Evidence for Nodal Superconductivity in LaFePO from Scanning SQUID Susceptometry

TL;DR

This study uses scanning SQUID susceptometry to measure the penetration depth in LaFePO, revealing linear temperature dependence indicative of nodal superconductivity, similar to cuprates.

Contribution

It demonstrates the effectiveness of scanning SQUID susceptometry for local measurements of penetration depth in superconductors, providing evidence for nodal superconductivity in LaFePO.

Findings

Penetration depth varies linearly with temperature from 0.36 to ~2 K.

Linear dependence suggests the presence of line nodes in the superconducting gap.

Results are similar to cuprate superconductors, indicating well-developed nodes.

Abstract

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