AbstractThe microscopic mechanism of heavy band formation, relevant for unconventional superconductivity in CeCoIn₅ and other Ce-based heavy fermion materials, depends strongly on the efficiency with which f electrons are delocalized from the rare earth sites and participate in a Kondo lattice. Replacing Ce³⁺ (4f¹, J = 5/2) with Sm³⁺ (4f⁵, J = 5/2), we show that a combination of the crystal electric field and on-site Coulomb repulsion causes SmCoIn₅ to exhibit a Γ₇ ground state similar to CeCoIn₅ with multiple f electrons. We show that with this single-ion ground state, SmCoIn₅ exhibits a temperature-induced valence crossover consistent with a Kondo scenario, leading to increased delocalization of f holes below a temperature scale set by the crystal field, T_v ≈ 60 K. Our result provides evidence that in the case of many f electrons, the crystal field remains the dominant tuning knob in controlling the efficiency of delocalization near a heavy fermion quantum critical point, and additionally clarifies that charge fluctuations play a general role in the ground state of “115” materials.