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UBC Theses and Dissertations

Electron-phonon mediated superconductivity probed by ARPES : from MgB2 to lithium-decorated graphene Ludbrook, Bartholomew Mears


This thesis traces a path from conventional superconductivity in a bulk material to the introduction of superconductivity and other novel phenomena in graphene. Magnesium diboride is a conventional superconductor, where the pairing is mediated by the electron-phonon coupling. ARPES (angle resolved photoemission spectroscopy) is shown to be an excellent probe to quantitatively study the momentum dependence of the electron-phonon coupling, demonstrating the origin of the distribution of superconducting gap sizes previously observed with other experimental techniques. Next, we exploit our understanding of the electron-phonon coupling to study how it can be tuned in a low dimensional system. It is shown that the electron-phonon coupling in graphene can be strongly enhanced by the deposition of alkali adatoms. High resolution, low temperature ARPES measurements provide the first experimental evidence of superconductivity in this two-dimensional system, showing a temperature dependent pairing gap, and an estimated Tc of ~ 6K. Finally, we present a study of another graphene-adatom system expected to show novel physics. Thallium on graphene has been predicted to enhance the spin-orbit coupling, leading to a robust topological insulator state. From ARPES measurements characterizing this system, we disentangle the long-range and short-range scattering contributions and show that thallium atoms act as surprisingly strong short-range scatterers. Our results are consistent with theoretical predictions for this system, indicating it is a good place to search for a two-dimensional topological insulator.

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