Josephson Interference in FeSe Visualized at the Atomic-scale

In a Josephson junction involving multi-band superconductors, inter-band and inter-junction Josephson coupling compete. Thus, causing frustration and spatial disjunction of superfluid densities among the superconducting condensates.Experimentally, a signature of such frustrated coupling is seen as quantum interference of Josephson currents from different tunneling channels. We explore these phenomena within the prototypical s+- wave superconductor FeSe. Atomic-resolution scanned Josephson tunneling microscopy (SJTM) allows unprecedented imaging of individual superconducting condensates and junction tuning; both capabilities which are unattainable in Josephson devices with fixed characteristics. We examine characteristic tunneling inequalities to quantitatively demonstrate frustrated Josephson tunneling. The relative transparency of the two parallel tunneling channels is found tunable and tends toward a 0-π transition with decreasing SJTM junction resistance. Simultaneous visualization of both condensates show anti-correlated superfluid modulations brought about by inter-band scattering. This study establishes SJTM as a powerful experimental method to motivate further research of multi-condensate superconductivity.