Observation of finite-Q superfluid density oscillation in FeSe

Superfluid density is an important parameter that measures the phase coherence strength of superconducting pairing. In cuprate superconductors, the low carrier concentration and short superconducting coherence length lead to strong microscopic inhomogeneity in the superfluid density. In iron-based superconductor FeTe_0.55Se_0.45, microscopic measurements have revealed strongly inhomogeneous superfluid density. Whether this inhomogeneity in superfluid density is an intrinsic property of iron-based superconductors requires further investigation. Here, we use Scanning Josephson Tunneling Microscopy (SJTM) to measure bulk FeSe single crystals and found that the superfluid density exhibits finite-Q spatial oscillations. Detailed analysis shows that the spatial oscillation pattern are closely related to the fermi surface and electronic behaviour of FeSe. The superfluid density oscillations are particularly pronounced near iron-site defects and weaken with distance, suggesting that magnetic defects are the cause of spatial modulation in superfluid density. Furthermore, we found that the superconducting coherence peak has the strongest correlation with superfluid density while other experimental observables has weak or no correlation. Our study suggests that, at least in FeSe, there is no intrinsic microscopic inhomogeneity of superfluid density , which should be distinct from cuprates.