Diverse forms of $\sigma$ bonding in two-dimensional Si allotropes:   Nematic orbitals in the $MoS_2$ structure
  The interplay of $sp^2$- and $sp^3$-type bonding defines silicon allotropes in two- and three-dimensional forms. A novel two-dimensional phase bearing structural resembleance to a single MoS$_2$ layer is found to possess a lower total energy than low-buckled silicene and to be stable in terms of its phonon dispersion relations. A new set of cigar-shaped, nematic orbitals originating from the Si $sp^2$ orbitals realizes bonding with a 6-fold coordination of the inner Si atoms of the layer. The identification of these nematic orbitals advocates diverse Si bonding configurations different from those of C atoms. 
Florian Gimbert, Chi-Cheng Lee, Rainer Friedlein, Antoine Fleurence,   Yukiko Yamada-Takamura and Taisuke Ozaki
cond-mat.mtrl-sci