Crystal field rules heavy fermion delocalization in SmCoIn5

Upper panel: Difference between the experimentally observed magnetization and calculated magnetization for different field directions. Middle panel: Temperature dependence of the a and c lattice parameters, and corresponding hyperbolic fits to the high temperature data. The departure from the expected form occurs near Tv. Lower panel: DC electrical resistance showing a departure from linear behavior at Tv. The solid line shows a fit to a hyperbolic form. Inset: resistance continues in a linear fashion up to T = 300 K. h Net carrier density from Hall resistance measurements with current applied along the a axis direction and field along c. Error bars represent fitted errors.

The microscopic mechanism of heavy band formation, relevant for unconventional super- conductivity 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, Tv ≈ 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.