Vortex lattice structure in BaFe2(As0.67P0.33)2 by the small-angle neutron scattering technique

TL;DR

This study used small-angle neutron scattering to investigate the vortex lattice in BaFe2(As0.67P0.33)2, revealing isotropic hexagonal structures, nodal superconducting gaps, and pinning effects influencing vortex arrangements.

Contribution

First SANS observation of vortex lattice in BFAP showing isotropic hexagonal symmetry and field-dependent nodal effects in the superconducting gap.

Findings

Record vortex lattice observation in pnictides (1-16 T)

Confirmation of nodal superconducting gap structure at low fields

Identification of pinning effects influencing vortex lattice orientation

Abstract

We have observed a magnetic vortex lattice (VL) in BaFe2(As_{0.67}P_{0.33})2 (BFAP) single crystals by small-angle neutron scattering (SANS). With the field along the c-axis, a nearly isotropic hexagonal VL was formed in the field range from 1 to 16 T, which is a record for this technique in the pnictides, 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{\deg}. 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.