Probing the physics of high-temperature superconductivity: Spin dynamics and pair-breaking effects

In this thesis we study two important aspects of the high critical temperature cuprate superconductors: spin dynamics in the superconducting state and pair-breaking effects in the pseudogap phase. Understanding magnetic correlations in the cuprates is essential to a full understanding of the underlying physics in these materials. And our studies of pair-breaking may provide a deeper understanding of the pseudogap origin. Indeed, these very intriguing pseudogap phenomena still remain a mystery which when solved will contribute an important piece to the jigsaw puzzle of high temperature superconductivity.; In our study of spin dynamics, we employ an RPA formalism with strong Coulomb repulsion. These calculations give a natural explanation of the observed evolution with frequency of neutron peaks from incommensurate to commensurate and back to incommensurate at higher frequencies found in underdoped YB ₂Cu₃O_6+x. We also explain related phenomena such as peak sharpening, and the spin gap observed in La_2−xSr _xCuO₄. We find these phenomena are generic manifestations of d-wave pairing symmetry, while the underlying Fermi surface topology plays a less important role.; We probe the physics of the pseudogap state by studying two different pair-breaking phenomena: magnetic fields and impurities. We generalize our theory of an extended BCS superconductivity in the clean case to include the effects of a magnetic field and impurities. In our physical picture the pseudogap state derives from the same pairing interaction which leads to superconductivity. The different responses of T_c and T* to these pair-breakers are due to a normal state gap at T_c which is absent at T*. This new physics cannot be obtained from common BCS intuition. Because of the presence of a pseudogap, there exist multiple length scales in the system, which only become equivalent in the BCS (weak coupling) regime.