  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.
