1, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
3, West Virginia University, Morgantown, West Virginia, United States
A remarkable demonstration of the importance of the atomic-scale properties of interfaces is found in monolayer FeSe on SrTiO3 that exhibits a superconducting gap consistent with a critical temperature an order of magnitude higher than that seen for the bulk material. Here a combination of aberration-corrected scanning transmission electron microscopy and quantum mechanical calculations is used to determine the atomic structure of the film and its interface with the substrate and to identify what drives the increase in Tc. A comparison of experimental images of the interfacial region with multislice STEM simulations based on the calculated structure shows essentially perfect agreement in all respects (structural and compositional) and leads us to conclude we have accurately determined the FeSe/ SrTiO3 interface. Our calculations show that the interlayer in the FeSe/SrTiO3 system is not merely a passive glue holding substrate and film together. We find that electrons in the interfacial layer (IL) form electronic distributions (at 0 K) that stabilize a small distortion of the FeSe lattice that has been shown to enhance Tc in a strained, freestanding monolayer of FeSe. The shear-like distortions in the FeSe monolayer that develop naturally from the interfacial layer are not seen in calculations of freestanding FeSe nor in FeSe on the usual TiO2-terminated SrTiO3 surface. The continued study of this system is expected to lead to insights into 2D superconductivity and other novel phenomena.