Neutron scattering study of the iron based superconductors

In most iron-based and copper-oxide superconductors, the T_c [superconducting critical temperature] gradually increases upon charge carrier doping or isovalent doping. In the under-doped regime of electron-doped BaFe₂As₂ [Barium Iron 2 Arsenic 2], the superconductivity appears before the complete suppression of AF [antiferromagnetic] long-range order, creating a SC-AF [superconducting-antiferromagnetic] coexisting area. The interplay between long-range magnetic order and superconductivity was studied using a triple-axis thermal neutron spectrometer under an in-plane magnetic field. The suppression of superconductivity and the enhancement of long-range AF order were discovered under 11 Tesla, suggesting the competing nature of these two phases and the itinerant nature of iron-pnictides. We also measured the evolution of the neutron spin resonance in under-doped BaFe_1.925Ni_0.075As₂ [Barium Iron 1.925 Nickel 0.075 As 2] and in over-doped BaFe_1.85Ni_0.15As₂ [Barium Iron 1.85 Nickel 0.15 As 2]. Combining the previous results, we were able to compare the evolution of the neutron spin resonance energies in both BaFe_2-xNi_xAs₂ [Barium Iron (2-x) Nickel (x) Arsenic 2] and BaFe_2-xCo_xAs₂ [Barium Iron (2-x) Cobalt (x) Arsenic 2].

In late 2010, another type of iron-based superconductor alkali iron selenide A_0.8Fe_1.6+ySe₂ [Alkali 0.8 Iron (1.6+y) Selenium 2 ] (or “245 system”) was discovered. It is a unique system among the family of unconventional (high-T_c [superconducting critical temperature]) superconductors because of its novel phase diagram, insulating parent compounds and huge magnetic moment. Using the inelastic time-of-flight technique, we are able to show that the parent compound can be described well by a local Heisenberg model. The follow up experiments on a superconducting sample (below and above T_c) and insulating samples (below and above T_n [Neel Temperature]) provide a more detailed comparison and show more evidence that the alkali iron selenide system is rather different from either itinerant iron-pnictides or local copper-oxide systems.