Structure and Magnetism in Superconducting Iron Chalcogenides

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Date/Time:Thursday, 19 Apr 2012 - Thursday, 19 Apr 2012
Location:PHYSICS Hall Room 5
Channel:Condensed Matter Physics
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Vivek Thampy Johns Hopkins Univ., Baltimore, MD

Iron superconductors are unique in that they provide a wide range of control parameters such as structure, chemical composition, pressure and impurities that can be tuned to provide insight into the underlying mechanisms of cooper pairing. In this talk, I will discuss the relationship of superconductivity with a) structure, b) impurities, and c) low energy magnetic excitations in the structurally simplest iron superconductors, (Fe$_{1+y}$Te$_{1-x}$Se$_x$). I will demonstrate the importance of the Te/Se height to superconductivity and show that it is consistent with the $s_{pm}$ pairing symmetry. I will discuss the pivotal role played by iron impurities, occupying interstitial sites in Fe$_{1+y}$Te$_{0.62}$Se$_{0.38}$, in the microscopic origin of the quasi-static magnetism at $(frac{1}{2},0)$. We used polarized and unpolarized neutron scattering, simulations of the scattering function based on structural data, and a semi-metallic 5-band model with super-exchange interactions with the interstitial iron to show the formation of magnetic polarons around the interstitial iron atoms which seed the observed $(frac{1}{2},0)$ magnetism. Though the quasi-static magnetism occurs at $(frac{1}{2},0)$, the low energy spin dynamics are dominated by the $(frac{1}{2},frac{1}{2})$ fluctuations, like other iron based superconductors. I will discuss the characteristics of the signature feature in the low energy inelastic neutron scattering spectrum - the so-called spin resonance.