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

We measure changes in the penetration depth lambda of the T-c approximate to 6 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 similar to 2 K, with a slope of 143 +/- 15 A degrees/K, suggesting line nodes in the superconducting order parameter. The linear dependence up to similar to T-c/3, similar to the cuprate superconductors, indicates well-developed nodes.