  Using first-principles Density Functional Theory calculations, we showed that
electronic and magnetic properties of bare and Ti adatom adsorbed single-layer
silicene and germanene, which are charged or exerted by a perpendicular
electric field are modified to attain new functionalities. In particular, when
exerted by a perpendicular electric field, the symmetry between the planes of
buckled atoms is broken to open a gap at the Dirac points. The occupation of
3d-orbitals of adsorbed Ti atom changes with charging or applied electric field
to induce significant changes of magnetic moment. We predict that neutral
silicene uniformly covered by Ti atoms becomes a half-metal at a specific value
of coverage and hence allows the transport of electrons in one spin direction,
but blocks the opposite direction. These calculated properties, however exhibit
a dependence on the size of the vacuum spacing between periodically repeating
silicene and germanene layers, if they are treated using plane wave basis set
within periodic boundary condition. We clarified the cause of this spurious
dependence and show that it can be eliminated by the use of local orbital basis
set.
