  Germanene, a graphene like single layer structure of Ge, has been shown to be
stable and recently grown on Pt and Au substrates. We show that a Ge adatom
adsorbed to germanene pushes down the host Ge atom underneath and forms a
dumbbell structure. This exothermic process occurs spontaneously. The
attractive dumbbell-dumbbell interaction favors high coverage of dumbbells.
This letter heralds stable new phases of germanene, which are constructed from
periodically repeating coverage of dumbbell structures and display diversity of
electronic and magnetic properties.
