Atomic-scale Frustrated Josephson Coupling and Multi-condensate Visualization in FeSe

Abstract: In a Josephson junction involving multi-band superconductors, competition between inter-band and inter-junction Josephson coupling gives rise to frustration and spatial disjunction of superfluid densities among superconducting condensates. Such frustrated coupling manifests as quantum interference of Josephson currents from different tunneling channels and becomes tunable if channel transparency can be varied. To explore these unconventional effects in the prototypical $s^\pm$-wave superconductor FeSe, we use atomic resolution scanned Josephson tunneling microscopy SJTM for condensate resolved imaging and junction tuning -- capabilities unattainable in macroscopic Josephson devices with fixed characteristics. We quantitatively demonstrate frustrated Josephson tunneling by examining two tunneling inequalities. The relative transparency of two parallel tunneling pathways is found tunable, revealing a tendency towards a 0-pi transition with decreasing SJTM junction resistance. Simultaneous visualization of both superconducting condensates reveals anti correlated superfluid modulations, highlighting the role of inter-band scattering. Our study establishes SJTM as a powerful tool enabling new research frontiers of multi condensate superconductivity.