dc.contributor.author Sprau, Peter Oliver dc.date.accessioned 2018-04-26T14:16:04Z dc.date.available 2018-04-26T14:16:04Z dc.date.issued 2017-08-30 dc.identifier.other Sprau_cornellgrad_0058F_10323 dc.identifier.other http://dissertations.umi.com/cornellgrad:10323 dc.identifier.other bibid: 10361432 dc.identifier.uri https://hdl.handle.net/1813/56771 dc.description.abstract FeSe is the focus of intense research interest because of its unusual non-magnetic nematic state and because it forms the basis for achieving the highest critical temperatures of any iron-based superconductor. However, its Cooper pairing mechanism has not been determined because an accurate knowledge of the momentum-space structure of superconducting energy gaps $\Delta_i(\vec{k})$ on the different electron-bands $E_i(\vec{k})$ does not exist. Here we use Bogoliubov quasiparticle interference (BQPI) imaging to determine the coherent Fermi surface geometry of the $\alpha$- and $\varepsilon$-bands surrounding the $\Gamma = (0, 0)$ and $X = (\pi / a_{Fe}, 0)$ points of FeSe, and to measure their superconducting energy gaps $\Delta_{\alpha}(\vec{k})$ and $\Delta_{\varepsilon}(\vec{k})$. We show directly that both gaps are extremely anisotropic but nodeless, and are aligned along orthogonal crystal axes. Moreover, by implementing a novel technique we demonstrate the sign change between $\Delta_{\alpha}(\vec{k})$ and $\Delta_{\varepsilon}(\vec{k})$. This complex configuration of $\Delta_{\alpha}(\vec{k})$ and $\Delta_{\varepsilon}(\vec{k})$, which was unanticipated within pairing theories for FeSe, reveals a unique form of superconductivity based on orbital selective Cooper pairing of electrons from the $d_{yz}$ orbitals of iron atoms. This new paradigm of orbital selectivity may be pivotal to understanding the microscopic interplay of quantum paramagnetism, nematicity and high temperature superconductivity. dc.language.iso en_US dc.subject superconductivity dc.subject Condensed matter physics dc.subject FeSe dc.subject orbital selectivity dc.subject Scanning Tunneling Microscopy dc.subject strong correlations dc.title Discovery of Orbital Selective Cooper Pairing in FeSe dc.type dissertation or thesis thesis.degree.discipline Physics thesis.degree.grantor Cornell University thesis.degree.level Doctor of Philosophy thesis.degree.name Ph. D., Physics dc.contributor.chair Davis, James C. dc.contributor.committeeMember McEuen, Paul L. dc.contributor.committeeMember Kim, Eun-Ah dcterms.license https://hdl.handle.net/1813/59810 dc.identifier.doi https://doi.org/10.7298/X4TB153Z
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