  Single-layer FeSe films with extremely expanded in-plane lattice constant of
3.99A are fabricated by epitaxially growing FeSe/Nb:SrTiO3/KTaO3
heterostructures, and studied by in situ angle-resolved photoemission
spectroscopy. Two elliptical electron pockets at the Brillion zone corner are
resolved with negligible hybridization between them, indicating the symmetry of
the low energy electronic structure remains intact as a free-standing
single-layer FeSe, although it is on a substrate. The superconducting gap
closes at a record high temperature of 70K 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 put strong constraints on the current theories, and support the
coexistence of both even and odd parity spin-singlet pairing channels as
classified by the lattice symmetry.
