Nonlocality and strong coupling in the heavy fermion superconductor (formula presented) A penetration depth study

We report measurements of the magnetic penetration depth (formula presented) in single crystals of (formula presented) down to (formula presented) using a tunnel-diode-based, self-inductive technique at 28 MHz. While the in-plane penetration depth tends to follow a power law, (formula presented) the data are better described as a crossover between linear (formula presented) and quadratic (formula presented) behavior, with (formula presented) the crossover temperature in the strong-coupling limit. The c-axis penetration depth (formula presented) is linear in T. Both the magnitude of (formula presented) 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.