Angle-Resolved Photoemission Spectroscopy Study of Spin Fluctuations in the Cuprate Superconductors

Description

This thesis makes significant advances towards an understanding of superconductivity in the cuprate family of unconventional, high-temperature superconductors. Even though the high-temperature superconductors were discovered over 35 years ago, there is not yet a general consensus on an acceptable theory of superconductivity in these materials. One of the early proposals suggested that collective magnetic excitations of the conduction electrons could lead them to form pairs, which in turn condense to form the superconducting state at a critical temperature Tc. Quantitative calculations of Tc using experimental data were, however, not available to verify the applicability of this magnetic mechanism. In this thesis, the author constructed an angle-resolved photoemission apparatus that could provide sufficiently accurate data of the electronic excitation spectra of samples in the normal state, data which was furthermore unusually devoid of any surface contamination. The author also applied the Bethe-Salpeter method to his uncommonly pristine and precise normal state data, and was able to predict the approximate superconducting transition temperatures of different samples. This rare combination of experiment with sophisticated theoretical calculations leads to the conclusion that antiferromagnetic correlations are a viable candidate for the pairing interaction in the cuprate superconductors. Supervisor's Foreword Acknowledgments Parts of This Thesis Have Been Published in the Following Journal Article Contents 1 Introduction References 2 Superconductivity and the Cuprates 2.1 BCS Superconductivity 2.2 Chemistry and Electronic Structure of the Undoped Cuprates 2.3 Cuprate Phase Diagram: Superconductivity and the Pseudogap 2.4 Magnetism in the Cuprates and Superconductivity References 3 Angle-Resolved Photoemission Spectroscopy 3.1 Basic Description 3.2 Limitations 3.3 Conservation Laws 3.4 The Spectral Function 3.5 Alternative Ways of Viewing Photoemission Data References 4 Experimental Details 4.1 System Overview 4.2 System Bakeout 4.3 Sample Manipulator 4.4 The Helium Lamp 4.4.1 Principles of Operation 4.4.2 Glow Discharge in the UVS 300 Model 4.4.3 Basic Notions on the Helium Spectrum 4.4.4 The Duoplasmatron 4.4.5 Modifications to the UVS 300 Lamp 4.5 Sample Preparation 4.6 System Characterization 4.6.1 Flux 4.6.2 Energy Resolution 4.6.3 Sample ``Aging'' References 5 Results 5.1 ARPES Data and the Spectral Function 5.2 The Bare and Interacting Spin Susceptibilities in RealFrequencies 5.3 Solving the BSE in Matsubara Frequencies 5.4 Uncertainties in the Eigenvalues 5.5 The Coupling Energies U and U0 References 6 Conclusions References A Solution of the Bethe-Salpeter Equation A.1 Fourier Techniques for Convolution Sums A.2 The Power Method A.3 Simple Tests of the Algorithm for the BSE References B Alternative Estimate of the Spin-Fermion Coupling from the Single-Particle Self-energy References C Additional Results for the OP91 Sample Reference D The Anomalous Green's Function References