Observation of flat bands due to band hybridization in the 3d -electron heavy-fermion compound CaCu3Ru4O12

We report angle-resolved photoemission spectroscopy and first-principles numerical calculations for the band-structure evolution of the $3d$ heavy-fermion compound $\mathrm{CaC}{\mathrm{u}}_{3}\mathrm{R}{\mathrm{u}}_{4}{\mathrm{O}}_{12}$. Below $T\phantom{\rule{0.28em}{0ex}}\ensuremath{\sim}\phantom{\rule{0.28em}{0ex}}200$ 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 level centered around the Brillouin-zone corner. Our results confirm the non-Kondo nature of the $\mathrm{CaC}{\mathrm{u}}_{3}\mathrm{R}{\mathrm{u}}_{4}{\mathrm{O}}_{12}$, in which the $\mathrm{Cu}\phantom{\rule{0.16em}{0ex}}3{d}_{\mathrm{xy}}$ electrons are less correlated and not in the Kondo limit. Comparison between theory and experiment also suggests that other mechanisms may be needed to give a full quantitative explanation of the peculiar properties in this material.