Nodal superconducting-gap structure in ferropnictide superconductor BaFe2(As0.7P0.3)2

Abstract

The Cooper pairs of conventional superconductors exhibit a nodeless s-wave symmetry, and most unconventional superconductors, including cuprates and heavy-fermion materials, exhibit nodal d-wave pairing. In contrast to both, angle-resolved photoemission spectroscopy measurements indicate that the iron-based superconductor BaFe2(As0.7P0.3)2 exhibits an unusual nodal s-wave pairing. The superconducting-gap distribution is a pivotal characteristic for a superconductor. Whereas the cuprates and conventional phonon-mediated superconductors are characterized by distinct d-wave and s-wave pairing symmetries with nodal and nodeless gap distributions respectively, the superconducting-gap distributions in iron-based superconductors are rather diversified. Whereas nodeless gap distributions have been directly observed in Ba1−xKxFe2As2, BaFe2−xCoxAs2, KxFe2−ySe2 and FeTe1−xSex (refs 1, 2, 3, 4), the signatures of nodal superconducting gaps have been reported in LaOFeP, LiFeP, KFe2As2, BaFe2(As1−xPx)2, BaFe2−xRuxAs2 and FeSe (refs 5, 6, 7, 8, 9, 10, 11, 12). We here report the angle-resolved photoemission spectroscopy measurements on the superconducting-gap structure of BaFe2(As0.7P0.3)2, and in particular the direct observation of a circular line node on the largest hole Fermi surface around the Z point at the Brillouin zone boundary. Our findings rule out a d-wave-pairing origin for the nodal gap, and establish the existence of nodes in iron pnictides under the s-wave pairing symmetry.