While electron-phonon coupling (EPC) provides a well-established framework for understanding conventional superconductors, its role in Fe-based materials, where strong electronic correlations and competing orders are prevalent, remains an open question. Here, we integrate first-principles dynamical mean-field theory calculations with the epitaxial growth of single-layer $\text{Fe}X$ ($X$=Se, S, Te) films on ${\mathrm{SrTiO}}_3$(001) substrates, which allows us to control the $\mathrm{Fe}{X}_4$ tetrahedral angle and electron correlations. Using scanning tunneling spectroscopy, we demonstrate a superconducting dome where the pairing gap peaks at a magic angle of the ${\mathrm{FeSe}}_4$ tetrahedron. Crucially, our calculations show that the electron coupling to the $A_{1g}$ phonon mode is maximized at this magic angle in the FeSe film. Compared with superconductivity in Cs/K doped bilayer FeSe/${\mathrm{SrTiO}}_3$, our finding demonstrates that the primary boost to superconducting $T_c$ in monolayer FeSe/${\mathrm{SrTiO}}_3$, arises from intralayer correlations that reinforce the EPC within FeSe film.

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• Multiband superconductivity
• Iron-based superconductors
• Angle-resolved photoemission spectroscopy
• Density functional theory
• Electron-correlation calculations
• Epitaxy
• First-principles calculations
• Green's function methods
• Many-body techniques
• Scanning tunneling microscopy
• Scanning tunneling spectroscopy