Doctoral Dissertations

Date of Award

12-2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Physics

Major Professor

Steven Johnston

Committee Members

Christian Batista, Elbio Dagotto, David Mandrus

Abstract

The microscopic properties of the phonon mediated conventional superconductors are well explained by the Bardeen-Cooper-Schrieffer (BCS) theory. However, a comprehensive description of the unconventional superconductivity such as the cuprates and iron-based superconductors is still under considerable debate. One of the theories proposed for explaining the pairing mechanism is the spin fluctuation exchange perhaps playing a leading role in inducing the unconventional superconductivity[1]. These materials are thought to be unconventional because they share certain commonalities that are absent in the conventional superconductors. The materials possess variety of phases upon doping including the superconductivity residing in close proximity to magnetism. They show an anisotropic superconducting order parameter. The transition temperatures tend to be higher than those predicted by the conventional electron-phonon couplings. These observations differ from the conventional superconductors indicating that the electron-phonon interactions may be non-factors or play a minimal role in paring. However, a significant phonon renormalization interpreted from photoemission spectroscopy[2], a pronounced isotope effect in cuprates[3], and the emergence of the replica band due to strong forward scattering phonons[4] raise questions regarding the role of phonons in these magnetic superconductors.McMillan and Rowell have studied the low energy excitations seen from a tunneling experiment to prove the electron-phonon coupling was indeed driving the superconductivity, and thus validated the BCS theory[5]. If the unconventional pairing is explained by the exchange of bosons, then one could use the extended version of the BCS theory such as the Eliashberg theory to describe properties of the superconductivity. In the present analysis, we have investigated the low energy features seen in various unconventional superconductors using a fully self-consistent algorithm. We find that the bosonic excitation picture has correctly explained many experimental features, though there were properties. The present analysis has enlightened the picture that the spin fluctuation exchange plays a pivotal role in mediating the pairing; we may further argue that the phonons can play an important role in renormalizing the electronic structure in cuprates; additionally, the forward scattering phonons can coexist with the spin fluctuation exchange to greatly enhance the transition temperature in a selected layered materials.

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