UCLA

Sr2RuO4 is the cleanest and most well-characterized example of unconventional superconductivity known to date. Early experimental reports found strong evidence for the “chiral p-wave” d = z(kx � iky) superconducting state, an electronic analog to the chiral A-phase of superfluid Helium-3. As a result, Sr2RuO4 was widely accepted as the paradigmatic example of a topological quasi-two-dimensional superconductor and this colored the analysis of experimental reports for over two decades. The NMR measurements presented in this thesis directly contradict this interpretation. A pronounced drop is observed in the 17O Knight shift, incompatible with the chiral p-wave state as well as a previous body of NMR work. The discrepancy is shown to arise from systematic heating of the sample due to the high amplitude NMR pulses. Through quantitative measurements of the residual Knight shift as a function of applied in-plane field we additionally derive an upper bound on the magnetic response of the superconducting condensate of less than 10% that of the normal state. This is sufficient to further rule out all pure p-wave order parameter candidates for Sr2RuO4 and provides strong evidence for even parity superconductivity. As such, these results represent a fundamental advancement in understanding the nature of superconductivity in this archetypal system.The normal metallic state of Sr2RuO4 is also a subject of interest due to its strong correlations as well as the proximity of the Fermi energy to a quasi-two-dimensional singularity in the density of states. Application of uniaxial stress is known to be able to tune the band structure through the singularity and is accompanied by profound changes to the physical properties, including a more than doubling of the superconducting critical temperature. We show by way of the 17O Knight shift that the Fermi liquid crossover scale in Sr2RuO4 can be driven to vanishing temperature with the application of in-plane uniaxial stress approaching the van Hove singularity. The behavior is then successfully described via the strain dependent dispersion of a non-interacting quasiparticle model. Finally, a recently reported magnetic phase appearing at applied stresses beyond the van Hove singularity is investigated with 17T1 measurements. Enhanced fluctuations are found in a small region of phase space near the purported line of transitions but do not extend to superconducting dome, making it unlikely they are important for the superconducting pairing mechanism.