Anisotropic Vortex Pinning in Single Crystal FeSe*

Iron-based superconductors are known for their complex interplay between magnetic, nematic, and superconducting order. FeSe is an iron-based superconductor with a structural transition at 90 K without the appearance of magnetic ordering. The lack of magnetic order allows us to probe the superconducting transition of an iron-based superconductor from a purely nematic phase. We study single crystal FeSe using a scanning SQUID microscope that has both a field coil to apply a local magnetic field and a pickup loop to measure the vortex response. We model the effect of the SQUID geometry on vortex motion in an anisotropic pinning potential and find that SQUID orientation has a significant effect. Our initial findings suggest that that the pinning potential anisotropy aligns with twin domain boundaries produced by the structural transition. We also check for correlation between spatial variation in diamagnetic response and in anisotropy. These studies will help elucidate whether the relationship between nematic and superconducting order in these materials is competing or enhancing.