Flat-band ferromagnetism in extended Δ-chain Hubbard models

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Abstract
We examined the magnetic property of newly extended Δ-chain Hubbard models which may be relevant to quantum atomic wires formed on solid surfaces. We rigorously show that the extended models exhibit so-called flat-band ferromagnetism, based on the Mielke-Tasaki theorem. Moreover, we numerically find that the ferromagnetic state is stable against some deviations from completely flat band, by using the exact diagonalization method. The present result will be also useful as a guiding principle to design magnetic materials that possibly display the flat-band ferromagnetism.