Measurement of an Enhanced Superconducting Phase and a Pronounced Anisotropy of the Energy Gap of a Strained FeSe Single Layer inFeSe/Nb:SrTiO3/KTaO3Heterostructures Using Photoemission Spectroscopy

Single-layer FeSe films with an extremely expanded in-plane lattice constant of $3.99\pm 0.02\AA$ are fabricated by epitaxially growing $\mathrm{FeSe}/\mathrm{Nb}:{\mathrm{SrTiO}}_3/{\mathrm{KTaO}}_3$ heterostructures and studied by in situ angle-resolved photoemission spectroscopy. Two elliptical electron pockets at the Brillouin zone corner are resolved with negligible hybridization between them, indicating that the symmetry of the low-energy electronic structure remains intact as a freestanding single-layer FeSe, although it is on a substrate. The superconducting gap closes at a record high temperature of 70 K for the iron-based superconductors. Intriguingly, the superconducting gap distribution is anisotropic but nodeless around the electron pockets, with minima at the crossings of the two pockets. Our results place strong constraints on current theories.