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Hunter Sims1 2 Donovan Leonard2 Axiel Birenbaum2 Zhouzhi Ge3 Lian Li3 Valentino Cooper2 Matthew Chisholm2 Sokrates Pantelides1 2

1, Vanderbilt University, Nashville, Tennessee, United States
2, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
3, West Virginia University, Morgantown, West Virginia, United States

It was recently demonstrated that monolayer FeSe on a SrTiO3 substrate is a superconductor with Tc between 60 and 100 K, compared to 8 K in bulk FeSe. This is in contrast to the expected behavior; thinning a superconductor typically reduces Tc. Similar results have been obtained in monolayer FeSe deposited on BaTiO3 and anatase TiO2. In contrast, Tc has been measured to be only 3.7 K in bi-layer FeSe deposited on graphene (extrapolated to about 2 K in a monolayer), pointing to the major role of the interface in enhancing superconductivity. Here we determine the atomic structure of an interfacial layer and identify its role in driving the increase in Tc using a combination of quantum mechanical calculations and scanning transmission electron microscopy. Within our DFT calculations, this interfacial layer hosts long-range magnetic and orbital order not found in the typical TiO2 surface termination nor in previously identified surface reconstructions. Our calculations suggest the presence of a weak magnetic coupling between the interfacial monolayer and the deposited FeSe film, yielding a rich phase space of possible magnetic states. Interactions between this interfacial monolayer and FeSe generate symmetry-breaking distortions in the film that are favorable for increasing Tc and are not present in other possible FeSe / STO interface structures. We propose that this may provide a path forward toward the design and enhancement of other two-dimensional superconductors.

DFT calculations were supported by U.S. DOE grant DE-FG02-09ER46554 and the McMinn endowment at Vanderbilt University (HS, STP). They were performed at the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 (HS, STP). Work at ORNL is sponsored by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division (DNL, AYB, VRC, MFC). Work at West Virginia University (ZG, LL) is supported by the U.S. National Science Foundation, Division of Materials Research (DMR-1335215).

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