We report angle-resolved photoemission spectroscopy and first-principles numerical calculations for the band structure evolution of the 3d heavy-fermion compound CaCu3Ru4O12. Below 200 K, we observed an emergent hybridization gap between the Cu 3d electron-like band and the Ru 4d hole-like band and the resulting flat band features near the Fermi energy centered around the Brillouin zone corner. Our results confirm the non-Kondo nature of CaCu3Ru4O12, in which the Cu 3dxy electrons are less correlated and not in the Kondo limit. Comparison between theory and experiment also suggests that other mechanism such as nonlocal interactions or spin fluctuations beyond the local dynamical mean-field theory may be needed in order to give a quantitative explanation of the peculiar properties in this material.
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