Local interplay between the superconducting gap and the Van Hove singularity in overdoped Bi2201

Mapping the local electronic structure on a nm scale has been vital to the understanding of cuprates. Experiments that probe the local DOS with STS have shown the superconducting gap to be inhomogeneous in these compounds [1]. More recently, ARPES experiments on cuprates have shown the gap to fill rather than close, but it is yet unclear what this process looks like locally [2]. Other STS results have shown that the DOS in overdoped Bi-2201 also contains a pronounced peak attributed to the Van Hove singularity [3]. Such a momentum space origin of the peak remains contested, however, as this peak is reported as similarly inhomogeneous in real space. Presented in this thesis is a phenomenological analysis of STS spectra of Bi2Sr2CuO6+δ (Bi-2201) in the far overdoped regime (OD12K and OD3K). Two different fitting models demonstrate that this supposed VHS is responsible for electron-hole asymmetry of the coherence peaks of the superconducting gap by having the peak energy be close to...Show moreMapping the local electronic structure on a nm scale has been vital to the understanding of cuprates. Experiments that probe the local DOS with STS have shown the superconducting gap to be inhomogeneous in these compounds [1]. More recently, ARPES experiments on cuprates have shown the gap to fill rather than close, but it is yet unclear what this process looks like locally [2]. Other STS results have shown that the DOS in overdoped Bi-2201 also contains a pronounced peak attributed to the Van Hove singularity [3]. Such a momentum space origin of the peak remains contested, however, as this peak is reported as similarly inhomogeneous in real space. Presented in this thesis is a phenomenological analysis of STS spectra of Bi2Sr2CuO6+δ (Bi-2201) in the far overdoped regime (OD12K and OD3K). Two different fitting models demonstrate that this supposed VHS is responsible for electron-hole asymmetry of the coherence peaks of the superconducting gap by having the peak energy be close to gap size Δ. It it also shown that the gap filling hypothesis holds up with the addition of a phenomenological peak, although the best model for how the gap closes stays undecided on. Aside from fitting, peak detection analysis supports the same conclusions about the VHS. A more general dimensionality reduction approach is applied to investigate relations between features in the local DOS, utilising Principle Component Analysis PCA) and Uniform Manifold Approximation and Projection (UMAP). However no conspicuous clusters were found in the STS data. This thesis paves the way for more complete models of the still poorly-understood LDOS of cuprates and how it varies in real space. Furthermore, the affirmation of a filling gap adds to the evidence pointing to high-temperature superconductivity being limited by phase coherence. [1] Ø. Fischer, M. Kugler, I. Maggio-aprile, C. Berthod, and C. Renner, Scanning tunneling spectroscopy of high-temperature superconductors, 79 (2007). [2] T. J. Reber, S. Parham, N. C. Plumb, Y. Cao, H. Li, Z. Sun, Q. Wang, H. Iwasawa, M. Arita, J. S. Wen, Z. J. Xu, G. D. Gu, Y. Yoshida, H. Eisaki, G. B. Arnold, and D. S. Dessau, Pairing, pair-breaking, and their roles in setting the Tc of cuprate high temperature superconductors, (2015). [3] A. Piriou, N. Jenkins, C. Berthod, I. Maggio-Aprile, and Fischer, First direct observation of the Van Hove singularity in the tunnelling spectra of cuprates, Nature Communications 2, 221 (2011).Show less

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Local interplay between the superconducting gap and the Van Hove singularity in overdoped Bi2201