In our previous paper (Phys. Rev. B {\bf 89}, 165430 (2014)) we have foundthat in graphene, in distinction to the four occupied bands, which can bedescribed by the simple tight-binding model (TBM) with four atomic orbitals peratom, the two lowest lying at the $\Gamma$-point unoccupied bands (one of themof a $\sigma$ type and the other of a $\pi$ type) can not be described by suchmodel. In the present work we suggest a minimalistic model for these two bands,based on (nearly) free electrons model (FEM), which correctly describes thesymmetry of these bands, their dispersion law and their localization withrespect to the graphene plane.
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机译：在我们之前的论文（Phys。Rev. B {\ bf 89}，165430（2014））中，我们发现在石墨烯中，与四个占据带不同，这可以通过简单的紧束缚模型（TBM）用四个原子轨道peratom，位于$ \ Gamma $点的未占用带中的最低两个（其中一个为$ \ sigma $类型，另一个为$ \ pi $类型）无法用此类模型描述。在目前的工作中，我们建议基于（几乎）自由电子模型（FEM）来针对这两个谱带的简约模型，该模型正确描述了这些谱带的对称性，其色散规律以及它们相对于石墨烯平面的定位。
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