Atomic-Site-Dependent Pairing Gap in Monolayer FeSe/SrTiO3(001)–(√13 × √13)

The interfacial FeSe/TiO_2−δ coupling induces high-temperature superconductivity in monolayer FeSe films. Using cryogenic atomically resolved scanning tunneling microscopy/spectroscopy, we obtained atomic-site dependent surface density of states, work function, and the pairing gap in the monolayer FeSe on the SrTiO₃(001)–(√13 × √13)–R33.7° surface. Our results disclosed the out-of-plane Se–Fe–Se triple layer gradient variation, switched DOS for Fe sites on and off TiO_5□, and inequivalent Fe sublattices, which gives global spatial modulation of pairing gap contaminants with the (√13 × √13) pattern. Moreover, the coherent lattice coupling induces strong inversion asymmetry and in-plane anisotropy in the monolayer FeSe, which is demonstrated to correlate with the particle–hole asymmetry in coherence peaks. These results disclose delicate atomic-scale correlations between pairing and lattice-electronic coupling in the Bardeen–Cooper–Schrieffer to Bose–Einstein condensation crossover regime, providing insights into understanding the pairing mechanism of multiorbital superconductivity.