Superconductivity above 30 K due to the introduction of oxygen in CaFeAsF

TL;DR

This paper reports the discovery of superconductivity above 30 K in CaFeAsF induced by oxygen doping through air annealing, highlighting a new route for hole-doped iron-based superconductors.

Contribution

It demonstrates that oxygen incorporation via annealing can induce high-temperature superconductivity in CaFeAsF, a novel approach in 1111 iron-based superconductors.

Findings

Superconductivity with Tc up to 30.7 K was achieved in CaFeAsF.

Oxygen doping alters Fe valence, leading to hole-type doping.

Superconductivity was confirmed in both single-crystalline and polycrystalline samples.

Abstract

Exploring new unconventional superconductors is of great value for both fundamental research and practical applications. It is a long-term challenge to develop and study more hole-doped superconductors in 1111 system of iron-based superconductors. Here we report the discovery of superconductivity with a critical transition temperature up to 30.7 K in the compound CaFeAsF by a post-annealing treatment in air atmosphere. The superconducting behaviors are verified in both the single-crystalline and polycrystalline samples by the resistance and magnetization measurements. The analysis by combining the depth-resolved time-of-flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS) measurements shows that the introduction of oxygen elements and the consequent changing in Fe valence by the annealing treatment may lead to the hole-type doping, which is the origin for the occurrence of superconductivity. Our results pave the way for further in-depth investigations on the hole-doped 1111 system in iron-based superconductors.