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Exploring new unconventional superconductors is of great value for bothfundamental research and practical applications. It is a long-term challenge todevelop and study more hole-doped superconductors in 1111 system of iron-basedsuperconductors. However, fifteen years after the discovery of iron-basedsuperconductors, it has become increasingly difficult to discover new membersin this system by conventional means. Here we report the discovery ofsuperconductivity with the critical transition temperature up to 30.7 K in theparent compound CaFeAsF by an annealing treatment in air atmosphere. Thesuperconducting behaviors are verified in both the single-crystalline andpolycrystalline samples by the resistance and magnetization measurements. Theanalysis by combining the depth-resolved time-of-flight secondary ion massspectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS) measurementsshow that the introduction of oxygen elements and the consequent changing in Fevalence by the annealing treatment may lead to the hole-type doping, which isthe origin for the occurrence of superconductivity. Our results provide a newroute to induce hole-doped superconductivity in Fe-based superconductors.

Superconductivity with $T_c$ up to 30.7 K in air-annealed CaFeAsF