Strong suppression of superconductivity by divalent ytterbium Kondo holes in CeCoIn${}_5$

To study the nature of partially substituted Yb ions in a Ce-based Kondo lattice, we fabricated high quality Ce${}_{1-x}$Yb${}_x$CoIn${}_5$ epitaxial thin films using molecular beam epitaxy. We find that the Yb substitution leads to a linear decrease of the unit cell volume—indicating that Yb ions are divalent, forming Kondo holes in Ce${}_{1-x}$Yb${}_x$CoIn${}_5$—and leads to a strong suppression of the superconductivity and Kondo coherence. These results, combined with measurements of the Hall effect, indicate that Yb ions act as nonmagnetic impurity scatterers in the coherent Kondo lattice without serious suppression of the antiferromagnetic fluctuations. These results are in stark contrast to previous studies performed using bulk single crystals, which claim the importance of valence fluctuations of Yb ions. The present work also highlights the suitability of epitaxial films in the study of the impurity effect on the Kondo lattice.

Figure 1

(a) X-ray diffraction $\phi$-scan data of the (115) peak for Ce${}_{0.8}$Yb${}_{0.2}$CoIn${}_5$. Inset: Streak patterns of the RHEED image during the crystal growth. (b) X-ray reciprocal lattice mapping for Ce${}_{0.8}$Yb${}_{0.2}$CoIn${}_5$ near the (115) peak. (c) Lattice parameters $a$ (filled green triangles) and $c$ (filled red squares and circles) for Ce${}_{1-x}$Yb${}_x$CoIn${}_5$ films. Here $c$ is determined by reciprocal mapping (squares) and $\theta –2\theta$ scan (circles). Open blue circles represent $c$ of bulk crystals reported in Ref. 6. The dashed lines are guides for the eyes. The inset shows $x$ determined by EDX, $x_{\mathrm{EDX}}$, vs $x$ determined by the evaporation ratio of Ce to Yb atoms.

Figure 2

Temperature dependence of the (a) resistivity and (b) Hall coefficient for Ce${}_{1-x}$Yb${}_x$CoIn${}_5$.

Figure 3

(a) $x$ dependence of $T_{\mathrm{coh}}$ normalized by the value at $x=0$ for thin films (filled red circles) and bulk crystals reported in Ref. 6 (open blue circles). (b) Temperature dependence of the resistivity at low temperatures. The dashed lines are the extrapolation above $T_c$.

Figure 4

(a) $x$ dependence of the residual resistivity. The dotted line is a guide for the eyes. (b) $x$ dependence of the superconducting transition temperature normalized by the value at $x=0$ for thin films (filled red circles) and bulk crystals [open blue squares (Ref. 6) and open circles (Ref. 8)]. (c) $T_c/T_{c0}$ plotted as a function of residual resistivity. The dotted line is the linear extrapolation from the low ${\rho }_0$ region. The solid curve represents the result fitted by Abrikosov-Gorkov (AG) theory.