A Study of Iron Chalcogenide Superconductivity and Cobaltite Magnetism

Date of Completion

January 2011

Keywords

Physics, Low Temperature|Physics, Solid State|Physics, Condensed Matter|Engineering, Materials Science

Degree

Ph.D.

Abstract

This dissertation focuses on two strongly correlated electron systems, including Fe-based superconductivity and cobaltite magnetism. ^ Iron-based superconductors, a new class of non-Cu-based high T c superconductors, have aroused great research interest since the discovery of LaO1-xF xFeAs by Hosono's group in 2008. The 11-type Fe-chalcogenides are structurally the simplest Fe-based superconductors, consisting only of FeX-buckled planes. We discovered a new superconducting member of the Fe-chalcogenide family: FeTeOx films. We carried out a systematic study on the crystal structure, electronic structure, transport and magnetic properties of both Fe1+xTe single crystal and FeTeOx thin films. X-ray absorption spectra show that oxygen incorporation changes the nominal Fe valence state dramatically, implying the important role of the Fe valence state in the superconductivity. Synchrotron and neutron diffraction experiments show a new structural transition at 12 K for superconducting FeTeOx films. Our Angle-resolved photoemission spectroscopy (ARPES) investigation reveals the presence of an energy gap (Δ ≈ 12 meV) for FeTe single crystal, which could be expected from the SDW order but that has never been observed in previous measurements. No SDW energy gap has been observed for the FeTeO x films. A density functional calculation has been employed in studying the oxygen incorporation effects on the crystal and electronic structure of FeTe. The most stable sites for the interstitial oxygen are found to be the unoccupied; symmetry-equivalent sites of Te. Also, we studied the strain effects and observed the suppression of superconductivity in FeSe filins under large tensile strain.^ An important feature of correlated materials is the presence of spontaneous phase separation driven by electronic interactions. We have studied the complex magnetism of SrCoOx, (2.5 ≤ x ≤ 3), and discovered a new magnetic phase separation for the higher oxygen concentration regime. Previously, it has been reported that SrCoOx structurally separates into two crystalline phases for 2.5 ≤ x ≤ 2.75, orthorhombic SrCoO2.5 and cubic SrCoO 2.75. We found here that for higher oxygen concentration, 2.88 ≤ x ≤ 3, SrCoOx exhibits a magnetic phase separation while maintaining a single crystallographic phase. The Tcs of two magnetic components correspond to SrCoO 2.88 and SrCoO3 with Tc = 220 K and 280 K, respectively. A phase diagram with four line phases is proposed for SrCoOx (2.5 ≤ x ≤ 3). ^

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