Abstract
In recent experiments, superconductivity and correlated insulating states were observed in twisted bilayer graphene (TBG) with small magic angles, which highlights the importance of the flat bands near Fermi energy. However, the moiré pattern of TBG consists of more than ten thousand carbon atoms that is not easy to handle with conventional methods. By density functional theory calculations, we obtain a flat band at EF in a novel carbon monolayer coined as cyclicgraphdiyne with the unit cell of eighteen atoms. By doping holes into cyclicgraphdiyne to make the flat band partially occupied, we find that cyclicgraphdiyne with 1/8, 1/4, 3/8 and 1/2 hole doping concentration shows ferromagnetism (half-metal) while the case without doping is nonmagnetic, indicating a hole-induced nonmagnetic-ferromagnetic transition. The calculated conductivity of cyclicgraphdiyne with 1/8, 1/4 and 3/8 hole doping concentration is much higher than that without doping or with 1/2 hole doping. These results make cyclicgraphdiyne really attractive. By studying several carbon monolayers, we find that a perfect flat band may occur in the lattices with both separated or corner-connected triangular motifs with only including nearest-neighboring hopping of electrons, and the dispersion of flat band can be tuned by next-nearest-neighboring hopping. Our results shed insightful light on the formation of flat band in TBG. The present study also poses an alternative way to manipulate magnetism through doping flat band in carbon materials.
Highlights
A twisted bilayer graphene (TBG) with small angles was predicted to have moiré pattern with a very large unit cell of more than ten thousand carbon atoms
By means of the first-principles calculations, we find that the flat band near the EF could appear in a carbon monolayer with unit cell of eighteen atoms, which is coined cyclicgraphdiyne
A flat band appears at EF in cyclicgraphdiyne
Summary
A twisted bilayer graphene (TBG) with small angles was predicted to have moiré pattern with a very large unit cell of more than ten thousand carbon atoms. The superconductivity and correlated insulating states at half-filling were observed in TBG, which highlight the importance of the flat bands near Fermi energy[5,6]. The topological properties of electronic band structure[9,10], ferromagnetic Mott state[11] and metal-insulator transition[12,13], extended Hubbard model, nematic superconductivity by density wave fluctuations[14,15,16] in TBG were addressed. By means of the first-principles calculations, we find that the flat band near the EF could appear in a carbon monolayer with unit cell of eighteen atoms, which is coined cyclicgraphdiyne. The present study provides a simple 2D platform of carbon atoms to probe the flat bands and magnetism in 2D systems
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