title: Evidence for a nodal-line superconducting state in LaFePO.
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{+/-} state where 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 approximately 100 mK. We find that lambda(T) varies approximately linearly with T strongly suggesting the presence of gap nodes in this compound. Taken together with other data, this suggests the gap function is not universal for all pnictide superconductors.
authors.affiliation: H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, BS8 1TL, United Kingdom.
authors.forename: J D
authors.initials: JD
authors.lastname: Fletcher
authors.forename: A
authors.initials: A
authors.lastname: Serafin
authors.forename: L
authors.initials: L
authors.lastname: Malone
authors.forename: J G
authors.initials: JG
authors.lastname: Analytis
authors.forename: J-H
authors.initials: JH
authors.lastname: Chu
authors.forename: A S
authors.initials: AS
authors.lastname: Erickson
authors.forename: I R
authors.initials: IR
authors.lastname: Fisher
authors.forename: A
authors.initials: A
authors.lastname: Carrington