Evolution of the superconducting energy gap structure concomitant with Fermi surface reconstruction in the heavy-fermion superconductor CeCoIn5

Abstract

The London penetration depth, $\hat{I}»(T)$ was measured in single crystals of Ce${}_{1-x}{R}_x$CoIn${}_5$, $R$=La, Nd and Yb down to 50~mK ($T_c/T\sim$50) using a tunnel-diode resonator. In the cleanest samples $\hat{I}”\hat{I}»(T)$ is best described by the power law, $\hat{I}”\hat{I}»(T)\propto T^n$, with $n\sim 1$, consistent with line nodes. Substitutions of Ce with La, Nd and Yb lead to similar monotonic suppressions of $T_c$, however the effects on $\hat{I}”\hat{I}»(T)$ differ. While La and Nd doping results in an increase of the exponent to $n\sim 2$, as expected for a dirty nodal superconductor, Yb doping leads to $n>3$, inconsistent with nodes, suggesting a change from nodal to nodeless superconductivity where Fermi surface topology changes were reported, implying that the nodal structure and Fermi surface topology are closely linked.