Epitaxial Growth of Honeycomb Monolayer CuSe with Dirac Nodal Line Fermions

2D transition metal chalcogenides have attracted tremendous attention due to their novel properties and potential applications. Although 2D transition metal dichalcogenides are easily fabricated due to their layer-stacked bulk phase, 2D transition metal monochalcogenides are difficult to obtain. Recently, a single atomic layer transition metal monochalcogenide (CuSe) with an intrinsic pattern of nanoscale triangular holes is fabricated on Cu(111). The first-principles calculations show that free-standing monolayer CuSe with holes is not stable, while hole-free CuSe is endowed with the Dirac nodal line fermion (DNLF), protected by mirror reflection symmetry. This very rare DNLF state is evidenced by topologically nontrivial edge states situated inside the spin-orbit coupling gaps. Motivated by the promising properties of hole-free honeycomb CuSe, monolayer CuSe is fabricated on Cu(111) surfaces by molecular beam epitaxy and confirmed success with high resolution scanning tunneling microscopy. The good agreement of angle resolved photoemission spectra with the calculated band structures of CuSe/Cu(111) demonstrates that the sample is monolayer CuSe with a honeycomb lattice. These results suggest that the honeycomb monolayer transition metal monochalcogenide can be a new platform to study 2D DNLFs.