Capping Layer Influence and Isotropic In-Plane Upper Critical field of the Superconductivity at the FeSe / SrTiO3 Interface


Author: Yanan Li, Ziqiao Wang, Run Xiao, Qi Li, Ke Wang, Anthony Richardella, Jian Wang, Nitin Samarth APS Physics Logo | © APS Physics Physical Review Materials
Date: 2021
Instruments: EVO 25/50 MBE, LT NANOPROBE Lab, LT NANOPROBE, Materials Innovation Platform (MIP)

Understanding the superconductivity at the interface of FeSe / SrTiO3 is a problem of great contemporary interest due to the significant increase in critical temperature (Tc) compared to that of bulk FeSe, as well as the possibility of an unconventional pairing mechanism and topological superconductivity. We report a study of the influence of a capping layer on superconductivity in thin films of FeSe grown on SrTiO3 using molecular beam epitaxy. We used in vacuo four-probe electrical resistance measurements and ex situ magnetotransport measurements to examine the effect of three capping layers that provide distinct charge transfer into FeSe: insulating FeTe, nonmetallic Te, and metallic Zr. Our results show that FeTe provides an optimal cap that barely influences the inherent Tc found in pristine FeSe / SrTiO3, while the transfer of holes from a nonmetallic Te cap completely suppresses superconductivity and leads to insulating behavior. Finally, we used ex situ magnetoresistance measurements in FeTe capped FeSe films to extract the angular dependence of the in-plane upper critical magnetic field. Our observations reveal an almost isotropic in-plane upper critical field, providing insight into the symmetry and pairing mechanism of high-temperature superconductivity in FeSe.