Degree Type

Dissertation

Date of Award

2015

Degree Name

Doctor of Philosophy

Department

Physics and Astronomy

First Advisor

Robert J. McQueeney

Second Advisor

Alan I. Goldman

Abstract

Unconventional superconductivity and antiferromagnetism are often found in close proximity to one another. For the series of compounds Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, which possesses both antiferromagnetism and superconductivity for the under-doped range of composition, this observation is certainly true. The close proximity, and in fact coexistence for under-doped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, of antiferromagnetism and superconductivity has encouraged speculation that antiferromagnetic spin fluctuations may mediate the electron pairing interaction in unconventional superconductors. Previous studies indicated that the spin fluctuations at optimally-doped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ are diffusive, while those at \bafeas are well defined spin wave excitations. Therefore, the nature of magnetic excitations in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ must change with the introduction of cobalt; but it is unclear if that change is merely a consequence of the loss of antiferromagnetic order, or a necessary ingredient for the appearance of superconductivity. To resolve this uncertainty, this work has been undertaken to study the spin fluctuations of five Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ compositions varying in cobalt concentration from lightly-doped to nearly optimally-doped as well as representative samples of other, non-superconducting, transition metal substituted \bafeas compounds. The spin fluctuations of these samples, in their antiferromagnetically ordered and (where possible) superconducting states have been studied via triple-axis and time-of-flight inelastic neutron scattering, and definitively determine the importance of spin fluctuations for superconductivity in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$.

Copyright Owner

Gregory Scott Tucker

Language

en

File Format

application/pdf

File Size

253 pages

Share

COinS