A two-dimensional borophene monolayer with ideal Dirac nodal-line fermions

As a relatively new member of two-dimensional materials, borophene has gained huge interest over the past years, especially in the field of discovering new topological materials, such as Dirac nodal line semimetals. Here, based on first-principles calculations, for the first time, we find a completely flat borophene monolayer (named χ_2/9) with ideal Dirac nodal line states around the Fermi level. A tight-binding model using the Slater-Koster approach is proposed to demonstrate that the unique electronic feature of χ_2/9 that mainly originated from the first-nearest neighbor interactions of the p orbitals of boron. According to our symmetry analysis, the Dirac nodal line in χ_2/9 is guaranteed by the out-of-plane mirror or C₂ rotational symmetry and the negligible p orbital coupling. The chemical bonding analysis reveals the rare electronic properties of this material, which can be attributed to the multicentered π bonds.