Condensed-Matter Physics & Materials Science Seminar

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, (Fe1+yTe1-xSex). I will demonstrate the importance of the Te/Se height to superconductivity and show that it is consistent with the s± pairing symmetry. I will discuss the pivotal role played by iron impurities, occupying interstitial sites in Fe1+yTe0.62Se0.38, in the microscopic origin of the quasi-static magnetism at (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 (1/2,0) magnetism. Though the quasi-static magnetism occurs at (1/2,0), the low energy spin dynamics are dominated by the (1/2,1/2) 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.