Capping layer influence and isotropic in-plane upper critical field of the superconductivity at the FeSe/SrTiO3 interface

TL;DR

This study investigates how different capping layers affect superconductivity in FeSe/SrTiO3 films, revealing that certain caps can suppress or preserve superconductivity and showing an almost isotropic in-plane upper critical field, which informs pairing mechanisms.

Contribution

It demonstrates how specific capping layers influence superconductivity and provides new insights into the pairing symmetry through magnetoresistance measurements.

Findings

FeTe capping preserves Tc in FeSe/SrTiO3.

Non-metallic Te capping suppresses superconductivity.

FeTe-capped films show isotropic in-plane critical magnetic field.

Abstract

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 magneto-transport measurements to examine the effect of three capping layers that provide distinctly different charge transfer into FeSe: compound FeTe, non-metallic 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 non-metallic 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.