Superconductivity below 20 K in heavily electron-doped surface layer of FeSe bulk crystal.

J J Seo,J D Denlinger,J K Jeong,B Y Kim,S -K Mo,B S Kim,Y K Kim,Jun Sung Kim,J M Ok,C Kim

doi:10.1038/ncomms11116

Abstract

A superconducting transition temperature (Tc) as high as 100 K was recently discovered in one monolayer FeSe grown on SrTiO3. The discovery ignited efforts to identify the mechanism for the markedly enhanced Tc from its bulk value of 8 K. There are two main views about the origin of the Tc enhancement: interfacial effects and/or excess electrons with strong electron correlation. Here, we report the observation of superconductivity below 20 K in surface electron-doped bulk FeSe. The doped surface layer possesses all the key spectroscopic aspects of the monolayer FeSe on SrTiO3. Without interfacial effects, the surface layer state has a moderate Tc of 20 K with a smaller gap opening of 4.2 meV. Our results show that excess electrons with strong correlation cannot induce the maximum Tc, which in turn reveals the need for interfacial effects to achieve the highest Tc in one monolayer FeSe on SrTiO3.

Highlights

A superconducting transition temperature (Tc) as high as 100 K was recently discovered in one monolayer FeSe grown on SrTiO3
We demonstrate that the induced state on bulk FeSe satisfies these three characteristics of 1 ML FeSe
We first show that the doping level achieved via surface electron doping can reach that of the 1 ML FeSe/STO

Summary

Introduction

A superconducting transition temperature (Tc) as high as 100 K was recently discovered in one monolayer FeSe grown on SrTiO3. The other possibility comes from the stabilization of an ordered state by the interface that should provide strong spin fluctuation when it is broken by electron doping This view is based on an earlier experimental observation that the phase transition temperature increases with less number of layers[8]. A simple way to address the issue would be to fabricate a free standing 1 ML FeSe with excess electrons It can clearly tell us if the interface effect is needed to achieve the enhanced superconductivity, but is practically impossible to achieve. In the electronic structure point of view, the induced state is found to possess all the key characteristic aspects of 1 ML FeSe/STO: heavy electron doping, reduced dimensionality (2D) and enhanced electron correlation strength. We demonstrate that the induced state on bulk FeSe satisfies these three characteristics of 1 ML FeSe

Methods

Results

Conclusion