  The superconducting gap is a pivotal character for a superconductor. While
the cuprates and conventional phonon-mediated superconductors are characterized
by distinct d-wave and s-wave pairing symmetry with nodal and nodeless gap
distribution respectively, the superconducting gap distributions in iron-based
superconductors are rather diversified. While nodeless gap distributions have
been directly observed in Ba1-xKxFe2As2, BaFe2-xCoxAs2, KxFe2-ySe2, and
FeTe1-xSex, the signatures of nodal superconducting gap have been reported in
LaOFeP, LiFeP, KFe2As2, BaFe2(As1-xPx)2, BaFe2-xRuxAs2 and FeSe. We here report
the angle resolved photoemission spectroscopy (ARPES) measurements on the
superconducting gap structure of BaFe2(As1-xPx)2 in the momentum space, and
particularly, the first direct observation of a circular line node on the
largest hole Fermi surface around the Z point at the Brillouin zone boundary.
Our data rules out the d-wave pairing origin of the nodal gap, and unify both
the nodaland nodeless gaps in iron pnictides under the s\pm pairing symmetry.

Superconductivity