  The microscopic mechanism of heavy band formation, relevant for
unconventional superconductivity in CeCoIn$_5$ 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 Ce$^{3+}$ ($4f^1$, $J=5/2$) with Sm$^{3+}$ ($4f^5$, $J=5/2$), we show
that a combination of crystal field and on-site Coulomb repulsion causes
SmCoIn$_5$ to exhibit a $\Gamma_7$ ground state similar to CeCoIn$_5$ with
multiple $f$ electrons. Remarkably, we also find that with this ground state,
SmCoIn$_5$ 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, $T_v$ $\approx$ 60 K. Our result
provides evidence that in the case of many $f$ electrons, the crystal field
remains the most important 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.
