Evidence for nodal superconductivity in LaFePO

In several iron-arsenide superconductors there is strong evidence for a fully gapped superconducting state consistent with either a conventional s-wave symmetry or an unusual $s_{\pm }$ state where there the gap changes sign between the electron and hole Fermi surface sheets. Here we report measurements of the penetration depth $\lambda (T)$ in very clean samples of the related iron-phosphide superconductor, LaFePO, at temperatures down to $\sim$ 100 mK. We find that $\lambda (T)$ varies almost perfectly linearly with $T$ strongly suggesting the presence of gap nodes in this compound. Taken together with other data, this suggests the gap function may not be generic to all pnictide superconductors.