Possible interface superconductivity in rare-earth-doped CaFe ₂ As ₂ and undoped CaFe ₂ As ₂

Among the many theoretical mechanisms proposed for higher T_c, the interfacial mechanism provides not only continual inspiration but also hope. Difficulties do exist, as most of the materials proposed to exhibit the interfacial mechanism are artificially formed heterostructures and are by nature delicate and easily disturbed by strain and change in the stoichiometry at the interface. The discovery of superconductivity in naturally assembled rare-earth(R)-doped CaFe₂As₂ (Ca122) with T_c up to 49 K, has brought an alternate route to tackle the problem. Detailed magnetization, resistivity, chemical composition, specific heat, and annealing studies have been systematically carried out on R-doped Ca122 single crystals with R = La, Ce, Pr, and Nd. The experimental observations lead us to the conjecture that the T_c enhancement may be related to naturally occurring chemical interfaces associated with defects. Most recently, we successfully induced superconductivity in undoped Ca122 with a T_c up to 25 K through proper thermal treatment. We later conducted systematic annealing for different time periods at a constant temperature of 350 ^∘C on the a singe crystal Ca122 sample quenched rapidly from 850 ^∘C to room temperature. The room temperature X-ray diffraction (XRD) shows an initial tetragonal structure (PI) with a c_I = 11.547(1) Å prior to any annealing and remains to be a tetragonal phase (PII) with only a slightly larger c_II = 11.702(2) Å following prolong annealing. Neither PI or PII phase is superconducting above 2 K. However, superconductivity was detected after intermediate annealing, when mixture of the two phase appears. The evolution of mixed phase as suggested by the XRD results is consistent with that shown by the XRD simulations with microstructures of stacking of the two phases. The observation has provided the most direct evidence of interfacial induced superconductivity in Ca122 to date and possibly in R-doped Ca122.