Superconductivity in Undoped CaFe₂As₂ Single Crystals at Ambient Pressure

Among the many theoretical mechanisms proposed for higher T_c, the interfacial mechanism provides not only continual inspiration but also hope. For pristine CaAs₂Fe₂ (Ca122) single crystals, previous studies have shown the existence of complex phase, but no superconductivity was reported at ambient pressure. Recently, we identified two different non-superconducting phases in Ca122 through thermal treatments. Such thermal treatments offer a reproducible, controllable, and reversible environment in which many characterization techniques can be applied. We successfully tuned a mixed-phase in Ca122 through a proper annealing process and induced the superconductivity with a T_c up to 25 K at ambient pressure[1]. The transport, magnetic, and room temperature X-ray results lead us to the conjecture that the origin of superconductivity arises from the interface of the mesoscopically stacked layers of PI and PII. To further understand the underlying mechanism, systematic low-temperature X-ray, STEM, and high-pressure measurements were conducted over a series of thermal treated Ca122 single crystals[2]. This work provides the first direct evidence of interfacial induced superconductivity in undoped Ca122 single crystals.