Non-Locality and Strong Coupling in the Heavy Fermion Superconductor CeCoIn$_{5}$: A Penetration Depth Study

TL;DR

This study measures the magnetic penetration depth in CeCoIn$_{5}$, revealing evidence of d-wave superconductivity with line nodes and a crossover in temperature dependence indicative of strong coupling effects.

Contribution

It provides detailed penetration depth measurements in CeCoIn$_{5}$, demonstrating a crossover behavior and confirming d-wave symmetry with line nodes along the c-axis.

Findings

In-plane penetration depth follows a T^{3/2} power law.

c-axis penetration depth is linear in T.

Evidence supports d-wave superconductivity with line nodes.

Abstract

We report measurements of the magnetic penetration depth $\lambda$ in single crystals of CeCoIn$_{5}$ down to $\sim$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, $\lambda_{//} \sim {\it T}^{3/2}$, the data are better described as a crossover between linear ({\it T} $\gg $ ${\it T}^\ast $) and quadratic ({\it T} $\ll {\it T}^\ast $) behavior, with ${\it T}^\ast $ the crossover temperature in the strong-coupling limit. The {\it c}-axis penetration depth $\lambda_{\perp} $ is linear in {\it T}, providing evidence that CeCoIn$_{5}$ is a {\it d}-wave superconductor with line nodes along the {\it c}-axis. The different temperature dependences of $\lambda_{//} $ and $\lambda_{\perp} $ rule out impurity effects as the source of ${\it T}^{\ast} $.