  Heavy-fermion materials are mostly rare-earth or actinide intermetallics with
very few exceptions in d-electron systems. The physical mechanism for these
d-electron heavy fermion systems remains unclear. Here by studying the
quadruple-perovskite CaCu3Ir4O12, we propose a symmetry-based mechanism that
may enforce heavy-fermion physics in d-electron systems. We show that electron
hoppings between neighboring Cu 3d-orbitals are strictly prohibited by the
crystal symmetry, so that Cu 3d-electrons can only become delocalized through
hybridization with other more itinerant bands, resembling that in typical
heavy-fermion rare-earth intermetallics. This provides a useful way to enforce
heavy-fermion physics in d-electron systems and may help future design of new
heavy-fermion materials.
