Possible Interface Enhanced Superconductivity in IRON Pnictides and Chalcogenides

The question of whether properly arranged interfaces can significantly enhance the superconducting transition temperature (T), and how such an enhancement can be achieved, make up one of the main challenges in superconductivity today. Despite many exciting discoveries in the investigation of interface superconductivity, it is still a highly debated area. This dissertation explores two possible interface-enhanced superconductivity systems: the rare-earth doped CaFe$2$As$2$ single crystal system [(Ca,R)122] with possible spontaneous phase separation due to defect-ordering and the ultra-thin FeSe film grown on SrTiO$3$ (STO) substrate by molecular beam epitaxy (MBE), where the associated interfaces are well defined. Before the discovery of superconductivity in these two systems, there were very few pieces of solid and clear experimental evidence that interface superconductivity could elevate T to a value that cannot be reached by either strain or doping. These two systems provide an opportunity to explore this issue.

In 2011, we discovered non-bulk superconductivity up to 49 K in single-crystalline CaFe$2$As$2$ via electron-doping by partial replacement of Ca by rare-earth elements (R = La, Ce, Pr, and Nd). This onset-T is higher than that for any known compounds consisting of one or more of its constituent elements of Ca, R, Fe and As in the (Ca,R)122 at ambient or under high pressure reported previously. After the establishment of the high T, extensive chemical and physical investigations of the (Ca,R)122 single crystals were carried out to reveal the origin of the high T in this system. We found extremely large magnetic anisotropy, doping-independence of T, and a close relationship between defect density and superconductivity. These observations suggest that the T enhancement may be associated with naturally occurring interfaces with different ordered defect structures and thus provide a possible new paradigm in the search for superconductors with a higher T.

It has been suggested that the unusually high T in the ultra-thin films of FeSe/STO system is associated with interfaces. The challenge lies in the determination of the onset T and the nature of the superconducting state. The extremely small magnetization signal associated with the superconducting phase is difficult to detect experimentally, and this lead to the development of improved experimental and analysis procedures. Detailed investigation of the superconductivity in eight 1-4 unit-cell (UC) FeSe films were performed, including magnetization and sheet resistance measurements over more than one and a half years. The results revealed the existence of the Meissner state below 20 K and an unusual superconducting mesostructure up to 45 K. A model, the mechanism of which is similar to that of the Andreev reflection between normal and superconducting carriers, is proposed to account for the observed magnetization. Phase angle analysis was used to experimentally elucidate the model. The complex superconducting structure observed is consistent with the challenges in synthesizing ultra-thin FeSe films with a superconducting temperature much higher than that of a bulk FeSe.