Renormalizations in unconventional superconducting states of Ce${}_{1-x}$Yb${}_x$CoIn${}_5$

We have measured the superconducting penetration depth~$\mathrm{\Lambda }(T)$ in the heavy-fermion/intermediate-valent superconducting alloy series~Ce${}_{1-x}$Yb${}_x$CoIn${}_5$ using transverse-field muon spin relaxation, to study the effect of intermediate-valent Yb doping on Fermi-liquid renormalization. From $\mathrm{\Lambda }(T)$ we determine the superfluid density ${\rho }_s(T)$, and find that it decreases continuously with increasing nominal Yb concentration~$x$, i.e., with increasing intermediate valence. The temperature-dependent renormalization of the “normal” fluid density~$\rho_N(T) = \rho_s(0) - \rho_s(T)$ in both the heavy-fermion and intermediate valence limits is proportional to the temperature-dependent renormalization of the specific heat. This indicates that the temperature-dependent Fermi-liquid Landau parameters of the superconducting quasiparticles entering the two different physical quantities are the same. These results represent an important advance in understanding of both intermediate valence and heavy-fermion phenomena in superconductors.