeeman effect. On the other hand, J. Jiang et al. 31 propose that the resistivity of WTe 2 is intrinsically small due to the prohibition of backscattering between spin-...

No abstract