Surface superconductivity emerged from disordered surface in undoped BaFe2As2

Abstract

Among the iron-based superconductors, ``122''-type compounds have been widely studied due to the availability of high-quality single crystals. However, due to lack of a natural cleavage plane, the cleaved surface is usually polarized, which allows doping change or structure modification on the surface. In this paper, we propose a simple method to successfully transform the 122-type parent compound $\mathrm{Ba}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ to a superconductor via room-temperature relaxation or uniaxial strain. Using scanning tunneling microscopy/spectroscopy, we demonstrate that this superconductivity results directly from the disordered surface morphologies. Based on the transport results and the disappearance of superconductivity with every new cleave, we conclude that the superconductivity only manifests at the most superficial surface. Our finding opens up an unexpected way to achieve superconductivity in undoped $\mathrm{Ba}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$, other than chemical doping, which may offer an alternative channel to understand the iron-based superconductivity and its connection with disorders.