Vortex lattice structure inBaFe2(As0.67P0.33)2via small-angle neutron scattering

Abstract

We have observed a magnetic vortex lattice (VL) in ${\mathrm{BaFe}}_{2}({\mathrm{As}}_{0.67}{\mathrm{P}}_{0.33}){}_{2}$ (BFAP) single crystals by small-angle 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 and 3 T, with the VL orientation changing by ${45}^{\ensuremath{\circ}}$. 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 suggests 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 angle-resolved photoemission spectroscopy results [Y. Zhang, Z. R. Ye, Q. Q. Ge, F. Chen, J. Jiang, M. Xu, B. P. Xie, and D. L. Feng, Nature Physics 8, 371 (2012).].