Spontaneous Symmetry Breaking and Dynamic Phase Transition in Monolayer
  Silicene

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Abstract

The (r3xr3)R30{\deg} honeycomb of silicene monolayer on Ag(111) was found to undergo a phase transition to two types of mirror-symmetric boundary-separated rhombic phases at temperatures below 40 K by scanning tunneling microscopy. The first-principles calculations reveal that weak interactions between silicene and Ag(111) drive the spontaneous ultra buckling in the monolayer silicene, forming two energy-degenerate and mirror-symmetric (r3xr3)R30{\deg} rhombic phases, in which the linear band dispersion near Dirac point (DP) and a significant gap opening (150 meV) at DP were induced. The low transition barrier between these two phases enables them interchangeable through dynamic flip-flop motion, resulting in the (r3xr3)R30{\deg} honeycomb structure observed at high temperature.

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Two and one-dimensional honeycomb structures of silicon and germanium

Salim Ciraci, Hasan Sahin, Ethem Akturk … (2008)

Based on first-principles calculations of structure optimization, phonon modes and finite temperature molecular dynamics, we predict that silicon and germanium have stable, two-dimensional, low-buckled, honeycomb structures. Similar to graphene, they are ambipolar and their charge carriers can behave like a massless Dirac fermions due to their pi- and pi*-bands which are crossed linearly at the Fermi level. In addition to these fundamental properties, bare and hydrogen passivated nanoribbons of Si and Ge show remarkable electronic and magnetic properties, which are size and orientation dependent. These properties offer interesting alternatives for the engineering of diverse nanodevices.

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Van der Waals density functionals applied to solids

(2013)

The van der Waals density functional (vdW-DF) of Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)] is a promising approach for including dispersion in approximate density functional theory exchange-correlation functionals. Indeed, an improved description of systems held by dispersion forces has been demonstrated in the literature. However, despite many applications, standard general tests on a broad range of materials are lacking. Here we calculate the lattice constants, bulk moduli, and atomization energies for a range of solids using the original vdW-DF and several of its offspring. We find that the original vdW-DF overestimates lattice constants in a similar manner to how it overestimates binding distances for gas phase dimers. However, some of the modified vdW functionals lead to average errors which are similar to those of PBE or better. Likewise, atomization energies that are slightly better than from PBE are obtained from the modified vdW-DFs. Although the tests reported here are for "hard" solids, not normally materials for which dispersion forces are thought to be important, we find a systematic improvement in cohesive properties for the alkali metals and alkali halides when non-local correlations are accounted for.