Antiferromagnetic spin ordering in the dissociative adsorption of H2 on Si(001): density-functional calculations

The dissociative adsorption of an H(2) molecule on the Si(001) surface, which has been experimentally identified in terms of dissociation on one side of two adjacent Si dimers, is investigated by spin polarized density-functional calculations within the generalized-gradient approximation. In contrast to the prevailing nonmagnetic configuration of charge ordering, we propose a new ground state where the two single dangling bonds (DBs) created by H(2) dissociation are antiferromagnetically coupled with each other. Such a spin ordering is found to be energetically favored over the previously proposed charge ordering. In the latter configuration, the buckling of the two DBs amounts to a height difference (Delta h) of 0.63 A, caused by a Jahn-Teller-like distortion, while in the former configuration, their buckling is almost suppressed to be Delta h=0.03 A as a consequence of spin polarization.