Date of Award

8-2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Physics

Major Professor

Pengcheng Dai

Committee Members

Adriana Moreo, Haidong Zhou, Veerle M. Keppens

Abstract

In most iron-based and copper-oxide superconductors, the Tc [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 BaFe1.925Ni0.075As2 [Barium Iron 1.925 Nickel 0.075 As 2] and in over-doped BaFe1.85Ni0.15As2 [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 BaFe2-xNixAs2 [Barium Iron (2-x) Nickel (x) Arsenic 2] and BaFe2-xCoxAs2 [Barium Iron (2-x) Cobalt (x) Arsenic 2].

In late 2010, another type of iron-based superconductor alkali iron selenide A0.8Fe1.6+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-Tc [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 Tc) and insulating samples (below and above Tn [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.

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