Si-Cmma: A silicon thin film with excellent stability and Dirac nodal loop

Abstract

Silicon thin film with both remarkable stability and graphenelike properties attract many research interests in materials science and condensed matter physics. In this paper, two new two-dimensional silicon allotropes (Si-Cmma and Si-Pmma) with excellent energetic stability and positive dynamical stability are proposed and investigated by first-principles calculations. Si-Pmma possesses the energy of about 203 meV/atom lower than that of silicene and it is a normal metallic phase. Si-Cmma is of about 198 meV/atom lower than silicene in energy and it is confirmed to be a nodal loop semimetallic material. Symmetry analysis reveals that the nodal loop in Si-Cmma is protected by the glide mirror and it belongs to class AI $+$ R of Altland-Zirnbauer symmetry classes. A further three-band $k\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{p}$ model is constructed to understand the formation of the nodal loop and its strong anisotropy.