Evidence for a Nodal Energy Gap in the Iron-Pnictide Superconductor LaFePO from Penetration Depth Measurements by Scanning SQUID Susceptometry

Abstract

We measure changes in the penetration depth $\lambda$ of the $T_c\approx 6\mathrm{K}$ superconductor LaFePO. In the process, scanning SQUID susceptometry is demonstrated as a technique for accurately measuring local temperature-dependent changes in $\lambda$, ideal for studying early or difficult-to-grow materials. $\lambda$ is found to vary linearly with temperatures from 0.36 to $\sim 2\mathrm{K}$, with a slope of $143\pm 15\AA /\mathrm{K}$, suggesting line nodes in the superconducting order parameter. The linear dependence up to $\sim T_c/3$, similar to the cuprate superconductors, indicates well-developed nodes.