Nuclear magnetic resonance study of the onset of superconductivity and low energy excitations in high temperature superconductors

NMR measurements are reported of the nuclear spin-lattice rate in a high temperature superconductor in various magnetic fields both in the normal state near T_c and at low temperatures in the mixed state. The magnetic field dependence is used as a tool to determine what processes dominate the energy spectrum of the low energy excitations.; In the normal state of near-optimally doped YBa₂Cu₃O ₇ we found coexistence of two pseudogaps. One pseudogap, that dominates the spin-susceptibility near q = (π, π), originates from high energy processes and is not intimately tied to superconductivity. The second pseudogap, evident in the spin-susceptibility away from q = (π, π), likely originates from superconducting fluctuations as a precursory effect to superconductivity.; To be able to efficiently measure slow relaxation rates at low temperatures and at high fields, we have developed a progressive saturation technique for measuring $T-11$ of nuclei whose Zeeman levels are unequally split by quadrupolar interactions.; In the mixed state we were able to account for the anomalously broad NMR linewidth by taking into account the field distribution arising from the vortex lattice and the field distribution arising from antiferromagnetic moments, decaying on the length scale of the superconducting coherence length, localized in and around the vortex core.; From the NMR rate measurement in the vortex state, we find that outside the vortex cores the energy spectrum of the quasiparticles is dominated by their interaction with supercurrents, i.e. their energy is Doppler shifted by a supercurrent momentum, as is expected for a superconductor with nodes in the gap. At high magnetic field we find that, in addition to the Doppler shift, the Zeeman energy of the quasiparticles becomes important. Furthermore, we infer that these quasiparticle are antiferromagnetically correlated.; In the vortex core region, we find that the NMR rate is strongly enhanced in high magnetic fields implying the existence of some sort of bound state in the vortex core.