Condensed-Matter Physics & Materials Science Seminar

"Structure of Iron-Chalcogenide Superconductors and Thin-films"

Presented by Hefei Hu, University of Illinois at Urbana-Champaign

Thursday, June 28, 2012, 11:00 am — Bldg. 480 - Conference Room

Many similarities have been noted between iron-based and cuprate high-Tc superconductors, but several novel features exist in iron-based superconductors, such as non-d-wave paring symmetry, superconductivity induced by isovalent doping, coexistence of superconducting (SC) and antiferromagnetic (AFM) phases. The isovalently doped system, Fe1+yTe1-xSex, among other iron-based superconductors, received special attention due to its simple crystal structure and its extra tuning parameter, excess Fe, for superconducting and magnetic properties. To search for the structural origin of the properties of iron chalcogenides, I have carried out scanning transmission electron microscopy(STEM) and electron energy loss spectroscopy(EELS) study of Fe1+yTexSe1-x single crystals as well as Fe1+yTe thin films grown by molecular beam epitaxy(MBE) and combined atomic resolution analysis with transport measurement.
In Fe1+yTexSe1-x single crystals, nano-scale phase separation with ~20%, or less, fluctuations in Te concentration from the average compositions is observed using STEM-EELS. The energy-loss near-edge structure (ELNES) of the Fe-L2,3 edge changes as the composition varies, especially the L3 and L2 ratio. The L3/L2 ratio is sensitive to the d-state occupancy of the Fe atom. Fluctuations in the Te concentration and the resulted changes in d-state occupancy, resulting from phase separation, are direct evidences of spatially varying charge transfer or effective charge doping in this isovalently doped system, which provides a structural explanation of the coexistence of SC and AFM phases.
The parent compound Fe1+yTe in thin-film form becomes superconducting with oxygen incorporation. For the superconducting epitaxial Fe1+yTe thin-film, local d-spacing analysis of the film shows that the strain from lattice mismatch with the substrate quickly relaxes beyond the first layer. Evidence of replacement of Te by O is observed by STEM-EELS, when the films are grown in oxygen atmospher

Hosted by: Jing Tao

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