We have examined as a Hubbard model on a chain of squares, which was as proposed by Yajima et al. as a model of an atomic quantum wire As/Si͑100͒, to show that the flat-band ferromagnetism according to a kind of Mielke-Tasaki mechanism should be realized for an appropriate band filling in such a nonfrustrated lattice. Reflecting the fact that the flat band is not a bottom one, the ferromagnetism vanishes, rather than intensifies, as the Hubbard U is increased. The exact-diagonalization method is used to show that the critical value of U is in a realistic range. We also discussed the robustness of the magnetism against the degradation of the flatness of the band.
The atomic and electronic structures of As atomic wires, which are formed by As adsorbates around a dangling-bond wire on a H-terminated Si͑100͒, are investigated by using first-principles calculations within the local-density-functional approach. By comparing the metastable geometries of the As atomic wires with those of the Ga atomic wires previously studied, we found that As atoms have a stronger tendency to form covalent bonds. Interestingly, the most stable As configuration has an energy band with no significant dispersion as the lowest empty surface state. Electron-doping into this flat band may open up the possibility to form a ferromagnetic atomic wire. To exemplify such doping, we show that a half-filled flat band can be realized, its flatness unchanged, when K adsorbates are supplied to the As wire.
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