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Robust flat bands in RCo5 (R=rare earth) compounds

The mechanism to realize the peculiar flat bands generally existing in RCo5 (R=rare earth) compounds is clarified by analyzing the first-principles band structures and the tight-binding model. These flat bands are constructed from the localized eigenstates, the existence of which is guaranteed by the partial cancelation between the intersite hopping amplitudes among the Co-3d states at the Kagome sites and those between the Kagome and honeycomb sites. Their relative positions to other bands can be controlled by varying the lattice parameters keeping their dispersion almost flat, which suggests the possibility of flat-band engineering.