Effects of charging and perpendicular electric field on the properties of silicene and germanene

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 subjected to a perpendicular electric field, can be modified to attain new functionalities. In particular, when subjected to a perpendicular electric field, buckled atoms have the symmetry between their planes broken, opening a gap at the Dirac points. The occupation of 3d orbitals of the adsorbed Ti atom changes with charging or applied electric field, inducing significant changes in magnetic moment. We predict neutral silicene uniformly covered by Ti atoms to become a half-metal at a specific value of coverage and hence allow the transport of electrons in one spin direction, but block 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 a plane wave basis set within periodic boundary conditions. We clarified the cause of this spurious dependence and show that it can be eliminated by the use of a local orbital basis set. (paper)