Charge fluctuations in the intermediate-valence ground state of SmCoIn5

The 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+ (4f(1), J = 5/2) with Sm3+ (4f(5), J = 5/2), we show that a combination of the crystal electric field and on-site Coulomb repulsion causes SmCoIn5 to exhibit a & UGamma;(7) 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 & AP; 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. In heavy fermion metals, exotic states such as superconductivity depend to a large extent on whether unpaired electrons can become delocalized from the magnetic sites. The authors demonstrate that the primary driving force behind delocalization is the local environment of the rare earth ions and the strength of the resulting charge fluctuations.