  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.
