X-ray spectroscopic investigation of crystal fields in ${\mathrm{Ce}}_2{\mathrm{Rh}}_{1-x}{\mathrm{Ir}}_x{\mathrm{In}}_8$ heavy fermions

The higher dimensionality in the crystal fields of the ${\mathrm{Ce}}_{2}M{\mathrm{In}}_8$ ($M=\mathrm{Rh},\mathrm{Ir}$) compounds and its interplay with hybridization and disorder are key ingredients to understand the complex phase diagrams by this family, which have been explored extensively by macroscopic techniques. Here, we present an investigation of the crystal-electric field schemes of ${\mathrm{Ce}}_2{\mathrm{Rh}}_{1-x}{\mathrm{Ir}}_x{\mathrm{In}}_8$ using x-ray absorption spectroscopy. Our full multiplet calculations for the $4f^1$ configuration of ${\mathrm{Ce}}^{3+}$ to describe the temperature-dependent linear dichroism in ${\mathrm{Ce}}_{2}M{\mathrm{In}}_8$ are consistent with a ${\Gamma }_7^1=\sqrt{1-{\alpha }^2}·|\mp \frac{3}{2}〉-\left|\alpha \right|·|\pm \frac{5}{2}〉$ ground state containing a predominant $|\pm 3/2〉$ contribution that increases further with $x$. This enhancement is believed to favor superconductivity in Ce-based heavy fermion materials, observed in previous results in the $\mathrm{Ce}M{\mathrm{In}}_5$ family. Our recent observations shed light on the unexpected emergence of the ambient-pressure superconducting dome in the center of the composition phase diagram and its subsequent suppression on the Ir-rich side due to the early onset of fluctuations associated with the structurally more disordered state, inferred from previous neutron magnetic diffraction experiments.