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...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
Unconventional superconductivity comes in a variety of forms, of which the high-Tc cuprates are one with a complex phase diagram containing rich physics. Over certain doping range, strange metal...Show moreUnconventional superconductivity comes in a variety of forms, of which the high-Tc cuprates are one with a complex phase diagram containing rich physics. Over certain doping range, strange metal behaviour is seen, for which a theoretical explanation is still lacking. This thesis reviews the theoretical advances in strange metal research and provides an overview of the experimental tools available. The experimental suggestions are based on the predictions of the AdS-CFT correspondence as a possible explanation for the strange metal peculiarities. This theory of holographic duality is characterised by the prediction of unconventional hydrodynamics in the strange metal cuprates. Therefore, several suggestions for hydrodynamic experimentation are given, firstly on the boundary effects for the electron current flow profile and secondly on viscous electron backflow after a nano-sized constriction. These are phenomena only yet observed in graphene. Thirdly, the sign-reversing Hall effect as seen in graphene possibly points towards unconventional hydrodynamics in the cuprates as well. This is followed by the description of preparatory experiments conducted on exfoliated flakes of Bi2Sr2CuO6+x, with a determined Tc of 10.0 K ± 0.3 K. These strange metal experiments confirm literature values and provide a solid platform for conducting future experiments on possible unconventional electron hydrodynamics in this material.Show less
Bi(2)Sr(2)Cu(1)O(6+x) is a high-temperature superconductor exhibiting strange metal behaviour. A strange metal shows linear resistivity over a long range of temperatures. The strange metal...Show moreBi(2)Sr(2)Cu(1)O(6+x) is a high-temperature superconductor exhibiting strange metal behaviour. A strange metal shows linear resistivity over a long range of temperatures. The strange metal behaviour can possibly be explained by the Anti-de Sitter (AdS)/Conformal Field Theory (CFT) correspondence. In order to investigate the correspondence, a method for reliably measuring the strange metal phase is required. Because measurements on macroscopic crystals deviate from the expected linear resistivity due to C-axis contribution, microscopic Bi(2)Sr(2)Cu(1)O(6+x) flakes had to be used for the measurements. Therefore macroscopic crystals were exfoliated and the resulting flakes were contacted with electron beam lithography. Observing the strange metal regime of Bi(2)Sr(2)Cu(1)O(6+x) under the superconducting dome furthermore requires high current densities and high magnetic fields. These prerequisites for breaking the superconducting phase were obtained by structuring the contacted flakes using a Focused Ion Beam. After successfully contacting the flakes, linear resistivity was actually observed. Calculations on a Hall-bar and a constriction then yielded values for the resistivity of Bi(2)Sr(2)Cu(1)O(6+x) in agreement with literature. By contacting flakes showing the strange metal behaviour the first step for research into the AdS/CFT correspondence has been taken.Show less
