Abstract
We have observed a magnetic vortex lattice (VL) in BaFe2(As0:67P0:33)2 single crystals by smallangle
neutron scattering. With the field along the c-axis, a nearly isotropic hexagonal VL was
formed in the field range from 1 to 16 T, and no symmetry changes in the VL were observed. The
temperature-dependence of the VL signal was measured and confirms the presence of (non d-wave)
nodes in the superconducting gap structure for measurements at 5 T and below. The nodal effects
were suppressed at high fields. At low fields, a VL reorientation transition was observed between 1 T
and 3 T, with the VL orientation changing by 45
◦
. Below 1 T, the VL structure was strongly affected
by pinning and the diffraction pattern had a fourfold symmetry. We suggest that this (and possibly
also the VL reorientation) is due to pinning to defects aligned with the crystal structure, rather than
being intrinsic. The temperature dependence of the scaled intensity suggest that BFAP possesses
at least one full gap and one nodal gap with circular symmetry. Judging from the symmetry, the
node structure should take the form of an ‘accidental’ circular line node, which is consistent with
recent ARPES results1.
