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

We report measurements of the magnetic penetration depth λ in single crystals of CeCoIn₅ down to ∼0.14 K using a tunnel-diode-based, self-inductive technique at 28 MHz. While the in-plane penetration depth tends to follow a power law, λ_∥∼T^3/2, the data are better described as a crossover between linear (T≫T*) and quadratic (T≪T*) behavior, with T* the crossover temperature in the strongr-coupling limit. The c-axis penetration depth λ_⊥ 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.