Degree Type

Dissertation

Date of Award

2013

Degree Name

Doctor of Philosophy

Department

Physics and Astronomy

First Advisor

Ruslan Prozorov

Abstract

\specialchapt{ABSTRACT}

Many iron-based superconductors undergo a tetragonal to orthorhombic change of their crystallographic lattice symmetry, as well as paramagnetic to anti-ferromagnetic ordering upon cooling through a characteristic temperature $T_N$. The anisotropic structure of the orthorhombic crystal symmetry would naturally lead one to expect to find in-plane electronic anisotropy.

Upon cooling through $T_s$, and going into the orthorhombic symmetry, crystals divide into many small \textit{twin domains}. Although crystallographically identical, the twin domains express four different rotations of the orthorhombic lattice within the $\bf{ab}$-plane making direct measurements along an individual orthorhombic axis impossible. This complication lead to the developement of uniaxial stress and strain detwinning, which makes one of the four domain rotations far more energetically favorable than the other three, to the extent that more than 90\% of the entire crystal volume may be represented by the dominant domain. Once in this $\textit{detwinned}$ state, measurements may be made along the individual orthorhombic axes, allowing one to probe in-plane anisotropy.

Following the developement of the detwinning technique, measurements of the in-plane resistivity anisotropy between the orthorhombic $a_o$ and $b_o$ axes were made. The results, however, turned out to be the opposite of what is predicted from simple models of electrical resistivity. Many different competing theories were developed to understand this unusual behavior. The goal of my doctoral research is to understand the validitiy of these different theories and discover the primary driving force behind this unexpected result.

My experiments on the effects of doping on the in-plane resistivity anisotropy yielded an interesting result that not only is there an assymetry between electron and hole doping, but also that the sign of the anisotropy changes sign with sufficient hole doping. This result, along with the tempreature dependence of the in-plane resistivity anisotropy, provide very strong evidence that the primary source is anisotropic scattering due to magnetic spin fluctuations.

Copyright Owner

Erick Blomberg

Language

en

File Format

application/pdf

File Size

93 pages

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