Microscopic Origin and Josephson Signatures of $d$-Wave Superconductivity in Monolayer FeSe

Abstract

Monolayer FeSe on SrTiO$_3$ exhibits high-temperature superconductivity with an electron-only Fermi surface, raising fundamental questions about its unconventional pairing mechanism. Motivated by recent Josephson scanning tunneling microscopy evidence for nodeless $d$-wave superconductivity, we investigate the microscopic role of the Fe $d_{xy}$ orbital using an experimentally constrained multiorbital theory. A tight-binding model fitted to quasiparticle interference and angle-resolved photoemission spectroscopy reveals substantial $d_{xy}$ character near the Fermi level and dominant nearest-neighbor hopping, in contrast to the conventional kinetic-frustration picture of iron-based superconductors. Both itinerant spin-fluctuation calculations and strong-coupling exchange approaches favor a robust $d$-wave pairing state, whose interband pairing components allow a fully gapped quasiparticle spectrum. We further establish a symmetry-based selection rule for spatial Josephson tunneling, revealing characteristic bond-centered maxima and mirror-enforced interference zeros that distinguish $d$-wave pairing from competing $s$-wave states. These symmetry-enforced nodal lines survive near Fe-site impurities, while a pronounced contrast between the $d$- and $s$-wave Josephson patterns persists around vortices despite the loss of mirror protection. Our results connect the unconventional $d_{xy}$ electronic structure to nodeless $d$-wave superconductivity and demonstrate the power of spatial Josephson interference for resolving pairing symmetry in multiorbital superconductors.