Symmetry-protected Dirac nodal line semimetals (DNLSs) receive much attention because of their exotic physical properties and potential applications in dissipationless spin devices. Several compounds (Ca3P2, Cu3PdN, etc.) have been found to be three-dimensional DNLSs, in which the topological Dirac nodal line fermions (DNLFs) can be driven to form a Weyl fermion, Dirac fermion, or other topological phase if the protected symmetry is broken. However, DNLFs in 2D atomic crystal have rarely been explored because of their high vulnerability to symmetry breaking, even in challenging cryogenic experiments. Here we propose that free-standing monolayer CuSe with honeycomb structure is endowed with the exotic 2D DNLF protected by mirror reflection symmetry, based on first-principles calculation. The DNLF state is evidenced by nontrivial edge states that arise as the crossing bands open the gaps with spin-orbit coupling. Motivated by the promising 2D DNLF feature of CuSe, we constructed monolayer CuSe on a Cu(111) surface by molecular beam epitaxy and confirmed success with scanning tunneling microscopy. The good agreement of angle resolved photoemission spectroscopy with the calculated band structures of CuSe/Cu(111) demonstrates that it is a monolayer CuSe with a distorted honeycomb lattice. Our theoretical and experimental results, considered together, establish planar transition-metal honeycomb structure as a new platform to study 2D DNLFs.
In collaboration with Lei Gao, Jia-Tao Sun, Jian-Chen Lu, Hang Li, Kai Qian, Shuai Zhang, Tian Qian, Hong Ding and Hong-Jun Gao from the Institute of Physics, CAS; X. Lin and Y. Y. Zhang from the University of CAS.