Charge Fluctuations in the Intermediate-valence Ground State of SmCoIn

The microscopic mechanism of heavy band formation, relevant for unconventional superconductivity in CeCoIn and other Ce-based heavy fermion materials, depends strongly on the efficiency with which electrons are delocalized from the rare earth sites and participate in a Kondo lattice. Replacing Ce (4,  = 5/2) with Sm (4,  = 5/2), we show that a combination of the crystal electric field and on-site Coulomb repulsion causes SmCoIn to exhibit a Γ ground state similar to CeCoIn with multiple electrons. We show that with this single-ion ground state, SmCoIn exhibits a temperature-induced valence crossover consistent with a Kondo scenario, leading to increased delocalization of holes below a temperature scale set by the crystal field,  ≈ 60 K. Our result provides evidence that in the case of many 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.