Probing nano-scale atomic and electronic structures of iron-based superconductors

The discovery of iron-based high-Tc superconductors has attracted renewed interests in unconventional superconductivity after the intense research in the past two decades on cuprates. Similar to the cuprate superconductors, the iron-based superconductors exhibit high T c that the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity has failed to predict. Furthermore, the iron-based high-T c superconductors demonstrate intrinsic properties that are distinct from the cuprates, including a different pairing symmetry, coexistence of superconducting and magnetic ordering, and emergence of superconductivity by isovalent doping. The iron-based high-Tc superconductors belong to the structure families of chalcogenides and pnictides. Crystal structures and bonding have large effects on superconductivity in these materials. Investigating atomic and electronic structures is thus essential to help understand their interesting properties.;In this thesis, I have investigated the bulk and thin-film form of iron-based superconductors by using a combination of scanning transmission electron microscopy and electron energy loss spectroscopy. The major results are summarized below.;In the isovalent doped systems, Fe1+yTe1-xSe x and BaFe2(PxAs1-x)2, we discovered nanometer-scale phase separation associated with chemical inhomogeneity. Direct evidence of phase separation was obtained from the Z-dependent image contrast recorded in a scanning transmission electron microscope (STEM) using a high angle annular dark field (HAADF) detector. By investigating energy loss near edge structure (ELNES) of the Fe-L2,3 edge recorded in electron energy loss spectroscopy (EELS) spectra, especially the L3/L 2 white-line intensity ratio, we show the d-state occupancy of the Fe changes with composition. The results here provide structural evidences which help to explain the coexistence of superconducting and magnetic ordering in these materials, as well as demonstrate a direct effect on the electronic structure by isovalent doping.;Oxygen annealing effect is studied next in single crystals of Fe 1.08Te0.55Se0.45. The as-grown sample with the tetragonal PbO-type structure is non-superconducting owing to the excess Fe beyond the stoichiometric content of 1. Superconductivity is induced after oxygen annealing with an onset and zero resistance transition temperature around 14.5 K and 11.5 K, respectively. The oxygen doping is evidenced by electron energy loss spectroscopy and accompanied by improved homogeneity in the remaining PbO-type phase, as well as an increase in the L3/L 2 intensity ratio of the Fe-L2,3 edge, indicating an increase in Fe valence. Local phase transformation from the tetragonal PbO-type phase to the hexagonal NiAs-type phase is also observed after oxygen annealing.;Epitaxial Fe1+yTe thin-films with sharp 1-2 interfacial layers grown by molecular beam epitaxy (MBE) on LaAlO3 become superconducting when exposed to oxygen after growth at elevated sample temperatures. The interfacial strain caused by lattice mismatch is not detected in the thin-film. Oxygen, occupying interstitial sites, is detected next to the Fe layers by electron energy loss spectroscopy (EELS). Density functional Theory (DFT) calculations suggest preferential occupancy of oxygen located above the center of the Fe square lattice, at the opposite side of Te. The oxygen induced superconductivity depends critically on film quality; excess of Fe during MBE growth leads to amorphous-like interfacial film, a decrease in (001) lattice spacing and loss of superconductivity. Our results show that the balance between hole doping of diffused oxygen and electron doping of excess Fe is essential for the emergence of superconductivity in Fe1+yTe films.