Charge fluctuations in the intermediate-valence ground state of SmCoIn${}_5$

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${}^{3+}$ ($4f^1$, $J=5\mathrm{/}2$) with Sm${}^{3+}$ ($4f^5$, $J=5\mathrm{/}2$), we show that a combination of crystal field and on-site Coulomb repulsion causes SmCoIn${}_5$ to exhibit a ${\mathrm{\Gamma }}_7$ 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.