Evidence for nodal superconductivity in LaFePO

TL;DR

This study provides evidence for nodal superconductivity in LaFePO through low-temperature penetration depth measurements, indicating the presence of gap nodes and suggesting diversity in gap structures among pnictide superconductors.

Contribution

First direct measurement of penetration depth in LaFePO showing linear temperature dependence, indicating nodal superconductivity.

Findings

Penetration depth varies linearly with temperature at low T.

Evidence suggests the presence of gap nodes in LaFePO.

Gap structure may differ among pnictide superconductors.

Abstract

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.