20 K superconductivity in heavily electron doped surface layer of FeSe bulk crystal

TL;DR

This study reports 20 K superconductivity in electron-doped surface layers of FeSe, indicating that interface effects are crucial for achieving higher Tc beyond electron doping alone.

Contribution

It demonstrates that excess electrons with strong correlation are insufficient for maximum Tc, emphasizing the importance of interface effects in FeSe superconductivity.

Findings

Surface layer exhibits key spectroscopic features of 1ML FeSe on STO.

Superconductivity with Tc of 20 K observed without interface effects.

Electron doping alone cannot account for the highest Tc in FeSe systems.

Abstract

A superconducting transition temperature Tc as high as 100 K was recently discovered in 1 monolayer (1ML) FeSe grown on SrTiO3 (STO). The discovery immediately ignited efforts to identify the mechanism for the dramatically enhanced Tc from its bulk value of 7 K. Currently, there are two main views on the origin of the enhanced Tc; in the first view, the enhancement comes from an interfacial effect while in the other it is from excess electrons with strong correlation strength. The issue is controversial and there are evidences that support each view. Finding the origin of the Tc enhancement could be the key to achieving even higher Tc and to identifying the microscopic mechanism for the superconductivity in iron-based materials. Here, we report the observation of 20 K superconductivity in the electron doped surface layer of FeSe. The electronic state of the surface layer possesses all the key spectroscopic aspects of the 1ML FeSe on STO. Without any interface effect, the surface layer state is found to have a moderate Tc of 20 K with a smaller gap opening of 4 meV. Our results clearly show that excess electrons with strong correlation strength alone cannot induce the maximum Tc, which in turn strongly suggests need for an interfacial effect to reach the enhanced Tc found in 1ML FeSe/STO.