The two orbitals consist of two types of bonds in α-graphdiyne: One is the bonding bonds (Figure 3a) and the other the antibonding bonds (Figure buy H 89 3b), which are located at the different carbons. As a recent study reported [23], the effective hopping term of the acetylenic linkages is much smaller than the direct hopping between the vertex atoms. This is because the covalent bonds are formed in these acetylenic linkages as illustrated in Figure 3, which subsequently weakens the hopping ability. Thus, the reduced hopping parameter is a natural consequence, which also agrees well with our above tight-binding theory. Future experiments can test this prediction directly.

Figure 3 Charge density distributions of two orbitals at the Dirac point. The (a) bonding and (b) antibonding bonds. The isovalues are set to 0.03

Å -3; 3 ×3 supercells are given for the sake of clarity. Conclusions In conclusion, we have predicted a novel carbon allotrope called α-graphdiyne, which has a similar Dirac cone to that of graphene. The lower Fermi velocity stems from its largest lattice constant compared with other current carbon allotropes. The effective hopping parameter of 0.45 eV is obtained through fitting the energy bands in the vicinity of Dirac points. The obtained Fermi velocity has a lower value of 0.11 ×106 m/s, which might have potential applications in quantum electrodynamics. Acknowledgements We would like to thank L. Huang (LZU, Lanzhou) for the valuable discussion. This work was supported AZD2014 cost by the National Basic Research Program of China under no. 2012CB933101,

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