Nonlocality and strong coupling in the heavy fermion superconductor CeCoIn5: A penetration depth study

We report measurements of the magnetic penetration depth $\ensuremath{\lambda}$ in single crystals of ${\mathrm{CeCoIn}}_{5}$ down to $\ensuremath{\sim}0.14\mathrm{K}$ using a tunnel-diode-based, self-inductive technique at 28 MHz. While the in-plane penetration depth tends to follow a power law, ${\ensuremath{\lambda}}_{//}\ensuremath{\sim}{T}^{3/2},$ the data are better described as a crossover between linear $(T\ensuremath{\gg}{T}^{*})$ and quadratic $(T\ensuremath{\ll}{T}^{*})$ behavior, with ${T}^{*}$ the crossover temperature in the strong-coupling limit. The c-axis penetration depth ${\ensuremath{\lambda}}_{\ensuremath{\perp}}$ is linear in T. Both the magnitude of ${T}^{*}$ and the different temperature dependencies in the two directions rule out impurity effects, but instead indicate that the penetration depth is governed by nonlocal electrodynamics in a d-wave superconductor with line nodes along the c axis. This is experimental confirmation of directional nonlocality, predicted theoretically by Kosztin and Leggett.