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 χ2/9) 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 χ2/9 that mainly originated from the first-nearest neighbor interactions of the pz orbitals of boron. According to our symmetry analysis, the Dirac nodal line in χ2/9 is guaranteed by the out-of-plane mirror or C2 rotational symmetry and the negligible pz orbital coupling. The chemical bonding analysis reveals the rare electronic properties of this material, which can be attributed to the multicentered π bonds.

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